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Hi All,

Well all the talk is of a heatwave this week courtesy of a peak in the jet stream that is going to pull warm air up from the continent. For this reason I personally don’t think it will be dry throughout because summer air from the continent has a habit of triggering off thunderstorms and I expect these towards the end of the week and possibly over the weekend. The graphic below, courtesy of Meteoblue, shows the way warm air is being funneled up.

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Whenever I hear the words heatwave, my thoughts immediately turn to Gravesend in Kent because when you have a forecast like this, it is often quoted as the hottest place in the country, so all the best to the lads and lasses in Kent that will bear the brunt of it I think 🙂

General Weather Situation

So as is usual in these situations it’s a case of heat building and because it is quite a narrow peak, it won’t be the same for everyone as we have a West-East split. So for Monday whereas we will have a dry start over England and Wales, there is rain affecting the north-west of Scotland and during the morning more rain will push in to the coast of Connacht and Munster. Further south and east it looks a settled warm day, but as that weak rain front pushes into the west coast of Wales and England later on this afternoon, that may thicken up the cloud cover over the north and west of England. Temperature-wise, expect low high teens over the west and north and low twenties in the south of England. Winds will be light and from the west, with an increase in wind in the west and north.

For Tuesday we have a hotter day on the cards with a loss in cloud cover. This is in place over The Midlands and south of England and clears north through the morning where they’ll still be some patchy rain / drizzle first off. By the afternoon temperatures are really ramping up and I’d expect mid-twenties in the south east of England. That weak rain front sitting out to the west may just push some light rain and patchy cloud up the west coast of Ireland in the afternoon and also across the south-west of England and South Wales. The wind will swing round to the south / south-east during Tuesday and that’s what is bringing the heat up.

By early morning Wednesday we have some more rain fronts pushing up from the south and primarily I expect them to affect the west coast of Ireland and the south west / north west of England during the morning. Through the morning this rain will push up into Scotland and looks likely to be heavy along the south west coastline. This band of rain may move eastwards through Wednesday morning to affect northern England and the north Midlands as well. Further south we will see bright sunshine and high temperatures again, maybe the highest of the week with 30°C likely, especially down in Kent. Of course where you have warm air and moisture you have the building blocks of thunderstorms and these may trigger off further north over Scotland on Wednesday evening. By the evening the vast % of that rain looks to clear the U.K mainland and Ireland.

Overnight into Thursday we have the risk of rain spilling up from the continent into Kent and The Home Counties. Now as you know trying to predict where it may end up falling sitting here on a Monday morning is not for the faint-hearted, but at this stage it looks like it may affect a line east of Portsmouth stretching up to The Wash. Save for some light rain over north east Scotland and Ireland, the rest of the weather picture looks clear on Thursday, but with more in the way of cloud cover, it’ll be substantially cooler, down to the low twenties north of The Home Counties. In the south and south-east, expect mid-twenties in the way of temperatures with a light westerly / southerly wind. Again the combination of moisture and warm / hot air is likely to trigger thunderstorms in the south east of England on Thursday. Away from this area, west and north, we look to have a warm, dry sunny day with temperatures feeling very comfortable in the high teens (Ireland / Scotland) to the low twenties in The Midlands and north of England.

Closing out the week things get a bit more complicated as there appears to be more in the way of moisture around across the west coast of Ireland, possibly The Midlands and later in the day  across Donegal, the north of England and Scotland. So with more cloud cover, moisture and warmth I expect more in the way of thunderstorms on Friday, especially as that heat is set to build again across the south of England and The Midlands during the day on Friday.  So Friday evening may end up being turbulent to say the least ! For Ireland that west coast rain will move slowly eastwards through the day to affect The Midlands, whether it’ll reach the east coast of Leinster by late afternoon remains to be seen. Scotland looks to start off dull with heavy cloud cover on Friday and that’ll thicken to give rain, some of it heavy from lunchtime. This rain will slowly move north eastwards and finish up across Aberdeenshire by close of play Friday.

So how are we looking for the weekend, an extended heatwave or more in the way of rain ???

Saturday looks to be a largely dry day, but there will be exceptions. The north east of Scotland keeps that rain overnight and through Saturday so a very wet spell of weather for that area. Elsewhere we look settled after Friday night’s thunderstorms, a little cooler, down to the low twenties across the south west, Midlands and north of England, a couple of degrees higher in the south east of England. Again there is a suggestion of rain pushing up from the continent to affect this area through Saturday afternoon and into the evening, but we’ll see. On Saturday evening there also looks to be a strong risk of rain coming into the south west of England and pushing up the M5 into Sunday morning. This band of rain may end up affecting The Midlands and the east of England, north of The Wash early on Sunday. During Sunday an Atlantic low pressure system begins to affect the north and west, so we will start to see more in the way of wind and rain for Ireland, the north west of England and Scotland later on Sunday. Further south, we will still be under the influence of the high pressure so it looks dry and settled, though it should be cooler but still with the risk of thunderstorms I think if the heat begins to build again.

Weather Outlook

Last week I said I had a ‘low degree of confidence’ on what is now this week’s forecast and looking to next week it’s tricky to call again. The reason is because we have an Atlantic low pushing up against a ridge of continental high pressure and depending on who wins the day, this will dictate our weather. Currently it looks like Ireland, the west and north west of England and Scotland will lie on the path of that low pressure system, so a mixture of sunshine and showers next week with a stronger wind. (the strongest over Scotland) Further south in a line drawn I think from The Severn Estuary across to The Wash, we will still have high pressure in charge, with possibly nice, sensible temperatures for the beginning of next week, but a chance that they’ll ramp up again as we progress through to the second part of the week. The orientation of that divide between low pressure and high pressure is sure to change as we progress through this week.

Agronomic Notes

This week we have a heatwave for some areas so I’ll start by briefly covering some of the areas of turfgrass management we should consider. Firstly, if you look at your Meteoturf module you may see a clear indicator to the arrival of plant stress signified by a dip in Growth Potential even though temperatures are high enough for plant growth.

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This means that the temperatures will reach a point on Wednesday when they will negatively impact the growth of Poa annua and this is reflected by a decreasing growth potential figure. This is one of the advantages of the Growth Potential calculation over Growth Degree Days as there is no decrease in the latter on Wednesday because it doesn’t have a ‘top out’ value.

So we can expect severe plant stress this week, so how are we best to deal with this ?

First-off moisture management and here we have a line to tread. During periods of high temperature the grass plant will cool itself by conversion of of internal plant moisture to water vapour (gas) and this is then released through the stomatal pores. If the level of soil moisture is too low it cannot replace moisture lost in this fashion and begins to wilt, a clear sign that the plant is under stress. You’ll notice this as foot-printing on the turf surface when there is insufficient cell turgor (internal water pressure that helps the plant to maintain its structure) to maintain the structural integrity of the plant (so the pressure of foot traffic cause the plant to flatten rather than spring back). This should serve as a warning, so one thing you can do before this happens is to cool the plant by syringing, that is applying fine droplets to the surface of the grass plant leaf. As this moisture is evaporated from the leaf it cools it, a bit like when you get out of a shower on a hot day, your body is cooled by the process of evaporation of moisture from your skin. Importantly with syringing, you are supplying the moisture rather than the plant.  This is an important point because the plant uses energy to facilitate the cooling process. Al Turgeon in his excellent book ” Turfgrass Management” quotes a figure of 570 calories of energy is required to evaporate 1 gram of water. The less the plant has to facilitate cooling in this manner, the more carbohydrate reserves it retains.

Secondly you must make sure that there is sufficient moisture in the rootzone to maintain the plant’s internal moisture status but it’s how you apply this that really matters. We know that irrigation systems tend to create areas on a turf surface with high levels of soil moisture and areas of low soil moisture (noses, ridges, etc). If you rely on overhead irrigation alone during hot spells of weather you will create a disparity in moisture levels, so this is where hand-watering is key. The idea is use your irrigation to give you a base replacement of moisture and then top up the drier areas with localised hand-watering.

During periods of hot weather, the enemy is surface organic matter because this will retain more moisture, become saturated and heat up. Once it is hot, a wet, high-organic matter content rootzone lose heat very slowly and so it may ‘cook’ the plant roots. So even though the rootzone is wet, it is not a good thing because it will not be able to dissipate this heat. The same is true of a compacted soil. So if you know you have high surface organic matter or higher than you’d like, be careful not to over-irrigate. Timing of applied irrigation is also pertinent because if a high-surface organic matter rootzone sits wet all night, it will retain that heat for longer and the plant will be less able to cool itself. (That is why we irrigate early morning rather than late at night, plus the leaf is not sitting wet for too long, a calling card for disease)

The other point to make when dealing with high organic matter is that when it is wet, it swells and this has the effect of raising the crown of the plant higher in the canopy. The crown is the division point of a plant, above the crown we have shoot development, below it we have root development. If the crown is sitting higher in the canopy because it is elevated, you have a much higher risk of scalping the turf because the mower ‘sits down’ into the spongy turf and effectively cuts lower plus of course the crown is sitting higher. If you cut the crown, you kill the plant, simple as that. This is often an issue on approaches and collars on golf greens.

So keep the grass plant cool if you can, maintain adequate, but not excessive soil moisture and look out for indications of plant wilt.

If you’re planning to apply a wetting agent this week, then I’d also suggest a tankmix with a good-quality biostimulant because without a doubt it does benefit the plant during periods like this in suppressing stress. I did some trials back in the summer of 2006 when we had similar temperatures (but for longer as they extended into August) and I could clearly see the plots where I had applied a combination of wetting agent and biostimulant because they looked the least stressed, even when the % of E.T replacement was very low. (< 40%)

Cultural work

All common sense stuff really but this week wouldn’t be the week for going out and verticutting, scarifying, etc and I’d also think twice about dressing and matting / brushing in unless you pick your day and have all the other boxes ticked in terms of moisture management (or are in an area where temperatures won’t be as high)

Nutrition and PGR usage

On outfield areas you really need your turf ‘buttoned down’ if you want a PGR to give you the most beneficial effect. What I mean by that is that you want your turf already under regulation before hot weather so the growth and clip rate is lower and hence water usage will be decreased. It’s a fact that a regulated turf will be more drought-tolerant than an unregulated one, especially if it is growing too quickly, which brings me neatly onto nutrition. During periods of hot weather you want to keep N input levels on the low side because more growth due to high N = more energy expended by the plant and critically more water required to support that growth. On the flip side you don’t want it hanging due to lack of N because you’re just playing into the hands of plant-parasitic nematodes and possibly Anthracnose, come August.

So just like irrigation we have a line to tread with nutrition, ‘little and often’ always works best for me.

Before I leave PGR’s, it’s worth commenting that during hot weather we know that the TE molecule breaks down faster and so longevity is affected. So if you’re on a regular greens program, tighten up your intervals, don’t increase your rate because it’s interval, not input rate that is key with TE usage on surfaces in the summer. There’s plenty of U.S research to support this as they’ve been at it for longer.

Ok that’s it for now, enjoy the heat, dodge the lightning and tread that line wisely 🙂

Mark Hunt