September 7th, 2025

Hi All,

Bore da !

No blog last week as I was up in the lovely Welsh town of Pwllheli, North Wales on the beautiful Llyn (should be an accent on the y btw) Peninsula. Well, actually I am still here as I watch the horizon disappear as the waves batter the shingle beach and the rain hammers on the window. 5 minutes later, it is lovely and the sun comes out, this is 'The Llyn'. I have deduced this area is pretty much unforecastable from a weather perspective being a peninsular situated in North Wales and I have had some ineresting chats with the locals on this very subject (mainly Cafe owners !!). My advice to anyone visiting is to ignore the weather forecast because if it is windy and blowy one side, you can easily cross to the other where it'll be quieter and calmer....

Agaricus augustus mushrooms at the end of The Llyn peninsular looking over the Irish Sea towards Dublin !

Nature has I think had a good year given the lack of rain and warmer than average temperatures. Up here in Pwllheli there are still Swallows and Martins feeding what I think will be their 3rd brood of the year. I have seen this before and all of the previous brood pitch in to feed the last arrivals before setting off for their long migration south. On a hike in Snowdonia, I sat on a wall in Beddgelert, whilst life went on around me and I watched a beautiful Dipper chasing insects underwater in the babbling Afon Colwyn, just upstream of its confluence with the Afon Glaslyn. They're special birds, capable of walking underwater to catch Caddis and other invertebrates. This one was obviously feeling a little less energetic as he just stuck his head under to have a look :)

It did make me smile as motorcycles passed through the town noisily and people busied themselves with their Flat Whites and Bara Brith, (guilty your honour) seemingly in a hurry to relax, ("JC") whilst nobody noticed this special bird going about its business, c'ept me of course.

On a negative nature note, I was out chasing elusive Bass on the fly in Pwllheli harbour when I noticed a bird paddling towards me. It was a bird that shouldn't have been where it was. A lovely Gannet paddled close to me and then walked out onto the shore, just a few metres away. I knew what the craic was sadly. It had come onto the land to die because it was suffering from bird flu (or H5N1 as it is officially known). I left it in peace, nothing could be done and by the next morning it was dead. This virus is still wreaking havoc amongst our seabird population sadly. Bugger.

Walking by the lovely Pwllheli Golf Club, pictured above, watching the greenkeeper cut the greens on a Friday afternoon and a flock of Oystercatchers solid tine the tees for free, I was struck by how this year, the start of meteorological autumn and autumn proper have pretty much been in harmony.

No sooner did we flick the page on the calendar than it started raining. In fact, September 2025 is already the 2nd wettest month of the year back home in Leicestershire, with only 7 days clicked off ! Quite some change but happily one that was forecast. One can almost hear the overseeders whirring into action as worn out areas of the golf course are renovated with the hope that we pick up some germination and grass coverage in time to withstand a winter of play.

And why not when the soil temperature remains nice and high to enable rapid germination, plus we are getting some rain to help things along and light levels are still favourable. The graph below shows the soil temperature over the last 2 weeks at our default, Thame, U.K location and despite the arrival of cooler conditions with that rain, we have only lost just over 1°C. Work away I say !

Onto the weather....

Well despite a change to a stronger jet stream and the arrival of Atlantic low pressure systems, the forecast probability remains poor. This weekend and the first part of next week, this area of North Wales and the east coast of Ireland was set for some very strong winds and rain as an intense low pressure system pushed in from The Atlantic. When I woke up on Saturday morning, I flicked onto the GFS and Meteoblue and those winds were gone, like 2 days before they were supposed to occur and were forecast. So again I'll slip in that caveat to my forecast for the coming week, things will change and you'll no doubt see forecast rainfall levels go up and down like....well you get the gist ! Looking at Meteoblue, they attach a predictability probability of 45% to a 4-day forecast and it certainly isn't because they aren't on top of their game. I feel we still aren't in familiar meteorological territory for reasons I don't completely understand.

So to me, this is how it looks.....

7-day GFS projection courtesy of tropicaltidbits.com

As we start the new week, we have a low pressure vs. high pressure battle taking place, with an intense low sitting off the north west coast of Ireland and Scotland and bringing strong winds and rain to those parts. Meanwhile further south we have warm and humid air being vectored up from the south of Europe. Crossing The North Sea to Denmark and we have warm and humid high pressure in place that I can only imagine is pushing disease pressure up skywards. For the start of the week, I think we will see a sunshine and showers type scenario across Ireland, England and Wales, before heavier rain pushes into Ireland overnight into Tuesday. I see Denmark staying warm and humid until mid-week when rain will push up from the south and then the west. The same happens to the U.K & Ireland mid-week, with a weak low pressure out in The Irish Sea pushing rain fronts across the respective countries along with a mild, south west airstream. So currently mid-week looks wet. This wet period of weather looks to extend into Thursday as that rain pushes across to Denmark, displacing the warm, humid air as it does so. By the end of next week, we are projected to be set fair in a westerly airstream, bringing cooler and unsettled weather to all areas and in due course knocking temperatures down to the high teens and probably where should be in a 'normal' September.

Now remember my caveat, no doubt this week you'll see rainfall projections change from day to day because of this initial uncertainty on the first part of the week and the battle between those pressure systems. The projection is for the cooler and more unsettled, Atlantic airstream to dominate though as we approach the end of the week.

I won't waste your time and mine on a forecast beyond that, other than to say, I don't think we have seen the last of warmth and humid air this September.

Smith Kerns Dollar Spot Probability - Summer / Autumn 2025 - Multiple locations

Agronomic Notes

As we come into the autumn, thoughts turn quickly to disease pressure, not just from our usual Microdochium nivale, but for this month in particular, Dollar Spot and Anthracnose. The arrival of rain combined with the presence of warm air means increased humidity and ultimately plant leaf wetness.

The weather pattern we need to watch out for is a settled, high pressure system.

It is this that will give us long periods of plant leaf wetness and potentially muggy, overnight temperatures. Currently we have an west-east split, with cooler and more unsettled weather to the west (Ireland) and warmer, more humid weather across the the southern half of the U.K and Scandinavia. If the GFS projection comes to pass, what we should see is windier, cooler weather dominating our weather picture and this will ultimately lower the disease pressure from the diseases mentioned above.

Why ?

Well, windier weather makes it harder for fungal mycelium to develop, it can also (between the showers) dry out the plant leaf and make dew formation less likely. Rain also knocks dew off the plant and prevents its formation.

A wet leaf is a wet leaf right ? Well no it isn't.

Dew forms across the whole leaf in the form of fine droplets, whereas rain tends to sit in a smaller number of larger droplets. The former is conducive to disease development, the latter isn't. So the inference is we may have the potential to control disease development by irrigation, something that is already being tried in Holland and other countries where pesticidal control options are either limited or not available at all, primarily for Dollar Spot control. Now hands up, I don't have personal experience of this practice and experience has taught me not to make assumptions without it being based on experience, observation and therefore factual data.

Anyone care to share their experiences of this potential practice ?

The mechanics of dew formation

Microdochium activity on the periphery of an earlier disease scar (dark halo effect) during a heavy dew event

Overnight dew formation as measured by a Davis leaf wetness sensor

The mechanics of dew formation are interesting to me. Dew requires a combination of humidity, temperature and wind for it to occur and remain. As we come into the autumn, the longevity of night length extends through to the winter solstice and conversely day length shortens. Dew tends to form at night because the air temperature drops towards a pre-calculated threshold called the 'Dewpoint'. At this point, water vapour in the air (present as a gas) converts to a liquid and forms a droplet on the leaf surface. This can only occur if there is sufficient water vapour present in the first place, we know this parameter as % relative humidity. The reality of dew formation on grass is actually a little different to the explanation above, but since this forms the basis for my disease prediction work, I regard that as my I.P :)

The graph above shows dew formation as described by a Davis leaf wetness sensor. This records the severity and duration of moisture formation on the sensor over a set time period, in this case 15 minutes. So looking at the vertical axis, we have the severity of dew formation with a value of 0 as a dry leaf, 8 as the dew formation threshold and 15 as a fully-saturated plant leaf.

Following the graph through the night, we can see the first recording is at 03:00am when we pick up the leaf wetness intensity increasing rapidly and wetting the sensor for 5 minutes of the 15 minute duration. Thereafter the sensor records a maximum severity of 15 for a maximum duration of 15 minutes right through till 09:30am when it drops off sharply. This is likely because the sun had risen, the temperature increased and the wind began to strengthen and therefore moisture was evaporated from the sensor.

From a practical perspective, given that the sun is up around 06:00am and your greens team are champing at the bit to remove the dew mechanically, we can see that even removing the dew at sunrise will result in more dew forming thereafter on the leaf till 09:30am, when evaporation of moisture was sufficient enough to dry down the leaf. This latter process is denoted by a component of E.T, that of evaporation. E.T is a combination of moisture loss from the plant and soil by evaporation and transpiration and we can utilise the evaporation component as predicting when this is sufficient enough to dry down the leaf.

In this case, the leaf dried down as a result of a cumulative E.T of 0.1mm. Given that a heavy dew is supposed to represent 2,000 litres per hectare and this figure is equivalent to 0.2mm of rainfall, then I think this is quite accurate. The problem comes later into the autumn and winter when our E.T drops considerably due to an increase in humidity. It is harder to evaporate moisture into a high humidity atmosphere (that's why in humid climates you see people sweating profusely because it doesn't / cannot evaporate off their clothing) and so dew duration increases, driving disease pressure upwards. That's why we have to be wary of those humid, high pressure systems we get in the autumn / winter, bringing light winds, high humidity, high overnight temperatures and long periods of plant leaf wetness.

Link to USGA Green Section article here

There's a great read in the latest USGA Green Section Record on the relative merits of Poa annua and Agrostis stolonifera. It presents a balanced case where and why both grasses have their strengths and weaknesses. I wouldn't agree with all of it personally (like Poa needing 3-4 pounds of N - equivalent to 150-200kg / N / Ha) but it did make me think.

If you look at the last 2 years worth of weather, we have had cool, humid and wet in 2024 and warm and dry in 2025. You could feasibly argue that 2024 was a Poa year and 2025 an Agrostis one, so is the way forward to ensure you have a mixture of both across your greens ? (I am talking Parkland here rather than Links which I accept is a very different environment). Is it an interesting discussion and especially if you happen to manage shade AND heavy winter play as well. One of the other interesting comments is that you have to aerate Agrostis stolonifera greens when they are actively growing so you get recovery. Makes sense of course, but one of the limiting factors to that grass species growth is light. So in next week's blog I will graph out 2024 daily DLI (Daily Light Interval) vs. 2025 daily DLI (Daily Light Interval) for a number of locations and we can look at when the sweet spot for aeration actually is.

Ok, that's me for now, the low tide beckons....

All the best.

Mark Hunt