GRASS AGRONOMICS – Differential Growth
The weather pattern of spring 2009 has produced the usual issue with differential growth between the three normal grass species present in the majority of golf greens in the UK and Ireland, that is Perennial Meadowgrass (Poa annua var. reptans), Annual Meadowgrass (Poa annua var. annua) and Browntop Bentgrass (Agrostis tenuis).
From late February to Mid-March, greens looked excellent by and large, having come through the winter snow in good health with an even sward composition. As moisture or, more precisely, lack of moisture began to exert stress on the grass during April, bentgrass continued to grow, whereas the growth rate of Poa began to slow and the annual biotype started diverting its carbohydrate reserves away from growth and into seedhead production.
Towards the end of April / start of May, the perennial biotypes of Poa annua followed suit, so now lot of greens are showing pale colour (due to the seedhead production diverting reserves away from older leaves, which subsequently pale off), and depending on cutting height / topdressing frequency, an uneven surface.
There are then two distinct reasons for an uneven surface, firstly as discussed and shown above, the production of seedheads brought on by stress and, secondly, the differential growth rates between Poa annua and Bentgrass.
Why is there a difference in growth between Poa annua and Bentgrass?
This is due in my belief to a fundamental difference in the requirement for moisture between the plant species, with Agrostis sp. able to produce better growth potential at lower moisture levels than either of the Poa annua variants. Agrostis as we know tends to have a creeping, more lateral growth habit whereas both Poa species are more upright in nature, either clumpy, as is the case for Poa annua var. annua, or more tighter bunch forming for Poa annua var. reptans. So at low soil moisture levels, we have a difference in growth rates and a difference in how the plants grow, upright or laterally.
There are a number of cultural practices that can be adopted to minimise the disruption caused by an uneven putting surface. Here are some, but not all of them:
Seedhead Production
A lot of greenkeepers seem to be under the misapprehension that use of plant-growth regulators containing Trinexapac-ethyl will inhibit the production of seedheads, this is patently not the case. To quote Joe Vargas / Al Turgeon from their excellent publication - Poa Annua – Physiology, Culture and Control of Annual Bluegrass (ISBN 0-471-47268-9), "Trinexapac-ethyl does not suppress seedheads, but does reduce their height".
I interpret the last part of this statement to mean that use of this PGR reduces the height of the panicle (shown right) upon which the seedhead is borne and thereby ‘pulls’ the seedhead lower into the canopy. This can be a mixed blessing, it will make the seedhead slightly less visible but, at the same time, harder to remove by cultural operations such as grooming, verticutting or brushing. These cultural operations can have some negative effects on the turf surface and the grass plant itself, particularly if it is under stress. Faced with a Poa annua plant that is growing slowly, showing signs of stress and producing seedheads, I would suggest opting for brushing rather than grooming or verticutting as a method of seedhead removal.
Cutting Height
A highly emotive issue because, without a doubt research and experience has shown that the imposition of a low cutting height greatly increases stress on the turf. The question is what defines ‘low’ in terms of cutting height ?In the UK and Ireland it is common practice to raise the cutting height for the winter to increases the plant’s potential to photosynthesise, protect the crown and maximise root development.
Over the last few years I have began to question this practice or, at the very least, the difference between winter cutting height and autumn / spring cutting height. Putting my head on the block, I believe that a well-managed golf green, with good aeration practices, topdressing and, therefore, fibre control should be able to go through a winter at 4 – 4.5mm quite happily with no negative consequences. Nowadays, we have good growth potential through to late autumn / early winter with soil temperatures typically remaining above 6°C till late November / early December. In turn, provided surface fibre levels are under control and properly integrated with topdressing, the plant’s potential for root development is unaffected in my view by this level of cutting height.
When the cutting height is raised higher than this, the difference between the respective grass species is exaggerated and, therefore, the potential for an uneven surface is increased, particularly as you come out of winter into the spring.
Early Aeration / Topdressing
Close on the heels of cutting height as an emotive issue for greenkeepers and golfers alike comes aeration. The requirement to manage surface fibre by hollow coring and subsequent topdressing generates friction between golfers, management and greenkeepers the world over and spring is the worst time for this in my view.
If you look at the graphs on the preceding pages, the only time when moisture and temperature have been present at levels conducive to growth was from mid-February to late mid-March. After that either soil temperature, soil moisture or both are growth-limiting factors.
Again, going against the trend, if spring is the only time available, I would suggest trying to hollow core as early in the year as possible, whenever the weather window presented itself, be it January or February. Golfers are not inclined to object as much at this time of year, the surface has more time to recover and, most importantly, there are more opportunities to lightly topdress, allow the grass to grow through and then repeat the process (obviously the greens should not be top dressed at the time of hollow coring).
Early aeration does rely however on two fundamental factors being in place – Firstly, good autumn / winter nutrition, so the greens are the healthiest they can be coming into the chosen period, without being either under or overfertilised and secondly, a preventative fungicide program has been implemented the previous autumn through October and November to minimise the disease population.
The use of light, frequent topdressing and rolling the turf surface are also great methods of introducing evenness into the putting surface and with the advent of Spinner topdressers, this process is quick and painless. It is however important to note that over a year the amount of dressing must be sufficient to dilute surface fibre and minimise the development of ‘bridged’, lateral rooting.
Irrigation
There is a marked reluctance to irrigate during the spring, with typical objections being "It’s too early to irrigate" and "It will knock the soil temperature back if I irrigate".
The reality is that if irrigation is used early in the morning or better still (but less practical I’ll admit) during the late morning, the soil temperature is only depressed for about 2 hours before rising to the same daytime high as before. Looking at the weather statistics for April 2009, we can see that the main reason the grass plant is under stress is lack of moisture due to the low rainfall level and the high daily evapotranspiration caused by the warm, windy, daytime conditions. The evapotranspiration (E.T.) rate, measured in mm per day is the amount of water lost from the soil surface to the atmosphere by evaporation. We can see that at the end of April, the E.T. readings measured at The Oxfordshire were running between 2.0 – 3.7mm per day. This is a high E.T. rate and nearly on a par with the levels measured on the same golf course during the last hot summer, 2006. (They varied between 2.0 and 4.5mm per day then).
In wetting agent trials I have used daily E.T. to provide an indication of how much irrigation should be utilised and have found that typically replacing 60% of daily E.T. provides a healthy turf, without excessive surface moisture. If this level is reduced to 30%, the grass plant begins to show symptoms of stress.
Taking the example of the last 10 days of April, the total water loss by E.T. was close to 31mm and during the same period, rainfall replaced 11.2mm, leaving a deficit of 19.8mm. 60% of 19.8mm is 11.88mm and this is the balance of soil moisture that the irrigation system should be providing during this period.
If the irrigation or rainfall does not provide this level of moisture the grass plant will go under stress and in the case of Poa, it will demonstrate an increased potential to seed and thereby present problems related to an uneven surface.
Nutrition
During this period of cold nights and warm days, plant nutrition is particularly tricky because even if there is adequate nutrient available to the grass plant provided by either granular or liquid application, if the grass plant is under stress, it will be inefficient at taking in this nutrient. Following on from the above paragraph on irrigation, if the rootzone is dry, the plant is unable to uptake nutrients efficiently and so again, the result is poor fertiliser performance. When a grass plant yellows off in drought conditions, it is due to its inability to uptake nitrogen from the soil due a lack of soil moisture.
By the same token, it is inadvisable to over-apply fertiliser when the plant is under stress because it over-utilises carbohydrate reserves and forces the plant to try to take up more moisture from the rootzone. The best management policy in my view is to apply light, frequent applications of liquid / soluble fertiliser, preferably based on ammonium sulphate and / or potassium nitrate, tank-mixed with an appropriate biostimulant and only a light rate of iron (s this acts as a dessicant)
Typically I would be aiming at 6kg of N and a maximum of 1.2kg of Fe per hectare per application. Again, like irrigation, if Poa is lacking in nutrient, it will show an increased potential to produce seedheads.
Summary
The above are just some of the management strategies that require consideration during this period of the spring and by no means are they all of them. The views expressed are based on my experience and I would be more than happy to hear from other people on management practices that they have found beneficial this spring.
|
