Tuesday, 13 October 2020

Genki 3 Way

A few posts back I talked about how I found it useful to compare a 30 day running average growth ratio to a 30 day running average nitrogen genki. This would tell me  how my grass was growing and how much nitrogen I had applied compared to the standard level which I have determined to be about 2/g nitrogen per square meter per month at a 100% growth potential.

After thinking and using that tool for a few months now, I wondered about the other nutrients. Last year I had issues with the grass not growing as expected and it turned out to be a phosphorus deficiency probably made worse by regular phosphite applications.

The growth ratio model showed that the grass was growing too slow and in reality it probably was as we suffered from issues caused by slow growing turf such as Anthracnose! The more experience I get with the growth ratio model, the more valuable it becomes as a tool to help me keep the growth rate where I want it and plant health issues to the minimum. Most issues I have had with the greens can be clearly seen when you look at how the grass is actually growing compared to how it should be growing.

I had soil tests and thought I had applied sufficient fertilizer but what I think may have happened was that as the roots shortened in the summer heat, the phosphorus in the surface layers of the soil were used up. The deeper rooted bentgrass was healthy and happy but the predominantly poa surfaces were suffering until phosphorus was applied.

This got me thinking about what it would look like to include a genki level for phosphorus and potassium as well. We know that potassium in cool season turfgrass is found at a ratio of about 0.5 compared to nitrogen and phosphorus is around 0.13. With this I can compare my applications to the standard amount. 

If the nitrogen genki is above 1 but growth is lower than expected, I can look to see where the other macronutrients are in respect to their standard amount. If growth is higher than expected I can stop applying nutrients altogether until the growth ratio returns to where I want it to be.

I currently look at 30 day running averages for everything but think that an annual running average might be appropriate for something like phosphorus which is used in a much smaller quantity than the other macros and is relatively immobile in the soil.

There are times when soil nutrient status won't impact growth rates and this is when either moisture or frost come into play. Without sufficient moisture grass won't grow and when there's frost, we don't get much growth either.

I wonder if something like this could replace soil testing for the high use macronutrient content in the soil? Soil testing provides a general snapshot but this provides an up the the day idea of how the grass is growing and how my macronutrient applications might be affecting the growth rates.

It could also potentially provide a more precise tool for managing bentgrass vs poa annua too. In my experience, the deeper rooted bentgrass has more access to nutrients and has always had a much higher growth rate than the poa. I hypothesize it my last post that it seems that fertilizer applications might be the best way to manage bentgrass over poa and this might be another tool that I can use.

As bentgrass populations rise the growth rates on those greens tends to go up. Again, this is probably due to the deeper roots having access to more nutrients in the soil and the tendency of bentgrass to grow more for a given amount of nutrient inputs. These two things combine to leave the greens with more bentgrass growing much quicker than those with mostly poa annua. Micah Woods hypothesized that the most sustainable grass might be the one with the highest growth rate for a given amount of inputs. I  agree.

As more and more bentgrass populates the putting greens I eventually need to start managing for bentgrass and not poa annua. Currently I need to keep both species happy so avoid a loss of turf quality. It's a fine balance and maybe this tool can help me do better with less guesswork. The practicality of managing greens to that extreme might prove to be a bit too ambitious but without the data you can't even begin to dream about doing anything like this.

Let's forget about that for a minute and look at the issues I had with growth in 2019 and how the genki level for the big 3 macronutients looked. For those sharp data people out there a few issues become glaringly obvious with models like this especially in the months with little to no growth. A light fertilizer application can throw that ratio wildly high as the ideal amount is close to zero and the closer to zero the ideal amount of fertilizer is, the more any amount of fertilizer applied will throw the model off towards infinite. You can see this impact on the chart below. Everything has this curved shape to it. This is kind of the reverse of the growth potential curve for my climate. Anyway.

Holy Shit! When you're not used to looking at something like this is looks like a rat's nest! That's ok and we will filter out some of the data to look at it individually. I've filtered the dates for the growing part of the season. You can still see how high some of the genki levels will go in the shoulder season.

It might look super inconsistent like this and it is! It will be interesting to see how increased awareness will impact my fertilizer applications going forward. Also if you want to criticize me, first show me your macronutrient and growth ratio chart. It might look like I have no clue what I'm doing and to be honest I really don't. The deeper you look into fertilizer use the more you will realize how much guesswork is involved. I'm just trying to reduce that guesswork.

The thick dark blue line is the growth ratio and as you can see, the growth rates in 2019 were less than I wanted them to be. Greens were fast but also anthracnose!

Here's the same chart but just with the N Genki 30 day and Growth ratio.

In the spring we were recovering a few greens that died due to civitas applications too late into the winter. Don't apply civitas in the winter unless it's warm and sunny.

The growth ratio was pretty good until June when growth rates typically pick up. As soon as the heat arrived, the turf growth didn't keep up to expectations. It's important to remember that this is a growth ratio, it isn't total yield. The growth rate stayed rather constant or slightly down but it should have been much higher. 

I've seen this before with nutrient deficiency issues. A neighboring course had growth stall after spring aerification and the more nitrogen they applied the worse things got. As they tried to increase growth (and the weather warmed up) they also increased the demand for the other soil nutrients and thus made the deficiency worse. It turned out that they were low in calcium and a week after a calcium application the greens were as good as ever despite what some people said (2 weeks to live my ass). This is exactly what happened to us last year. As soon as the demand for nutrients went up, the deficiency got worse. The total yield and growth ratio for June was much lower than we wanted and we paid the price.

For the entire year we were applying much more nitrogen than we needed to try and drive growth rates when in reality we were probably making things worse.

Now let's look at K Genki. 

For the most part, the K genki is high all year long. In August when the anthracnose started to get bad I started applying higher N and K as this is recommended in most of the literature. In reality these nutrients only have an impact on anthracnose if they are deficient. They weren't deficient. I haven't read anywhere that low phosphorus can make anthracnose worse. In reality I don't think it matters. To someone that measures growth like I do it's not about any nutrient in particular, it's about growth plain and simple. If the grass is growing at an adequate rate it probably won't have many issues. If your grass is growing too slow and it's poa annua, watch out!

Now, let's look at the P genki below. I've also included a weekly running average growth ratio to see the impacts of P on the growth rate in more detail.

Again, crazy high genki in the shoulder seasons. Ignore that probably.

For most of the season the genki level for P was zero. I wasn't applying much P because my soil test showed that I had enough. The issue I think was with the soil testing depth. The depth was down to 10cm but the roots at this time were barely 5cm due to a hot dry summer and poa being the shitty grass that it is. The roots issue probably compounded itself because the growth rates were too low. Low shoot growth rate and probably a low root growth rate too.

Mid August I applied a bit of P. You can see a big jump in the 7 day growth ratio. I could also tell a big difference in the turf appearance but the growth ratio started declining again. Another P app in early September and BAM, another big jump in the growth ratio. After that the ratio's trend towards infinity but it was safe to say I have finally figured it out.

I was also having trouble determining how much P was enough. With the genki P you can easily see how much is enough. That first application I made in August was only 25% of the monthly P requirements. Once I applied more than a month's worth we finally saw good sustained growth.

The growth tapers off through October but this is due to other factors like raising the height of cut and less frequent mowing after aerification etc.

You also notice that the genki goes up even after application dates and this is because the nutrient demands are decreasing each day as it gets cooler so what was a month's amount of fertilizer in early October, is a year's worth of fertilizer at the end of October. I don't know enough about modeling to fix these issues yet. Maybe one day I'll learn and improve this.

Fast forward to 2020.

Here's the N genki and monthly growth ratio.

Right away you can see that the growth ratio is much better if a little high. I wasn't going to risk things this year and I wasn't sure how all this really worked so I was playing it safe.

In the spring the N genki and growth ratio closely mirror each other. On an adequately fertilized turfgrass soil N generally is the limiting nutrient so you can see that seems to be the case for the Spring of 2020.

When we get into August the typical mineralization event occurs which releases nutrients from the soil organic matter. This is something I've observed over many years at my last golf course property. I was relieved to see it again this year. N genki has remained low this year as we have had more than sufficient growth.

This shows me that if you growth ratio is low and N genki is high, you need to look elsewhere for the cause of the low growth assuming you aren't using crazy growth regulators or lack of water to slow growth. I do use Primo Maxx on a 200 GDD interval and this is worked into the growth model I have made.

What about the K genki?

I was going to make damn sure that K wasn't the limiting nutrient this year as can be seen above. I wasn't using this tool until early October 2020 so I wasn't aware of how my K applications looked compared to growth.

In September you notice that the K genki and growth ratio seem to be similar. The N genki was around 0.5 at this time but K was even lower. Eventually the growth ratio went down but started trending upward after a mid September K application. As we were getting great growth due to mineralization I assume there was more than enough free nitrogen in the soil. Perhaps K was starting to become a limiting nutrient here because there may have been a growth response. K is relatively free in the soil and is used in relatively high quantities. We were removing lots of nutrients from clippings in August and any K that was mineralized may have been lost or something. There's no way to know this for sure but I can speculate.

Now that growth is slowing for the Fall, you can see the K genki trend upward.

Let's look at the P genki

I am definitely overcompensating here! Again, I wasn't acutely aware of this relationship until early October 2020 so I was being careful. Applying way more than is needed is easy with nutrients like P which are used at a rate of only about 13% of the amount of nitrogen. It really doesn't take much. I don't see any relationship with growth and P this year as it should be! If all nutrients genki levels are above 1, I don't think you should see a growth ratio below 1 assuming that there aren't other factors like moisture restriction or growth regulators or frost etc.

Again, trending upward now that growth and nutrient demand is decreasing.

Now that I have this tool I can use it to manage nutrients with finer precision and I wonder if it has made soil nutrient testing obsolete? Did I just invent a new way to fertilize turfgrass? I think that soil nutrient testing is still important to get a snapshot of actual soil nutrient levels but even the MLSN is probably too high in many regards. This method could get you even lower and more precise.....especially with some refinement.

Below is a close up of all the data but only the last month.

The monthly growth ratio is perfect and the greens are perfect (except for the million or so holes currently in them). You can see a big jump in the weekly growth ratio on Oct 12th as we cut them for the first time following aerification.

P and K genki levels are well above 1 so I'm not worried about them at all. In a few days the monthly P genki will drop to zero as it will have been a month since the last application. I won't worry about P until I get less than ideal growth and both P and K genki are above 1. You can see a jump in the growth ratio following the P and K application on September 17th suggesting that they may have been the limiting nutrient as the soil mineralization occurs. I doubt soil mineralization happens perfectly at a ratio that the plants can use so maybe we are getting a good nitrogen release but P and K are lagging or not sufficient?

N genki is currently around 0.5 where it has been for the last 2.5 months. There is still more than enough nitrogen available for the growth we need. As the weather cools down the N genki will naturally go up as you can already see it starting to trend that way. We apply 0.5g N for monthly maintenance as we don't close for the winter. This seems to help during the low to no growth periods of the year and is recommended for disease management.

If the growth ratio begins to dip below where I want it and it's not due to frost or frozen conditions I will apply more nitrogen and see what happens. In the future I plan on adding other nutrients like calcium magnesium and sulfur. It would also be cool to apply some machine learning to this data to figure out what the best running average would be to use. One day maybe.

Now that I have this tool the growth and fertilizer applications for 2021 should be perfect right? We will just have to wait and see I guess. Of course I always come up with this stuff at the end of the season and am forced to wait 7 or 8 months to try it out in the field!


Tuesday, 22 September 2020

How to find success with overseeding new bentgrass varieties.


I think it's important to always continue to evaluate what you think you know in golf maintenance. I'm about to take a pretty hard turn with this blog post. It's because the new bentgrass varieties that we now have are completely different than those old varieties. With these drastic improvements come new ways that we can incorporate them into our courses to begin to enjoy the benefits that improved varieties offer golf course playing surfaces and your budget.

Bentgrass needs full sun. Bull shit. Some of the best establishment of bentgrass I have seen has been on my shady greens probably because the seedlings didn't dry out due to the shady conditions. Even under moderate to extreme shade it did just fine.

Bentgrass doesn't like disruption. Not from what I've seen. Don't get me wrong, I totally love the disturbance theory but in order for the bentgrass seedlings to have a hope in hell of making it to a mature plant you need to make space. Ideally you make that space but other times it's due to poa checking out from disease, too hot or too cold, weakening after going to seed or whatever else bullshit reason the poa decided to die this time. Good turf managers can kill poa in all 4 seasons.

Bentgrass is slow to wake up in the spring. Wrong. The only grass that I mow in the spring is bentgrass. The poa is sunken and feeling the hurt from almost no winter fertilization.

You can't overseed bentgrass into poa....wrong. I have never sodded any bentgrass and have greens that are now mostly bentgrass. I guess I'm doing it wrong?

How to successfully seed bentgrass into poa annua.

Don't limit water. Immature bentgrass plants don't have deep roots so don't let them dry out or throw seed when you can keep it wet. You'll need to wait for the bentgrass to mature before you can dial back the water.

Use fertilizer to put the hurt on poa. After years of working to optimize my fertilizer applications to provide just the right amount I have found that poa really doesn't like it when you back off fertilizer rates. Not just nitrogen, but everything. It loves fertilizer and maybe the main reason that poa is so widespread on golf courses is that we generally have been applying more fertilizer than is required because understanding how much is enough is really really hard to do. Use a standard fertilizer amount of 2 g nitrogen/m^2 to start. This will keep the poa alive but not as competitive as the bentgrass. If you go lower, the bentgrass will lose its competitive edge and moss will take over! Even bentgrass needs some fertilizer but it needs much much less than poa so this difference in needs can help you promote bentgrass over poa annua especially at the start.

When weather favors poa over bentgrass your only tool is soil fertility. If you have some cool wet weather, keep the fertilizer in the shed and watch the poa struggle. When conditions favor bentgrass, push that growth!

Maximize disruption when throwing down seed. BIG holes, and lots of them. When we seeded bentgrass last fall I poked millions of small, tightly spaced holes and put seed and sand into them. While we had a good germination in these holes, the plants the matured in many instances were in the deep tine holes that were much larger in diameter and spaced further apart. Give the plant room to mature, then let it do what it was designed to do. Spread out into the poa that is weak due to low fertilizer rates.

Bentgrass spreading to fill in the void (left) vs poa on the right. Bentgrass easily dominates these cup cutter sized holes.

Poa died in this dry fairway opening up space for bentgrass seedlings

It's hard to see in a picture but the 2cm diameter bentgrass plants are in old deep tine holes.

Be patient.
If you want instant results then buy some sod. Otherwise you need to be patient and throw seed at every opportunity. I've talked about overseeding and microdose applications already on this blog so go read those posts if you are thinking about overseeding . Generally it will take up to 2 seasons before you hard work starts to pay off. Don't let up, keep throwing that seed!

Let the improved plant do the dirty work. With pest control products that use systemic resistance etc you can give the bentgrass even more of an edge over the poa. As the newer bentgrass varieties have more disease resistance than unimproved poa annua you can boost is further with ISR products further giving the bentgrass the advantage. Of course, disease can quickly wipe out your poa especially with diseases like anthracnose but a small advantage over years can and will result in more bentgrass.

Once you get a good amount of plants to mature, get out of it's way. Let it do what it was designed to do. Dry it down every now and then, stretch those fungicide intervals, forget to fertilize them for a few months etc.

3 years after seeding into winter damage this green is now mostly bentgrass. With this much bentgrass coverage you can start to afford to seriously neglect the poa

Monday, 21 September 2020

Genki vs Growth Ratio is a game changer

Smokey skies and high growth rates go hand in hand for some reason.

I've experimented with how to best visualize growth rate data in a way that helps me make informed decisions about how much fertilizer I should apply so that the grass grows at the right rate without growing too fast or too slow because both of these circumstances can lead to less than ideal conditions on the golf course.

A few years ago I came us with the turfgrass speedo. (NSFW link haha).This helped me compare actual growth to ideal growth but that was it. I couldn't see how any of the tools I had to manipulate growth were impacting that growth.

A few months back Jon Merchant shared how he visualized his #clipvol data combined with the turfgrass genki. To me this was a game changer. It instantly allows you to compare your growth rate and fertilizer rates compared to the "standard amount." The standard amount is something that you can determine as a turfgrass manager based on your experience and the needs of your turfgrass to produce the desired playing surfaces for your facility.

This standard amount is multiplied by the Pace Turf growth potential daily. For me the standard nitrogen amount is 2 g/m^2 per month which results in a clipping yield of 625 ml/m^2 per month. Multiplied by the growth potential (which is always less than 100%) will result in ideal yield and nitrogen rates always being lower than this standard amount. You can use all sorts of high tech ways to determine these rates but I would just recommend experimenting for what works best for you. At this point I have all kinds of variable such as pgrs on the final factor. Over-complicating an imperfect model is for nerds.

To determine how much clippings you should expect for a given amount of nitrogen you can take the amount of nitrogen applied per month in grams per square meter and divide that by 0.064 (approx mass of dried clippings per ml) and then divide that again by 4%. I then multiply that by 80% because I don't expect all of the nitrogen that I apply to be used by the plant. There are losses that happen so deal with it. This is something that I have noticed over the years. If I get 80% of the nitrogen I apply back in the clippings I am doing pretty good although that could be improved with this genki vs growth ratio tool.

This is all very approximate but gives you a pretty good ballpark amount to work with. Remember, all models are wrong but some of them are useful. I think this one might be useful.

The genki takes the amount of nitrogen that you have applied and compares that to the standard amount. This gives you a ratio or percentage.

The growth ratio compares the actual growth rate to the ideal growth rate and also gives you a ratio or percentage.

To smooth out the data I use a 30 day running average. Fertilizer and growth rates don't respect the boundaries of days or months so I find running averages useful for turfgrass management because they are smoothed out. Fertilizer that you applied in late July won't just be used in July. It will probably have lasting affects well into August so a running average will help you see that carryover from month to month much better.

As we are comparing two percentage figures we can easily plot them both on the same graph.

On the chart above you can see the genki (blue line) is at 50% for the last 30 days (60 if you include the previous month). This means I am applying half as much fertilizer (nitrogen) as would be recommended by the growth potential. The growth ratio (red line) is much higher in late August and stabilizes in mid September but is again climbing higher than expected.

This tool make it easier for me to fertilize based on actual conditions and not just the date. In August the growth rate was much higher than expected so no (less) fertilizer was needed. The cause of this excess growth isn't exactly known but I could speculate that it was soil microorganisms mineralizing organic matter in the soil. Either way, I backed off the fertilizer apps. I didn't stop fertilizing completely as I always like to keep some fertilizer in the system to prevent growth rates from fluctuating too drastically. Having a genki of 50% avoids these massive fluctuations in my opinion. It also keeps the crummy poa barely alive while allowing the bentgrass to move and make up ground.

As mid September arrived the growth ratios stabilized to exactly where I wanted them to be. This was timely as we were hosting a provincial championship that week so we were easily able to achieve our target green speeds and had minimal growth to manage during that event. We also had almost 7 days of smoke cover during that week so for whatever reason, the growth ratio started to climb back up despite keeping the genki at or below 50%.

As we enter the Fall season I like to back off on my weekly green sprays so if I apply the forecast genki for the coming week but only apply it every 2 weeks I can reliably keep the genki at 50% if that's where I want it. If I want to bring it up to 100% I apply double the weekly forecast limit.

The above graphic might give the illusion that I have my shit figured out but I am only now just starting to maybe get good at this because until I put these two ratios together on a single graph it was hard for me to connect my fertilizer applications to the growth rate intuitively.

The chart below takes my growth rate and fertilizer data going back 2 seasons. You can see that at times I am apply way more fertilizer than the models would recommend I apply. This is mostly due to impatience in the spring. I've tired of grass that doesn't grow and I want to cut something! I don't have growth data for 2018 because that was before I was at this course but you can see that the previous superintendent also liked to get things moving in the spring. It's not just me! In 2019 we were also trying to recover 5 dead greens.

After playing with this stuff for years now I can say that it's actually pretty good. In 2019 you can see that the growth ratio was below 100% for most of the golf season. While this was at first great for green speeds, it eventually resulted in severe anthracnose. While applying more than the standard amount of nitrogen to the greens, they didn't grow as expected due to another nutrient deficiency ( I think). This year after soil testing we got that under control and growth has been almost perfect (slightly high) and the disease issues on greens have been almost non-existent.

If I could only have one chart to look at each day this would be it. I think in the future it would be great to improve upon the Pace Turf growth potential model with machine learning. I know there are some already working on this but even so, the temperature based GP model is pretty darn good and a drastic improvement over a calendar based approach.

Thursday, 20 August 2020

Accidental Microdose Overseeding

Last fall I started a small overseeding study to see which turf type would be suitable for our fairways as "if you aren't overseeding improved turfgrass varieties you will invariably get unimproved turfgrass" like poa annua which requires more water, fertilizer and attention than we can give it.

In the study I was testing 3 types of turfgrass. Perennial ryegrass, turf type tall fescue and creeping bentgrass. I couldn't test all the varieties of each species so I chose blends of the species which isn't important for the purpose of this post. All I wanted to see was how each species would establish with virtually no effort other than spreading the seed on the ground. I had plots on dry fairways, wet fairways, shady areas, sunny areas etc.

The seeding rates were super low because the idea was to put improved varieties everywhere for a low cost and then let them spread out over time. The seed germinated in 10 days.

Seed testing plots

Around the same time I noticed some really nice looking grass on one of our tees. A closer inspection showed it to be creeping bentgrass. The previous superintendent assured me that no creeping bentgrass was ever seeded on tees, or anywhere other than greens during the winterkill episode of early 2017 where we had record low temperatures and lots of ice!

That shit is dead.....February 2017 at my previous course

This spring we applied very little fertilizer to the fairways due to the uncertainty surrounding covid19 with the hope of reducing yield so that I could manage all the mowing on the course by myself. This made the poa annua pretty angry. There was a lot more poa here than at my previous course probably due to  a better irrigation system and historically higher resource availability (fertilizer and wetting agents). As the poa was looking so bad some weeds started to establish but I also noticed many patches of really nice looking grass which turned out to be creeping bentgrass. 

Creeping bentgrass is happy and weed free even under low input maintenance

Where did all this bentgrass come from though? After a while it dawned on me. The seed was tracked on golfer's feet from the greens that were overseeded in 2017! Fairways following the greens that were hit the hardest by the winterkill had the most bentgrass!

The only good grass on the 11th fairway is bentgrass! The 10th green was hit hard in 2017...hmmm

So it's highly likely that all the bentgrass in the fairways was accidentally tracked onto the fairways from the greens 3 years ago.

Back to my overseeding study. Not surprisingly the results weren't great. I didn't do a formal count or anything but there is almost no visible difference on the tttf and ryegrass plots. On the plots seeded with bentgrass there was some visible bentgrass plants visible after almost 1 season but it doesn't matter. I have my answer to what turf species will establish across a wide range of conditions on our fairways.

Minimal bentgrass established with ultra low seed rate and no seedbed preparation in very challenging conditions with hardly adequate irrigation and not enough fertilizer in my overseeding study. Believe it or not, this is a fantastic result. Now that the few bentgrass plants are mature they will easily spread out to cover this entire area which will significantly improve conditions here.

The more I think about it and observe this accidental overseeding the more in makes sense that bentgrass is a great candidate for fairway overseeding.

The seed is super small. This makes it possible for you to get even coverage of a large area with not that much seed. Sure bentgrass seed is expensive but if you calculate it on a cost per seed basis it's actually quite cheap ($0.000037 per seed). The small seed also makes it easier to get good seed to soil contact as it easily falls between the existing turfgrass plants. I also like to think that because the seed is so small, the birds will eat less of it than other turfgrass species but who knows.

If you apply bentgrass seed with a purpose built seeding machine you will be forced to apply a high rate of seed but if you think outside the box you can apply the seed almost infinitely low in rate. For me I applied it in my study at a low rate of 5g/m^2 and while the results aren't shocking if you are looking for instant total coverage, there is more than enough plants surviving to maturity allowing the grass to spread over the next few years.

To get the super low seeding rates I used a push rotary spreader with the gate full closed. The seed is so small that it would work it's way through the closed gate edges at the perfect amount! Spacing was 1m or less as the seed doesn't go far as it is so small and light.

By the numbers you can get really good coverage with very little seed. Let's say we want to get 1 seed per cm^2 (5 seeds per square inch approx). That is 10,000 seeds per square meter or 100,000,000 seeds per hectare.

There are 13,200,000 bentgrass seeds per kg. 1 pail of seed is generally 11 kg so that pail will give you 1 seed per cm^2 on 1.45 ha of turf! For many recommended overseeding rates that is still an ultra super low rate. For me I say why not go lower....way lower! Here's why.

Improved bentgrass varieties are a superior plant. They have many desirable traits that native grasses don't but one of the best things about improved bentgrass (and unimproved bentgrass) is its spreading growth habit. Bentgrass will cover a lot of ground if it's allowed to. It seems that with the newer bentgrass varieties, some disruption in the form of divots, ballmarks, disease scars and winterkill!

Bentgrass spreading across an old cup cutter plug on the left vs Poa on the right. The real question here is why isn't there more poa germinating in the cup cutter scar after 8 months?

Take this patch of bentgrass in the following picture. It's about 30cm ( 12") across and was likely established in the spring of 2017. It's highly probably this this patch is from a single parent plant and has spread about 10cm (5cm in each direction) per year for the past 3 years. In order to get good bentgrass coverage we don't necessarily need a lot of seed, we need a little bit of seed everywhere and all the time.

A few meters away from the above picture is the middle of the fairway where most golfers walk....

Oh shit, we accidentally have predominantly bentgrass fairways! Imagine how good they would be today if we were trying! Even if we were trying very little....like a few dozen Kg of seed a year on the entire course. Hook a cheap pull behind rotary spreader behind your fairway mower once a month with a pail of seed in it and the gate set to closed! Bam!

Accidental seeding most likely established in old divots on a shaded fairway

If we had to wait for enough resources to afford to overseed our fairways at the recommended rates we might be waiting for a long time. In that time we will see water and fertilizer use go up and conditions going down. Instead I think it might be better to get the improved genetics everywhere ASAP and then work on letting the grass establish over time hitting the entire course lightly every year multiple times. 

I like the multiple times a year approach as I never like to "put all my eggs in one basket." If you decide to go big on the overseeding strategy and it rains 100mm the following week you just watched a significant investment flow down the drain. If the poa is still too competetive you will also be wasting your money on a big overseed. If you put a little out often there is less financial and weather related risk.

When you go big on overseeding or even bigger with a full on renovation you might want to push growth with extra fertilizer. This creates high growth rates and lots of thatch on the new bentgrass surfaces. With the overseeding method described above, you actually apply less fertilizer to reduce the competitive ability of the poa annua. I haven't noticed any visual difference in thatch on the bentgrass vs poa areas. 

I think the biggest risk is not overseeding at all because the reduction in resources required with these newer turf varieties is significant which is especially important as resources like water are harder and more expensive to find. In 5 years this could have been the best $2000 I ever spent! 😜

So what have you got to lose? Half ass overseed your fairways with bentgrass today and thank me in 3-10 years haha.