June 22nd, 2026

Hi All,

As we start this week, the media is consumed with another round of political circus in London and the prospect of an amber heatwave (maybe soon to be upgraded to a red) this week. This will be the second heatwave of 2026, the first gave us record temperatures at the end of May and this one could well break the 50-year old record from June 1976 for maximum air temperature in June.

GFS images courtesy of tropicaltidbits.com

The mechanism is practically identical as we can see from the images above courtesy of tropicaltidbits.com. The jet stream lifts northwards into the classic peak pattern I have discussed many times before and then a column of warm air from Africa extends northwards across southern and central Europe to fill below this peak.

Why are we seeing record temperatures ?

Well, it is quite simple, the air spilling up from the equator is warmer than it has been before and it is crossing seas that are significantly warmer than they have been before. The temperature differential between the Arctic and the equator is less, this means less energy is driving the jet stream and so it slows and like a current in a river, when it slows, it meanders. It meanders into peaks and troughs. The only consolation for us in an Island climate is that currently these events are short-lived.

So how long is this one likely to last ?

So, as we can see from the ECMWF projections above for temperature / pressure and precipitation, we look to have this heatwave in place for the best part of this week, with the hot air plume gradually shifting eastwards and pushing into The Netherlands, Germany and Denmark towards the latter stages of this week. The record air temperature for England in June is 35.6°C, set on June 29th, 1957 and June 28th, 1976. For Denmark, the record is 35.9°C set on June 25th, 2006. I can see both countries records going this week.

As the hot air plume moves eastwards, an Atlantic low pressure will try to push in across Ireland and the west from Friday, so that'll introduce cooler and wetter air into the weather picture, first for Ireland, then for the west of the U.K. How much progress it makes eastwards is still up for debate, but it will shift the wind direction to W / NW regardless and that'll cool things down to the low twenties. For Denmark, that heat will prevail over the weekend and then start to drop southwards at the start of next week as low pressure asserts itself in the weather picture.

The ECMWF and GFS both show that Atlantic low pressure making a renewed push eastwards at the start of next week so that suggests the arrival of cooler, wetter air for us all in the first half of next week.

So, we look to have 4 days or so to 'survive' before we see a turnaround to cooler and wetter weather at the start of July (potentially). Now of course things will change with the above, but that's how it looks to me. I'd also respectfully like to point out that this heatwave was predicted by the ECMWF / GFS models at the start of last week, so we had 8-9 days notice. Plenty of time to get a plan in place.

Agronomic Notes

One of the interesting points for me is how grass behaves during a heatwave.

Firstly, we know that the predicted 35-37°C temperatures, cool season grasses will stop producing leaf growth (and incidentally probably 10°C lower than that for root growth), so that means our Fescues, Bents, Poa annua and Ryegrass will stop growing. On this note, I'd be interested to you guys and girls who are measuring leaf clippings if this is what you actually see ?

Certainly if we look at the Growth Potential model for this week we can see a significant dip in projected cool season C3 grass growth. Interestingly, I have included the same graph for warm season C4 grasses like Bermuda and you can see it is almost optimal. The south of England is becoming a transition zone eh ?

I should also point out that I am using 18°C as my optimum temperature for cool season grass growth, so the drop in growth is more significant than the standard formulae that uses 20°C.

Would I be applying a PGR this week to greens ?

Personally, I'd either have applied earlier (knowing this was coming) or I'd skip it to the end of the week knowing that the grass is likely to be more receptive to a PGR and I'm heading into less stressy weather. That's my take on it.

It is probably worth looking at what happens to our cool season grass during periods of excessively hot temperature, so I lifted a slide from one of my presentations.

As the temperature increases, the photosynthetic efficiency decreases, so the plant's ability to produce energy decreases. At the same time however there is an additional cost for a cool season grass at high temperatures, the plant expends more energy respiring and controlling water loss, so it is a double whammy, less energy produced and more energy consumed. The NAR (Net Assimilation Rate) therefore deceases as temperatures increase.

Whilst we are talking about respiration and water loss, I have included an image from one of our soon-to-be-introduced graphing functions on Prodata Reports which shows the hourly E.T loss on May 26th, 2026 during last month's heatwave. You can see the hourly E.T peaked at 14:00-15:00pm but continued onto 17:00-18:00 before dropping signficiantly. So this is the time period when the plant experiences the most stress.

Now the temperature threshold isn't the same for all cool season grasses, I have Poa annua reaching this impasse at 27-28°C and Perennial ryegrass / Creeping Bentgrass around 32°C. So we lose growth for a short period, no great shakes. Well not quite, because if we receive heavy wear at the same time and the plant doesn't get the chance to recover, we lose sward density. That leaves voids in the sward for annual Poa and moss to exploit. Or if you are hosting say a big tennis tournament, your baselines and courts wear out faster. That's the rub with trying to grow cool season grasses in our modern-day climate during record high temperatures.

Before I sign off today I'd like you to consider our animals and specifically birds this week. There are a lot of young birds / fledglings around at present, my garden is full. I have young Great Tits, Robins, Blue Tits and Blackbirds and crucially they need both water to drink and to bath in to cool their bodies during a heatwave. (just like we do). I use a plant pot tray and regularly keep it topped up as well as a bird bath. Apart from saving their lives, they provide a great and sometimes laughable spectacle as they bathe.(just like we do)

All the best this week, protect your skin and work / exercise sensibly in the heat.

Mark Hunt