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  • Writer's pictureMark Hunt

April 25th - A tricky month so far for Poa...

Hi All,

Apologies for the absence of a post last week, my Dad lost his long fight against dementia so I gave the blog a rest for a week. Mum and Dad were a competitive pair, they first met playing for their respective countries at Ping Pong, Dad for England, mum for Denmark. Dad passed away on exactly the same day as my mum did two years ago and I can't help but think there was an element of competition between them even when it came to this. It did make me smile.

It's a lovely time of year to walk in the countryside with the Hedgerows and verges springing into life. The Bluebell stands look particularly good this year and Cow Parsley and Campion will soon be joining them. Still don't think we have the numbers of Swallows and Martins we normally have. Sure they're here now but not in the usual numbers.

April has been a good month so far if you needed to dry up but many didn't and in my mind it has played out exactly as we have seen other April's, cool and very dry. There's another factor that has been at play in April and it is often un-reported and that's evapotranspiration or E.T as it is known. This is the process where moisture is lost from the grass plant itself and from the soil surface and when it reaches high daily levels, the plant begins to adapt physiologically in order to curb water loss. A high level would be 3-4 mm per day at this time of year but I have measured nearly 7 mm at its height in the summer. Think about that, 7 mm of moisture lost over a 24-hour period, that is a lot of water, that's about 71,000 litres across a hectare, lost in a day.

General Weather Situation

So we start this week with high pressure very much in charge as it was last week and that means we will continue the dry theme most likely through to the end of April and beyond. It doesn't mean however that we will be completely dry as there are some showers on the GFS output, it's just a question of whether you pick one up or not.

So for Monday we start pretty dull because of that cloud base blowing off The North Sea and already it is pushing some showers across the U.K. There's a band moving from the north east of Scotland and another across the north east of England, both of these will move south and west. There's also a few showers tickling the feet of Kerry across The Irish Sea. These showers will continue through the day and push in off The North Sea. A real case of hit and miss though, some will get them, some won't. We will continue to have that north east wind with us and that'll cap the temperatures down to the mid-teens across the U.K & Ireland. That high pressure is also affecting the continent with frequent frosts and a continuing dry spell across The North Sea in Denmark. If any of you watched Formula 1 or MotoGP at the weekend, you'd have seen very wet conditions and that's because low pressure systems are passing south of the U.K and Scandinavia and into southern Europe due to the blocking action of the high pressure system. Skies will clear later and that'll allow temperatures to drop to low single figures with the threat of more ground frost.

The rest of this week sees the settled, dry, north easterly wind pattern continue across the U.K, however the risk of showers will decrease from Tuesday onwards and we are set in an northerly / north easterly wind direction, cloudier and cooler across North Sea coasts and sunnier across Ireland and the west. The big swings in day and night temperature will continue with low single figures and the threat of ground frost in places continuing all through this week.

As we progress through the week, the wind will lessen as high pressure centres across the U.K but that's our only real consolation. For Ireland the pattern is the same though the temperatures will be cooler with only 11-13°C likely through the week. Scotland as so often happens at this time of year will be slightly warmer than England and Wales with temperatures pushing up to 14-16°C through the week and Thursday being the sunniest day.

The only real change in the weather takes place at the weekend when a rain front, yes, a rain front pushes into the north west of Ireland on Saturday morning. This front will move eastwards across Ireland on Saturday and during Saturday afternoon, it'll also introduce some showers across the west and south west of Scotland. These showers will push south across the north of England overnight into Sunday and they'll also be some rain across The South West early on Sunday morning. Sunday will see this unsettled weather extend to the south and east of England with a showery outlook overnight into Sunday clearing Ireland and Scotland as it moves across the south of England. Not shedloads of rain and some areas will remain dry, but at least a bit of rain. With this weekend change comes a temporary swing round in the wind direction to south westerly and that means some milder nights and more consistent growth for a few days.

Weather Outlook

Does the longer-term GFS outlook show a change in the weather for the end of April ?

Well in a word....No...

High pressure looks set to continue its reign into the start of May but there will be a subtle change in that it will sit out to the west of Ireland and that'll mean it will pull down a more northerly airstream, rather than an easterly. So I see the cool theme continuing into May but maybe with some slightly better night temperatures to help things along. Now of course longer-term projections are subject to more variability but both GFS and ECMWF agree on high pressure continuing into the start of May. Mystic Megging it, my money is on a change around the 9th and 10th of May.

Despite the prognosis for high pressure to continue it doesn't mean it'll be totally dry. Next Monday there's some rain in the forecast for the southern half of the U.K and a further showery outlook for Tuesday. Closing out next week we see a rain front push into the north west of Ireland and Scotland so not totally dry. The northerly aspect to the weather will continue for the continent next week so cool and dry is the current theme for them.

Grass Agronomics

As I have commented before, April can be a problematic month for growing grass and one of the reasons is high E.T days. Now we often pick up easterly winds during April and these winds tend not to subside at night so we have breezy conditions for longer across a day and this often contributes to high daily E.T at this time of year. It also makes it harder to irrigate in a consistent fashion because the strong winds affect sprinkler performance and consistent coverage.

This type of weather also affects plant growth differentially, with some grass species like Bentgrass growing better than Poa annua. This can lead to problems in terms of inconsistent surfaces.

So first up let's see how April '22 has panned out so far and why it's been another tricky one from a grass growth perspective.

All the data shown is from Sevenoaks, Kent, U.K

So from the chart above we can see that aside from a spell of unsettled weather lasting between the 4th - 9th of April, the remainder of the month has been characterised by no rain and consistently high (for the time of year) daily E.T. If you add the total rainfall and subtract the total E.T, we have a deficit of 48 mm month to date, just shy of 2" in old money.

Of course this calculation doesn't allow for the soil moisture status, so I've used data from a Terros 12 sensor fitted to an EnviroMonitor node and communicating with a Davis Vantage Pro weather station to look at the effects of this rainfall and E.T pattern on soil moisture status. The probe is positioned at 100 mm depth and the area is not irrigated so it would lend itself for comparison with an un-irrigated fairway, rough, sports pitch.

So first up let's look at the rainfall and E.T pattern and how it affected the theoretical soil moisture surplus / deficit.

So we can see in the first part of the month, the rainfall and E.T just about balanced each other out but from the 10th of April onwards, the rain stopped, the E.T ramped up and the soil moisture deficit grew to its current level of - 48 mm up until and including yesterday.

The next graph we look at the effect of this deficit on actual soil moisture levels on an un-irrigated area of heavy loam / clay soil.

The relationship between soil moisture and E.T

The soil moisture level at the start of the month was 37.3 % (volumetric moisture content) and by and large it stayed around this level until the 10th of April with the rainfall topping up what was lost.

From then on, the constant E.T dropped the moisture level from 38.5% on the 10th of April to 19.2 % by the 24th of April. At this point I would expect to start to see signs of desiccation and the beginning of plant stress. So 42 mm of E.T from the 10th of April to the 24th dropped the soil moisture content 19.3%. So that's roughly 0.5% of soil moisture lost for every 1 mm of E.T.

Now this is a heavy soil and as such it is more efficient at retaining moisture. I've done the same calculation on sandier rootzones and I reckon it works out as 1% of moisture is lost for every 1 mm of E.T.

The effects of this moisture dynamic on growth

If we graph out the growth potential for this location, you can see the topsy-turvy nature of growth during April with frosts at the start of the month and from the 8-10th curtailing growth. Bearing in mind this is the south of England, it is probably the best scenario I could look at. Even then we have had a daily G.P at around 30% of optimum over the last week and that is a reflection of cold nights holding back the temperature.

Differential Growth

If you dovetail the E.T and growth potential data in together, we get a differential effect on grass growth and for me it generates an interesting discussion.

Poa just doesn't like this type of weather....

So with Poa now switching into seedhead mode, it is diverting most of its carbohydrate reserves to seed production rather than new root and shoot growth. It is logical then that its growth rate naturally declines during the seedhead flush and as such it tends to sit down in the sward. Bentgrass on the other hand, be it Browntop or Creeping is a different animal, it likes dry weather and good light levels and we have had plenty of both this April. So Bentgrass is in growth mode and you'll see the characteristically thick leaf produced by this plant vs. Poa annua in a mixed Poa / Bent sward. If the cutting height is over 3.5 mm you tend to get woolly, thick bentgrass projecting over a canopy of semi-dormant Poa annua. Throw in seedheads extended on a panicle and you have all the constituents for a bumpy surface. Of course across 18 greens, some will have a higher bentgrass content than others and by virtue of the above, these greens will be growing stronger than more Poa-dominated ones. From a golfers perspective this translates to faster and slower greens and more inconsistency in the surface.

It is why I think April is one of the hardest months to grow grass in the U.K and Ireland. Expectations rise proportionally with daytime temperatures and the ability to manage this period of growth differential often challenges the best superintendents out there.

Can PGR's help ?

Well again this is a slightly contentious subject because as we know, plant growth regulators do not affect all grass species in the same way and in my view Poa annua is the most-affected plant species. So you could easily argue that why should you apply a PGR when Mother Nature is providing its own natural growth regulator free of charge to Poa ?

It is a valid argument but there's (as always) a counter to it. If you apply PGR's earlier, before a dry spell like the one we are currently enduring, do you make the plant more able to survive this without going under stress because you have regulated it ?

So it isn't growing, utilising soil moisture, etc at the same rate and maybe it isn't expressing the seedhead to as large a degree ?

Of course the above debate has been heightened with the arrival of Prohexadione-calcium as a second PGR alongside Trinexapac-ethyl. This appears to be more Poa-orientated and end-users are applying it earlier than TE. By doing so is it preventing seedhead expression in the canopy by holding back the Poa ? I would be interested in your feedback as always. Drop me a line on

If so though it follows that the differential between suppressed Poa annua and less suppressed Bentgrass would be greater and therefore to avoid the same issue (poor surface uniformity) I think you'd have to drop the cutting height. Bentgrass as we know looks really woolly and cabbage-like at higher heights of cut and the only way to avoid this is to cut lower. When I say lower I mean between 3-3.5 mm rather than below this but even here we have admit its horses for courses. I think PGR's can help manage this situation but you have to be ahead of the curve to get the best benefit. Now that is a tricky call.

Ok, that's me for another week, please feel free to post a comment or drop me a line on your experiences with early season PGR usage.

All the best.

Mark Hunt

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