August 5th


Image courtesy of

Hi All,

The screenshot above of the current position of the jet stream, high and low pressure systems and temperature tells the story of August 2019 I think, at least for the next 10 days. A low-lying jet stream is allowing Atlantic low pressure systems to dominate and so we will see a succession push in over the next 7-10 days bringing cooler and wetter, unsettled weather to the fore. This will have some positive and negative consequences for turf management, more on that later.

After naturalising my garden last year, it’s been great to see that my resident Hedgehogs have started a family and we now have a family of four plus two from last year, so that’s six in a comparatively small garden. Just some well-sited Hedgehog houses (under ferns and the like), a bit of bedding (though they’ll bring their own), some holes cut through fences and gateways to allow access and nightly feeding (which does cost I’ll admit) and these lot are happy. No Hedgepigs in decline here Chris Packham 😛

General Weather Situation

Bit of a grim one for what I know is a holiday month for many of you but it’s not too bad.

So Monday starts with low pressure in charge and so we will see plenty of showers moving across the U.K and Ireland during the course of the day. Some of these will be snap showers accompanied by a rumble of thunder but most of the electrical storms went through last night. Since this is an Atlantic low pressure system, most of the rain will be tracking west and north across Connacht and Northern Ireland and across Central Scotland through the course of the day with the showers consolidating to longer spells of rain this afternoon / evening. South of this rain front across the south of Ireland and central and southern England / Wales we will see a line of showers moving across from the west of the U.K into central areas through the morning and again we may see some consolidation through the afternoon into longer spells of rain. They will be few and far between though. Northern England may just miss the bulk of the northern and southern showers later in the day. Temperature-wise, it’ll still feel pleasant so high teens to low twenties for Wales and England and down in the high teens for Ireland and Scotland with the heavier cloud base and rain. The wind will be moderate to strong and from the west / south-west.

After a cooler night than of late during which temperatures will dip down to low double figures, we will see more rain from the off across Ireland and Scotland. During Tuesday morning the bulk of this rain will be more northerly and westerly-orientated but as we go into the afternoon that rain will consolidate and shift southwards across central and southern areas of the U.K. So dry initially for central and eastern areas of the U.K and wet across Ireland, Wales, the west of the U.K and Scotland before that rain pushes east in the 2nd half of the day to cover all areas. Again breezy from the south-west with a strong to moderate wind and temperatures a couple of degrees lower for the south of the U.K, high teens and maybe just nudging into the twenties early doors before the cloud pushes in.

Onto Wednesday and mid-week beckons promising a drier day for many areas than Tuesday. With low pressure still about we will have a continuation of showers across the west of Ireland, the south-west of England, South Wales possibly as well and definitely across the north-west of England and Scotland. Some of these showers in the north will be heavy. There will be though plenty of areas starting off dry on Wednesday and with some hazy sunshine about it will be a pleasant 1st half of the day for the north of England, Midlands, central and southern England. During the afternoon though we see a familiar pattern developing with showers pushing eastwards across northern and central areas of the U.K, the same with Ireland. So a wetter 2nd half of the day is likely but as that rain moves eastwards we will see it clear the west of Ireland, Wales and The South West to give some sunny intervals to end the day. By dusk that rain should be clearing the west and central areas of Scotland and Ireland with just some showers across the east of Leinster likely.

Image courtesy of

Thursday sees that low pressure exiting stage right as it moves off into Scandinavia but as you can see from the SLP graphic above, another low pressure is heading our way from, yes you’ve guessed it, The Bay of Biscay. Now currently it looks like we won’t see landfall till later on Thursday evening into the south of Ireland and The South West so Thursday could be the driest day of the week for many.  So a pleasant day beckons for the U.K and Ireland then on Thursday with lighter winds and some sunshine but we will see more in the way of cloud build from the west through the day. By Thursday night that low is projected to bring heavy rain into the south of Ireland and south / south west of England, Wales before moving north and east through the night. I think with the tightness of the isobars as well it’ll be very windy with it overnight into Friday. A bit pointless predicting wind direction because it’ll depend where you are with respect to the low pressure. Similar temperatures to the rest of the week, high teens and just nudging 20°C in the south.

Image courtesy of Meteoblue

Closing out the week on Friday and that low pressure system is projected to be right over us as the image above shows for 6 a.m. Friday morning. so a very wet day for many to close out the week as that low pressure tracks slowly north and eastwards. By lunchtime the heaviest of that rain will be over Scotland and the north of England, north of Ireland leaving behind a brighter, showery picture for central and southern regions of England, Ireland and Wales. Pretty windy as well, mainly from the south west over the U.K but Ireland will see north easterlies as they’ll be on the other side of the low pressure. (see above) Temperature-wise high teens under the rain and nudging a little higher as skies clear across the south later.

So with low pressure across the U.K at the end of the week, it’s probably not surprising that Saturday’s forecast is a tad unsettled. With the centre of the low pressure moving out into The North Sea that means the U.K and Ireland will pick up a cooler north-westerly wind and that’ll rattle in showers on Saturday. These showers and longer spells of rain will affect the north and west of England / Ireland and Scotland with Wales in the firing line as well. A bit of disagreement how far south these showers will push though so central and southern England may see less during Saturday morning with more pushing south p.m. maybe. A sunnier and warmer day for England on Sunday as more of those showers stay confined to the north and west. So continuing unsettled then for Ireland, Wales, Scotland and the north of England, particularly across westerly facing coasts.

Weather Outlook

Well after an unsettled week this week do we see a lift in the jet stream and a return to some drier, warmer, summer weather ?

Well in a word…No.

Next week doesn’t however look to start off too bad with a sunshine and showers scenario for Monday and Tuesday and some pleasantly warm temperatures. By Wednesday though the week looks to be on a downhill slide as a low pressure system is set to bring rain, some of it heavy to Ireland initially before moving eastwards to affect all areas. Thursday sees that low pressure move through but by Friday another one is on its way so become increasingly wet and windy for the end of next week / weekend. Sorry.

Agronomic Notes

Since this is the first blog of the month it is of course customary to look back on last month, July and see how it panned out ?

GDD Summary – UK Location – Thame, Oxfordshire

Looking at the GDD total for July, 381.5 is 12% lower than last year and reflects the cooler, wetter interlude we had in the last week of the month. Interestingly if we compare it to last year from an E.T and rainfall perspective in 2018 this location had 132.7mm E.T loss and 16.4mm of rainfall vs. 108.4mm E.T loss and 18.6mm rainfall in 2019.

2019 continues GDD cumulative-wise to only being a so-so year from a temperature perspective, slipping behind 2018 and as suggested last week, following a very similar pattern to 2014, specifically I think during August.

GDD / Rainfall Summary – UK Locations 

Well I am amazed to see dear old Market Harborough top out as both the warmest and wettest location in July 2019 from our limited data set. That rainfall total was helped on its way by 31.5mm on the 28th July, a day when it rained solid dawn to dusk and didn’t top 15°C. Temperature-wise we hit 36.6°C on the 25th, so we had our warmest and wettest days of July within 4 days of each other. That’s a similar pattern for other locations and marks out the end of July as a prime trigger point for Anthracnose in my books as discussed last week. When you compare the GDD for Fife with Okehampton, so our furthest north and south locations, there’s an 18% swing in temperature from south to north !

GDD / Rainfall Summary – Irish Locations 

A pretty busy Irish chart tells a number of stories ;

For some locations particularly along the east side of Ireland, July 2019 was a pretty dry month with the rain not arriving till the end of the month. Cork, Wexford, Killiney, Dublin and Donabate were all around 30 – 40mm of rainfall for the month. Across the west of Ireland the stats tell a familiar story with Valentia and Claremorris the two wettest locations. Still love Mayo though but then I don’t have to live there and put up with the rain I know. Growth-wise the results were pretty consistent across Ireland with very little variation between sites but I think in some Irish locations during July 2019, it would have been moisture (or lack of it) rather than temperature that was a growth-limiting factor. Interestingly if you compare the average GDD of the U.K locations with the average of the Irish there’s a near 20% reduction in temperature across the Irish Sea.

Using GDD Data – PGR Applications

Now we know the Americans like to do things differently and they calculate their GDD stats using a base temperature of 0°C rather than the 6°C that I have utilised since I began working with GDD.

Why the difference ?

Well I can’t speak from their perspective but my thinking is this is supposed to be a growth model and in my experience foliar growth of grass doesn’t take place below 6°C so why count it in a model ? Anyway that’s not the point of this section of the blog.

Every summer I get feedback from good Superintendents on the efficacy or rather inefficacy of their Trinexapac-ethyl applications (TE) PGR applications. The contention is that they aren’t providing the suppression they did in past years. Now I’ve heard this feedback from Superintendents from the other side of the pond when sitting in GCSAA classes and it goes along the line that applying the same rate of TE at the same frequency doesn’t appear to give the same growth suppression.

There are lots of variables involved here because we know from the excellent article “Avoiding the rebound”  (read it here) that TE has a finite life within the grass plant and breaks down faster at higher temperatures. So during the warmer days of summer we have to be careful on our application frequencies. Now the data from the article talks about applying every 200GDD to bentgrass greens calculating the GDD figure using a base of 0°C rather than the 6°C. If you do the maths that correlates closely with applying every 130GDD using a base temperature of 6°C.

So I thought I’d pick 3 locations and run the maths working on applying every 130GDD during July 2019 and starting on July 1st.

Here’s how the data looks…


So for the Cork location, if I applied a PGR on the 1st July and worked on re-applying every 130GDD, the next application would be required 13 days later and the next application only 12 after that. For Northampton, the stats are slightly different with the 1st application lasting 12 days and the next one only 11 days. Finally for Fife, the first application lasts 14 days and the next one again only 11 days.

So if you’re applying a PGR every 14 days the potential duration of rebound is ;


1st application – 1 day  / 2nd application – 2 days


1st application – 2 days  / 2nd application – 3 days


1st application – 0 days  / 2nd application – 3 days

What I mean by potential duration of rebound of say 3 days is that the PGR will have reached the end of its longevity after 11 days and won’t be re-applied again until day 14, so there are 3 days when the grass plant is rebounding from the effect of the PGR. As the paper comments, the amount of rebound will be proportional to the amount of regulation so we theoretically will see a growth flush for 3 days before the plant is regulated again.

I mentioned there are lots of variables and one of them is surely the amount of growth you are regulating. Now this is more accurately described by Growth Potential rather than GDD because the formula for G.P as we know has a ‘top out’ optimum temperature above which grass growth declines rather than increases.

So lets look at the pattern of growth during July 2019 from a Growth Potential perspective ;

You can see the big dip at the end of the month (25th July) when we hit those really high temperatures decreasing growth. That said, apart from that dip, conditions from a purely temperature perspective were near optimum for growth for most of the month, so we know the PGR was having to regulate significant growth.

So here’s a question, when we look at using a PGR from an application rate and frequency perspective do we take into account the amount of growth we are trying to regulate ?

Are PGR’s like TE becoming less effective or is the growth rate or type of growth less able to be regulated by the PGR itself ?

Of course temperature isn’t the only growth-limiting factor, moisture is one as well but I’m assuming we are irrigating and monitoring the water content of the rootzone accordingly 🙂

Are PGR’s applying a selection pressure to our surfaces ?

I’ve talked about this one before but there’s no harm in re-visiting it in my mind.

So let’s assume our greens are bent and poa annua and we are maybe overseeding with ryegrass (perish the thought from some quarters but it’s a fact of modern day greenkeeping)

Now we know TE works differentially across those 3 grass species, i.e. it doesn’t regulate them all to the same degree with Poa annua the most regulated, then bentgrass and then ryegrass. So in my mind it follows that surely the PGR will encourage the less regulated species to out-grow the most regulated ? So Poa annua would be out-competed by bentgrass and ryegrass in the above example.

On a pure Poa green we know we have lots of different biotypes of Poa annua across a sward, you can see that clearly at seedhead time. So I think it’s pretty safe to assume we would have the same differential response to a PGR application across a Poa green. That is some of the Poa biotypes would be more regulated than others and therefore out-competed ? It follows then that we would eventually move to a sward where the dominant Poa annua biotype was less regulated by a PGR application. So are we seeing selection pressure being applied by a PGR and is that one of the reasons why the same rate and frequency of application is less effective ?

Dovetail that in with the rebound effect and potentially greater growth from the grass plant to regulate in the first place and it might explain why a PGR isn’t as effective as it once was maybe ?

Food for thought ?

I keep remembering the words of that song…”Things that make you go hmmm”

You get a lot of that in this job don’t you ? 🙂

All the best for the coming week.

Mark Hunt