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  • Writer's pictureMark Hunt

May 22nd, 2023

Hi All,

If you read the tabloids / media, summer has begun and we are set fair for 3 weeks of sunshine and heat courtesy of an 'African Plume' (thanks Dave). Well I don't think it will be that quite that clear cut. It is true we have a lethargic and high-lying jet stream and this normally sets the scene for warm air to push up from the continent but we are in May still, we must remind ourselves of that and so equally likely to pull down some cooler northerly / easterly air at the same time.

I am following the debate at The Chelsea Flower Show with interest. They are debating whether a re-wilded garden is actually gardening at all. (You can read about it here). When I moved in to my current house with its very small rear garden, it was mostly decking and concrete patio slabs and to my eye, abhorrent. A desert for nature, just like gravel drives, block paving, decking and that worst offender, artificial grass. Equally I look at gardens with knee-length grass and and wonder if that really is the optimum environment for nature ?

As I type this now looking at my lawn, (The image above is from last year and features a bit of Yorkshire Fog 'removal')

I have Blackbirds feeding their 2nd brood of young, some of these have nested in the garden, secure in my mainly cat-free environment, Hedge Sparrows, Blue Tits and Robins as well. In the winter it is home to Pied and Grey Wagtails. How do these birds feed when the grass is a foot high ? High grass ala C.Packham, also offers a great environment for cat predation and Ticks. Above my 'reasonably well kept lawn', the air is full of insects, not just pollinators but midges, the staple diet for birds and Bats, the latter of which are here again as the evenings stretch out. I have Wood Mice and of course Hedgehogs. My point is that you can produce a nice garden which embraces nature without jumping on the rewilding bandwagon. There is a middle ground. The same is true for golf courses. we can manage them but also encourage nature by providing a critical land corridor away from the monosward, that is modern agriculture. It is a great opportunity.

GFS projections are courtesy of

Onto the weather, so are we likely to stay dry and sunny for the foreseeable ?

General Weather Situation - 22nd May

Well yes, the outlook is mainly dry but we will have a little wobble as we proceed through the weekend as the wind turns northerly for a time and pulls more cloud cover down and maybe the odd shower with it. This week though we are set fair from a dryness perspective with very little rain around in the projections. We will keep that northerly wind though, veering between north westerly to north easterly and as I mentioned above, they'll be a spell at northerly as well. That means we won't see a heatwave because that northerly wind direction will keep day time temperatures around the 19-22°C sort of ballpark and night time temperatures in single figures. This will have a knock on effect from an evapotranspiration perspective which I will talk about later.

So cool mornings will be a feature of this week and then the temperature will rise quickly as the sun's heat warms the air pushing the temperature up. I think the warmest temperatures will be across Wales and England, with Ireland and Scotland running 3-4°C lower during the day at around 18-19°C maximum.

Weather Outlook - w/c 29th May

There's a possible fly in the ointment I'll be keeping an eye on at the weekend / start of next week with a low pressure projected to form off the east coast of Scotland during the latter part of the weekend. This will then funnel down a more northerly wind aspect and the possibility of some rain across the north east and east of Scotland on Saturday pm and then maybe some showers for the eastern coast of England on Sunday into Monday. Further showers are likely across the north of Scotland on Tuesday as well and there is a chance some of these showers will move further south. Thereafter next week, high pressure is projected to reassert itself and we flip back to dry and settled with warming temperatures building from the south west from mid-week onwards. So that's how we look currently. The jet stream is very fragmented and weak, fair game for a period of settled weather but it may not be as clear cut as forecasters are saying.

Agronomic Notes

So we are now approaching the tipping point of the year, when rainfall amounts normally begin to decline and evapotranspiration (ETo) begins to ramp up.

Up to now it has been an interesting year when you look at the relationship between the two, as little as a couple of weeks ago, Greenkeeping social media was full of pictures of flooded turf, wet surfaces, clippings and cries of "No more rain please". Now we begin to see surfaces dry out as temperature, solar radiation increases and humidity drops (allowing more water to move from the surface of the grass plant and the soil into the atmosphere). Now the game moves to balancing the water requirements of the plant against water loss and that's where knowing your ETo is critical.

Evapotranspiration is probably one of the most important parameters you can measure using a weather station in relation to turfgrass management, but it isn't just as straight-forward as saying, "OK, I've lost 3.5mm by ET today, so that's what I need to replace by irrigation to keep my grass plant happy"

The reason why it isn't just a case of replacing reported ET is to do with how the figure is calculated using the FAO Penman-Monteith Equation, otherwise known as ETo. It is a very complex calculation and one that using meteorological data standardised at 2m height for a grass crop of 0.12m height, that's 12cm or 120mm. You can read about it in great detail here

Incidentally, that's why you won't get useful E.T data if you mount your weather station on top of a building. It will over-report by some margin because humidity, temperature, wind speed, etc will all be very different at height.

Now, we are obviously cutting quite a way below that so the dynamic at say 3mm in calculating ETo is quite different from the standardised 120mm. Temperature and humidity for one are very different. The reported ETo figure turns out to be an over-estimation of the actual ET in a turfgrass situation and so when you're irrigating, rather than replacing 100% of ET loss, you are looking to replace a lower %, let's say 60% for the sake of argument.

The exact % will depend upon many variables not least soil type (how easily water is held or lost from the soil, with heavy soils retaining more and sandier soils losing more) , root depth (deeper rooting means more efficient uptake of moisture), cutting height (the lower the cutting height, the faster the grass plant goes under stress related to moisture loss and of course the shallower the root system) and last but by no means least, grass species vs. water requirement.

As a turfgrass manager, it is critically important to understand not only your daily ETo figure, but how it can change through a day, a week or a month. Now you could argue that if you monitor soil moisture levels with a moisture meter, you don't have to understand your ETo. I'd argue the opposite actually.

Let's say you go out at 06:00am and do your soil moisture readings, where they are 6 hours, 8 hours, 10 hours later will depend upon the potential for moisture to be lost from the rootzone by ETo. On a low ETo day, they may not decrease by much at all. On a high ETo day, you could lose 6% moisture in a matter of hours and the plant be at wilt point before you know it.

You can see how the change in weather has affected the rate of moisture loss by ETo on our climatic report summary from The Oxfordshire ;

In the last 2 days, they have recorded ETo levels > 4.0 mm per day which I normally use as a threshold for plant stress.

To put that into perspective, last year their E.To maxed out at 6.73 mm per day during last July's heatwave. So even though it wasn't hot over the last couple of days, the combination of a strong wind and low atmospheric humidity has really ramped up the ETo levels significantly.

So if you looked at your surfaces this morning and thought, wow they have dried out significantly, that's why....

At the end of the day, a turfgrass manager should know what % of ETo they want to replace and how their surfaces behave vs. a certain ETo and that applies equally to a golf green or a cricket square.

Looking at the year so far and how rainfall and ETo has shaped up across the U.K & Ireland is quite interesting, especially February when some areas were so dry, they actually lost more moisture by evapotranspiration than they received by rainfall. You can also see how May-to-date is the tipping point between rainfall and ETo and how Sevenoaks and Shannon are following a pretty similar pattern y.t.d in terms of rainfall and E.To.

Again it is that central-eastern belt of England that is experiencing the lowest rainfall and the highest ETo and that's invariably where plant stress will start first.

OK, that's it for another week, enjoy the lack of rain and the ETo !!!!!!

All the best.....

Mark Hunt

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