The Great Top-Dressing Debate

In a month or so thoughts will start to turn to the autumn renovation program or the “closing of the green” as many clubs call it. Bowling clubs throughout the UK will take delivery of between 3 and 10 tonnes of very expensive top-dressing compost, which will be applied to the green after hollow tining or some other aeration operation, in the belief that this will ensure that the green is in perfect condition next season. However, too much top-dressing can actually  harm the green and in many cases, clubs simply shouldn’t be doing it at all…but why?

Localised Dry Patch

Over the last 20 years Localised Dry Patch (LDP) has become a major problem on bowling greens, and although this is not wholly attributable to top-dressing, the excessive use of sand in the top-dressing mix has caused water retention problems on a lot of greens.

It is of course desirable to have a free draining soil profile on a bowling green to help to encourage deep rooting of the grasses and to maintain a reasonable green speed for play. However, continued application of bulk sandy dressings is of limited benefit to most greens and actually harmful to many due to their already high sand content and related lack of soil microbial activity.

Thatch

Natural plant decomposition results in a build up of organic material (thatch) at the soil surface, which will become much more water retentive than desired. However, the answer to this does not lie with dilution of the organic layer with huge amounts of sand, but rather in reducing this layer through judicious and very regular aeration and core removal.

Soil Exchange

Where the soil is less than perfect for fine turf production, soil exchange programs consisting of hollow tining followed by top-dressing with a more desirable growing medium will still be required, but this is an entirely different subject.

Green Levels versus Surface Smoothness

There is a great difference between “Surface Smoothing” and “Surface Levelling”. Surface smoothness in this context relates to very minor discrepancies in the surface which can be rectified by a combination of surface aeration, rolling and light top-dressing.

Surface levels on the other hand cannot generally be greatly improved through even “Heavy” top dressing work. This term relates to much more severe changes in level which can be measured by laser survey and can be seen to have a visible affect on a wood’s traverse across the green.

“Heavy Topdressing” usually defined as dressings over 6 tonnes will not have a dramatic effect on surface levels and are more in keeping with the type of operation required by new greens for the first 2 to 3 seasons to achieve the final levels not ironed out during construction.

I say this because if you do the calculations, a 10 tonne dressing over an average green (1400 m2) will result in a maximum coverage of 4mm, which is only suitable for smoothing out the smallest of imperfections.

Irrigation

Going back to the main issue of greens drying out, the average bowling club in the UK is finding it difficult to find the money to irrigate the green sufficiently during dry weather. Again when we look at the numbers, the average automatic irrigation system throws out approximately 1mm of water for every 2 minutes of run time.

Now I visit a lot of bowling clubs and I know that many of them rely on “rule of thumb” measurements like 4 minutes per head etc when timing irrigation. Well, on average a green will lose 25mm of moisture to evapo-transpiration (a measure of the combined effects of soil evaporation and plant transpiration) and that’s from a healthy green in a normal dry week. This doesn’t take account of excessively high sand content, drying winds or existing dry patch problems etc.

To simply replace the moisture lost from one day’s evapo-transpiration you need to run your irrigation system for 8 minutes per head, that’s double and in some cases several times what many clubs are doing. Over a 7 day period this equates to 50 minutes of run time per head.

Catch 22

Turning back to Localised Dry Patch. This is a condition that causes large sections of the green surface to turn brown due to lack of moisture. No amount of irrigation will make these areas re-wet. They are literally “hydrophobic” or water repellent. Careful use of wetting agents and hand watering can make some improvement, but usually it takes a wet winter to bring about full re-wetting. The main trouble with LDP is its tendency to make the green bumpy. This happens when the baked dry thatch layer on top of the soil starts to shrink below the surface of the surrounding healthy turf. Irrigation makes the problem worse as the healthy areas grow more and the dry areas recede further.

To crown it all, continued heavy use of the irrigation system in the desperate effort to bring these areas back to life, starts to encourage thatch fungus, which eventually sinks and causes an even more uneven surface.