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  • Writer's pictureMark Hunt

August 22nd

Hi All,

Well for some the drought broke last week and rain arrived to quench the parched turf. For others, they received very little, such is the variability attached to summer rainfall events. Incredibly frustrating I know. We were sitting on 1.8 mm until a storm came up the Welland Valley and dumped a delightful 15.7 mm overnight.

The storm track (image right) and lightning strikes (image left) can clearly be seen in these snapshots I took from last week's main rainfall event using

This location in Essex picked up 26 mm and the rainfall rate peaked at 88 mm per hour, that's just over 3 1/2" per hour in old money.

Our default location at Thame, Oxfordshire picked up the storms later that day and although it returned a similar amount of rain (22 mm), most of it came in one great dump at 23:00 when 19.4 mm fell at a peak rain rate of 172 mm per hour, that's just under a 7" per hour rain rate. !!!!

No wonder bunkers were washed out and areas flooded temporarily !

One point I would make is I've seen some people on FB lambast the forecasters for predicting rainfall and then they receive only a fraction of the forecasted total or none at all. This is the reality of trying to forecast a super-variable outcome, that of summer rainfall falling from storm cells. It is clear that it is far from an exact science and I am afraid you just have to accept it as that. In my experience, greenkeepers and groundsman have 4-5 weather sites that they utilise and they settle on the one that gives them the forecast they want to get in the next 24 hours or so and complain when they don't get it 😊

On a serious note, I do appreciate though that it makes product applications, etc very tricky particularly when you're looking at a breakdown event after a period of minimal rainfall. You apply a surfactant and biostimulant to fairways expecting rain and it doesn't come. That is deeply frustrating. This week our rainfall has gone from a predicted 17 mm, to 6 mm and now less than a mm. That is what forecasters are up against and we just have to accept it.

As I said before, last weeks rain event wasn't likely to be a drought ender, more a marking of the card and a halt (in some locations) to the continual downward spiral of no rainfall and high temperatures and daily E.T.

So now we look at the trickier question of when our next meaningful rainfall is likely to present to us.

General Weather Situation

The first thing I'd say about our general weather situation is that it is extremely volatile in terms of outlook beyond 5 days. I heard the Radio 4 weatherman meteorologically shuffle his feet when he got towards the end of the week early this morning so let's have a big caveat here as we look at the same.

So this week we have an Atlantic low pressure sitting off the west coast of Ireland and that's going to feed in rain across the U.K & Ireland for the first half of the week. The animated GIF above from highlights the predicted rainfall pattern so that's our first staging point for where and when we may get rain this week.

As we go through this week we will see temperatures pick up across England and Wales from low twenties at the start of the week to mid-high twenties by Wednesday, peaking around 27 °C across the south on Wednesday before a change in wind direction from south westerly to north sees temperatures drop back to the early twenties for the tail end of the week. As we approach the weekend, the weather settles down as high pressure comes into play for the Bank Holiday. For Ireland, your temperature peaks today in the low twenties and then drops back into the high teens, maybe just nudging 20 °C later in the week as high pressure asserts itself over the U.K & Ireland. Scotland will be cooler with a much wetter first half of the week, but I'd expect it to sit in the high teens by the weekend and like Ireland, just nudge into the twenties.

So a warm and potentially wetter for some, first half of the week, before things start to settle down with less risk of rain at the end of the week, a cooler wind and pleasant, settled temperatures for the weekend.

Weather Outlook

So I think there's a low level of probability attached to the above GFS prediction for next weeks weather and that's because it has been changing almost daily and there's a lack of agreement across the models. Just to give you a flavour, below are the GFS and ECMWF projections for how the temperature and pressure systems will stack up at the start of next week. As you can see there's widespread disagreement and that's why I think the prediction probability is poor.

Presently it looks like we will have a settled start to next week. There's a small risk of a temporary breakdown across the south from Tuesday dependent on a Bay of Biscay low pressure extending its influence into the south of England but I think it'll settle down thereafter. So overall a dry and settled week is I think more likely than not as we tip-toe into September. Further than that we could see an Atlantic low dominate but it's so far away and the probability, so low, it's probably not worth mentioning.

Agronomic Notes

Why organic matter is your enemy in a drought scenario and how it affects recovery....

Some of you will have started to see recovery after last weeks rain and some will still be waiting (sorry) but one thing is for sure is that the degree of recovery will to a large extent be dictated by your organic matter levels.

Here I'm principally talking about on complexes, approaches, tees, fairways and outfield turf rather than greens though the same principles apply.

First off, we have to understand that organic matter behaves differently during periods of high temperature compared to rootzone / soil. In some cases, it heats up much faster and to a higher level, often running temperatures that are +5 °C higher than the air temperature above. Secondly, and perhaps most importantly, a condensed fibre layer causes the grass plants roots to grow laterally (sideways) in zone A rather than downward into zone B and critically, into the rootzone or soil layer below.

This a key point because if the roots are confined to the surface organic matter layer, they will effectively 'cook' during periods of high air temperature and this process is made even worse if the area is over-irrigated so the thatch layer is holding water as the temperature rises.

The graph below shows data taken from a Davis Vantage Pro2 weather station using a EnviroMonitor node to run supplementary sensors. One of these is the Terros 12 which measures soil moisture, temperature and salinity. The probe was placed horizontally around 100mm below the turf surface (see above) in a loam soil with no excessive surface later of organic matter. Location was Sevenoaks, Kent (Cheers Mark)

The air temperature peaked at 39.2°C on the 19th of July, whereas the soil temperature peaked at 24.5°C, some 15°C lower. That is where you want your plant roots to be and not frying in surface organic matter. Interestingly the graph also shows how the soil holds onto the temperature at night. Now I'd expect a sandier soil to drop / lose temperature faster at night and also heat up quicker during the day.

Sometimes a sandy soil is a benefit, sometimes it is not.

Another negative side effect of organic matter accumulation is that the crown of the grass plant is elevated above the turf canopy rather than contained (and protected) within it. You'll remember from last week how I described the crown of the grass plant as the heart of the plant. An elevated crown is subject to localised temperature extremes during high temperatures increasing plant stress and decreasing the potential ability of the plant to regenerate thereafter.

If the grass plants roots are confined to the organic matter layer then the potential for turf damage / loss is much higher. So when you look at areas that don't recover from this years drought, it is likely that excess organic matter will be top of the list.

Damage / recovery potential is of course also a function of plant species.

So the annual Poa biotype which typically is very shallow rooted tends to check out quicker / sooner / more consistently than the perennial Poa biotype. Poa annua itself is far more susceptible to loss of turf cover during high temperatures than bentgrass, be that Browntop or Creeping. Lolium perenne and particularly the later cultivars bred with drought tolerance in mind really shine I think when you look at how well they cope with a drought.

Many years ago I dealt with a golf club who had very acidic clay soil fairways with next to no microbial activity. These fairways were characterised by an almost cardboard-natured thatch layer in the surface. During a particularly hot summer, they lost huge areas of fairway cover and that autumn, they aerated and overseeded using a 30/70 rye / fescue mix, figuring that the Fescue was more drought tolerant. At the time that was my viewpoint. Over the autumn they had a fantastic take and by the spring the next year were sitting pretty or so I thought. That July, the temperatures pushed up to 36°C and they lost grass cover again. I was puzzled by this, I thought we had done everything right. On examination, it was the Fescue component that had checked out and only the fine-leaved Perennial ryegrass survived. When I took some cores I found the Fescue roots had not penetrated the organic matter layer into the clay soil below but the ryegrass roots had and this was the reason why one species survived and the other didn't.

Now of course established Fescue is deep rooted and very drought tolerant as a consequence but in the first season after overseeding, it is susceptible to loss during periods of high temperature / low rainfall due to slower root development. Once the roots are down then grand and of course if you don't have a surface organic matter issue, even better.

Bearing in mind that our last significant drought was only 4 years ago back in 2018 and many people were forced to overseed large areas, it will be interesting to assess the effectiveness of that work 4 years later. Hopefully it has provided resilience to the type of extremes we have / are, faced / facing this year and will no doubt again do in the future. If it hasn't then you may need to ask yourself why before repeating the operation and throwing good money after bad and deal with the cause not the symptom.

Have a good week and all the best.

Mark Hunt

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