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  • Writer's pictureMark Hunt

May 9th - Summer comes early ?

Hi All,

I simply love this time of year when Mother Nature reminds me what a lucky chap I am to be living in Leicestershire and Rutland. This walkway on the Hambleton Peninsular at Rutland Water was alive with bird song as Sedge Warblers, Blackcaps and ChiffChaffs competed for territory. Keck (Cow Parsley) that was scarcely a few inches high last week is now coming into flower and Campion alongside it. You can almost feel everything growing in front of your eyes. Cracking.

I heard an article on the Beeb last week that said that Great Tits are laying eggs 3 weeks earlier than they used to, to time the arrival of their chicks to their principle food source, the caterpillars that are found on Oak trees. This suggests the Oaks must be in leaf earlier as well to provide a food source for the birds. The study has been going since 1947 and is only 15 miles odd from our default GDD location, The Oxfordshire at Thame. This intrigues me because I don't think our spring weather is significantly warmer so I don't feel nature can be reacting just to that. Maybe it's more to do with the fact the months the other side of the winter (October, November and December) are that much milder nowadays with a pronounced lack of frost ? Or maybe it's the lesser number of frost days across the whole winter ?

I charted out the total GDD from January - April inclusive from 2005 to 2022 below and you'd think that this would show a steady progression if Spring was indeed getting warmer and nature was reacting to that, but there's no discernible trend that I can see.

Maybe if I have time I'll try and access some data from the 60's and 70's and see how they compare ?

General Weather Situation

So this week we have low pressure bringing some much needed rain to some, although some locations had a nice drop Friday night into Saturday last week. The following week could see our first hot temperatures of the year as a warm air plume pushes up from southern Europe, more on that later.

So Monday sees a rain front already pushing into and across Ireland and vectoring north and east into Scotland with some of that rain on the heavy side. It is the usual Atlantic low tracking though which means below this rain front will be a sunny and warm day for central and southern areas of the U.K, with temperatures in the low twenties. During the second half of the day, the front will push into North Wales but as it does so, it'll fizzle out leaving the heavy stuff over the north and west of Scotland and the north of Ireland. A real contrast in temperatures then with Scotland in the low teens under that thick cloud and heavy rain. Ireland in the mid-teens as the rain clears north and east through the day and further south, low twenties. The wind will be moderate to strong and from the south / south west.

Tuesday sees that rain consolidate overnight into a diagonal band (/) that sinks southwards across the southern half of the U.K during the early morning. This band of rain will move slowly and not depart the south east of England until late afternoon so we should see an amount of rain for most areas tomorrow. Away from this band, we will see showers across the north and west of the U.K and Ireland, with Scotland again seeing rain for the west and central areas. So a duller and cooler day for the southern half of the U.K, with more in the way of cloud and temperatures in the high teens. Scotland again sits cool under that thicker cloud with Ireland in the mid-teens again in their sunshine and showers weather. After that rain clears Wales earlier in the day, you should be set for mid to high-teens temperatures and some spells of sunshine. The wind will shift to a more westerly aspect.

Wednesday sees more rain arrive in the early hours of the morning and as predicted last week, this rain will have a more southerly aspect to it pushing into Wales and The South West tomorrow morning and moving eastwards. There will also be rain across the north west and west of Scotland. As we progress through the day this band of rain will slowly move south and east and lessen in intensity as it does so. So it'll be a case of some places get a nice drop, others miss it completely. A good bit cooler down south mid-week as the thicker cloud knocks the temperatures back. Away from this rain band the north, north west and Scotland will see showers on western-facing coasts during the morning but these will dissipate during the second half of the day. Either looks to be mostly dry, some showers through the second part of the day but plenty of sunshine. Mid-teens again for Ireland and staying cooler across Scotland as well with the thicker cloud base.

Onto Thursday and a much drier day in prospect for most of the U.K & Ireland. I say most because they'll be some showers affecting the west coast of Scotland and these will intensify and drift southwards as we go through the day. Otherwise for Ireland, Wales and England, a pleasant day with plenty of sunshine and temperatures in the mid to high teens with the best temperatures further south. Still cool across Scotland under that thicker cloud I'm afraid. Remaining breezy from the west.

Closing out the week on Friday and overnight that rain front pushes south into Northern Ireland, The Borders, northern England and North Wales. Through the morning this rain moves eastwards but fizzles out as it does so. So cloudy with rain initially for the north and north west but clearing through the day. Ireland, England and Wales have another reasonable day but some sunshine but the wind will veer towards the north west and this will make things feel a good bit cooler, especially across the south of the U.K.

The outlook for the weekend for all areas is dry with plenty of sunshine. That wind direction is on the move again, this time across to the north east and eventually easterly by Sunday but there doesn't look to be any rain on the horizon, in fact the opposite I'd say. Temperature-wise, nothing too high, mid to high teens and feeling pleasantly warm out of the wind.

Weather Outlook

Next week looks like potentially developing into a heat plume event as high pressure builds under a peak pattern in the jet stream and pulls warm air up from Africa. The top GFS projection is for next Monday and the one below for next Wednesday. So I think we could well see low to mid-twenties next week for some areas dependent on the trajectory of the hot air plume. You'll note however that Ireland is affected by an Atlantic low pressure system so here it'll be cooler and more unsettled. So next week looks like a mirror image of this week, cool and unsettled with rain for the north and north west vs. warm,, sunny and mainly dry for the southern half of the U.K. As temperatures build across the south and south east, I can see some thundery outbreaks building towards the end of the week and as we approach the weekend, there's a risk of a BOB forming (Bay of Biscay low pressure system). If this does indeed come to pass it could point towards a very wet weekend after next for the southern half of England. Beyond that I can see more warmth for the end of May but that's yonks away at present.

Agronomic Notes

Ok, first up on the menu in this week's agronomic blog is to drag myself back to the number crunching and look at how growth and rainfall shaped up in April. Now I covered this for the Thame location last week but I wanted to get my U.K and Irish summaries up to date, so here they are starting with the U.K.

From a growth potential perspective, the average monthly total G.P was 10.3, which if you think optimum is 30 days of 1.0 (optimum growth potential in a day = 1), in other words, a score of 30, this represents about 33% of optimum growth. (average daily G.P = 0.33) Nothing to shout about then when a good growth day has a G.P of 0.6 or above, so we can see across the board, April 2022 was on the cool side. You can also see the north eastern side of the country was the coolest which is typical (Fife in this case)

From a rainfall perspective, April 2022 across the board was on the dry side, with the wettest location that of Dumbarton at 83.8 mm and the driest, Sevenoaks down at 10.4 mm and a peppering of monthly rainfall totals around 20 mm, so April was dry. As I've stated before, growing grass in April is challenging without even taking into account the effect of pests like Leatherjackets and Microdochium nivale. Speaking of which, the 15th through to the 19th of April showed a peak on the Smiths Kerns disease model around 15-20%, which is high enough in my experience to initiate disease activity from our less than favourite pathogen !

All in all a challenging month I think we can safely say !

Hopping across The Irish Sea, I have updated the stats for both March and April and thanks to the providers, much appreciated.

So here's how we look across Ireland ;

From a growth perspective across Ireland, the average total G.P was 8.2 out of a total of 30, so that translates to a 0.27 average daily G.P or 27% of optimum, slightly behind the U.K. So again we can conclude that April 2022 was on the slow side, growth-wise across Ireland., The southern half of the country fared better than the north by the looks of it growth-wise, which figures. Rainfall-wise, a pretty dry month but not as dry as the U.K, nonetheless, the east and south eastern locations came in the driest, down through Meath, Wicklow and Wexford.

Whereas April has followed a similar pattern across the U.K & Ireland, I expect that to change as we go through May because Ireland looks set to be influenced more by Atlantic low pressure systems than the U.K, so I think we will see a wider variation in rainfall and temperature. You only have to look at today's situation (shown above) with the rainfall across Ireland and Scotland to see why I say that.

Anthracnose Trigger Potential

I'm looking at next week's hot air plume and wondering if it will be hot enough to trigger some Anthracnose spore germination ? Traditionally we look for 2-3 days ~ 25°C to initiate germination of the spores themselves but then critically we also need some high humidity as well to allow the fungus to develop on the leaf surface and then penetrate into the plant itself whereupon it tends to sit in a biotrophic (resting) state waiting for a stress trigger event. It would be really early to see Anthracnose causing turfgrass symptoms in late May / early June but it has happened in Scotland before when they've had a unseasonably warm May. I'll be keeping an eye on this one and updating next week when we see if the heat plume is on still on track (The Daily Express will most likely launch into one of their "100 days of scorching temperatures likely next week" headlines 😂😂)

Now let's talk about light....

Now we are coming into early May I thought it would be a good exercise to look at DLI (Daily Light Intervals) from October 1st, 2021 till the present day because in my mind it shapes some interesting agronomic debate.

Now I've talked about this before in terms of the lack of real-world data we have for specific species and their DLI requirement. The oft-quoted figures you see are a total DLI of 11 for Lolium perenne (Perennial ryegrass) and 30 for Agrostis Stolonifera (Creeping bentgrass).

Just to recap briefly, the Daily Light Interval or DLI for short, is held to be the minimum amount of PAR light (measured in the rather odd unit of mols of light per m2 over a 24-hour period) required for healthy turfgrass growth. PAR light stands for photosynthetically active radiation and represents the part of the light spectrum (wavelength) that plants utilise for photosynthesis. Different grass species have different DLI requirements and unfortunately there's very little published data on cool season grasses, especially Browntop Bentgrass.

You can find lot's of interesting information about light levels and shade in the USGA RECORD article here .

The league table of light requirements and therefore shade tolerance is held to be this ;

Now you can see that Browntop Bentgrass (or Colonial Bentgrass as they call it) is placed right down at the right end of the graphic close to Fescue and Poa annua which is held to be the most shade tolerant grass species and therefore has the lowest DLI requirement. Creeping bentgrass on the other hand is well up the pecking order because it has low shade tolerance and therefore a higher DLI requirement.

So we know creeping bentgrass has a DLI requirement of 30 and L.perenne, 11.

I then graphed out the actual measured DLI figures from a Davis Vantage Pro Groweather station utilising an EnviroMonitor Node fitted with an Apogee PAR sensor. The location was in The Midlands, sort of centre of the U.K.

Here's how the graph looked for last winter up to today ;

So taking Lolium perenne first, we can see once we get to the beginning of November, 2021, the DLI dropped below the required level to promote healthy grass growth and it didn't exceed this level till early February. In fact it wasn't till early March when it consistently exceeded a DLI requirement of 11 (yellow line).

So if we are growing ryegrass on tees, fairways and sportspitches, we can see that the balance will tip away from L.olium perenne for roughly 4 months of the year during the winter. This explains why my extremely-shaded, back lawn degenerates to a thin stand of mainly Poa annua for the winter but looks absolutely spiffing at the moment.

If we look at Agrostis stolonifera, we can see that even by early October we are well under the magic DLI figure of 30 and we didn't approach / exceed this level till mid-March and consistently till mid-April. So that's 6 months of the year or close to it when the light level isn't high enough to promote good growth from this grass species. It doesn't mean it won't grow, rather that its photosynthetic efficiency is compromised. Chuck in lots of winter play and you can see why A.stolonifera tends to call it a day unless play levels are regulated.

The balance between Poa annua and Bentgrass....

So whatever your spring and summer overseeding program, you must reconcile yourself to the fact that in our current climate, the DLI balance will tip towards Poa annua for at least 4 months of the year on putting greens and that's without any shade !

It would I think be fascinating to know where Browntop Bentgrass comes out from a DLI perspective. I suspect its DLI requirement is close to 15 - 20 mols per m2 per 24 hours. I say this because you can see a point when Browntop and particularly old cultivars of Browntop kick into growth despite the fact that it is still cold, but light levels are picking up.(see image above) Of course within a species there is held to be considerable difference between cultivars and their shade tolerance or lack of it, but again, hard data is hard to find !

If I am right, then we are locked into a specific dynamic in terms of grass species dominance on our putting greens with a clear battle between Bentgrass and Poa annua.

We then have to accept that there is a period of the year (November to the beginning of March typically) when the balance is tipped toward Poa annua from a DLI perspective and that it follows that the only way to keep Bentgrass, when light levels are sub-optimal from a photosynthetic perspective, is to lower the amount of regenerative growth required to counter-balance wear and tear. In other words we have to limit play during periods of low DLI weather if we are to keep bentgrass as a component of our sward.

Now there's an unpopular proposition to leave you with ;P

All the best..

Mark Hunt

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