The black plastic technique

Some weeds and grasses which grow from stolons or rhizomes are very difficult to control with organic techniques. These plants include Kikuyu (Penniseteum clandestinum), buffalo (Stenotaphrum secundatum) and couch (Cynodon dactylon).

The black plastic technique can be very useful for these difficult weeds and is completely effective in some situations. Used in combination with other techniques, such as pH or fertility modification, digging and mulching, the black plastic technique is probably the best option for good control of stoloniferous plants without herbicides. The system works best in hot weather and may only be completely effective in summer or autumn in southern Australia.

I first discovered this technique intuitively when gardening around a house built on short poles. The Kikuyu from one side of the yard had grown under the house and I started pulling at one piece. It had grown diagonally under the building, going right through to the other side. I pulled it up and wrapped it around my arm, much like rolling up a hose or a rope. At the other extreme there was a healthy two and a half metres of vigorously growing Kikuyu. I was amazed at the persistence of the Kikuyu, which just kept growing through, searching for the light. On reflection, this incident lead to me developing the technique described below.

Some basic botany

Buffalo and couch grow from stolons and Kikuyu grows from rhizomes and stolons. For simplicity, the term rhizome is used throughout this article, but the same techniques work for Kikuyu and couch.

A rhizome is a specially adapted root which stores starch for future growth and is well supplied with nodes, or potential growing points. A stolon is an adaptation from a stem, it travels along above the ground, and the nodes give rise to roots and shoots. In an established plantation there can be a lot of rhizome or stolon and consequently lots of stored starch. This is one reason that just pulling or mowing will not be an effective control. The plant simply re-grows from reserves stored underground. Another reason is because grasses which have this adaptation are also very effective at photosynthesis. As soon as the first green tips appear they rapidly produce starch and store it away in the underground parts. If this isn’t enough, the rhizomes tend to break at the node points, so that many small parts are left under the surface - each with the potential to regrow.

Some rhizomes may go deep, but in most situations they are found in the top 150 mm of soil. Vertical, fibrous roots go down from the rhizomes and in some cases rhizomes or roots may reach 4 metres depth (in the case of kikuyu), but these roots are chiefly for obtaining water and produce little or no regrowth.

The rhizomes must be weakened and/or removed. Digging is effective on some soil types and when soil is at the correct moisture content. When these conditions apply, the rhizomes will readily give up their grip on soil and can be pulled away without leaving too many sections (with growing points) in the ground. In heavy clay soils or at low moisture levels hand pulling will leave too many sections of rhizome in the ground for effective control. The same situation applies to mechanical control with an appropriate tool such as the Roterra or a ground-driven cultivator (also known as a Rollison cultivator).

Simple instructions for laying the plastic

Before laying the black plastic, adjust the mower to the lowest setting and mow all grass and weeds to the ground. Water the area well. The plant must be actively growing; any stresses that slow growth, such as low temperatures or dry soil will slow down the effectiveness of this technique.

Lay down the plastic next. If possible, go right to the edge of the infestation. If you cannot lay plastic to the end, or to a natural edge such as a track or pathway, you will need to treat a large minimum area. This is because the rhizomes form a large mass of connected (and even self-grafted) root and will transfer starch from exposed green tips back to the rhizomes. Alternatively you can trench or slice deeply around the edge of the treated area, but you will have to maintain this barrier during treatment.

Hold the plastic down by trenching and throwing soil over the edge of the sheet, or with logs, bricks and other heavy objects.

Leave the plastic on thew ground as long as possible. Depending on the season (whether the grasses are growing vigorously), the amount of underground parts and the thickness of the plastic, this will probably be at least three to five weeks. Check under the plastic occasionally to monitor growth of the grass.

The plant will regrow under the plastic mulch. The rhizomes give rise to new shoots, which wander around under the plastic, trying to find the light. The new growth will be "etiolated". This term refers to long internodal spaces (a long distance between nodes or swellings on the stem) and the bleached white colour. Most plants will do this when deprived of adequate light.

There may be a touch of green, as even black plastic will not prevent all sunlight from getting through, but the new growth will be predominantly white or a pale yellow.

The pale colour is due to lack of chlorophyll.

Chlorophyll is the enzyme responsible for photosynthesis. It is usually green in land based plants, but may also be red. Chlorophyll is only produced by the plant in response to exposure to sunlight (otherwise it would be a waste of energy).

Chlorophyll is also a UV protectant. This is why the black plastic technique works so well.

The plant has now expended much of the stored energy from rhizomes in producing a great length of etiolated growth, which wanders around under the plastic looking for light.

Wait until a very hot day, the hotter the better. Some time around or just after midday, in the hottest part of the day, pull back the black plastic.

The tender, pale, etiolated growth will be severely sunburnt and will die. Leave it exposed for the whole day or for a second day and then mow it to the ground again. Do not mow too early, as such is the tenacity of these plants that even the nodes in the stressed, etiolated growth may still be capable of developing.

This technique is really a form of "thermal" or heat treatment, as it is the hot rays of the sun which actually kill the etiolated parts.

Dig down and visually inspect the underground parts. Healthy rhizomes are white, turgid (firm) and full or solid to the feel. Severely stressed or expended rhizomes are not bright white and may appear flaky or empty when squeezed.

The technique is unlikely to be completely effective the first time, but may be repeated again, or the rhizomes may have been weakened to the point where hand digging or thick mulching will be effective.

In very difficult situations, such as under shady trees or in winter (Southern Australia) the technique may need to be repeated two or three times.

Trouble shooting

Soil moisture must be adequate before the plastic is placed; otherwise the grasses will not be stimulated to grow.

"Hot" manures or fertilisers may also be added prior to laying down plastic as they may stimulate growth too.

The plastic must be held down tightly at the edges, preferably trenched in or covered with soil, to exclude as much light as possible.

Punctures to the plastic may be repaired by laying another piece over the top - but do this quickly as once the grasses start to photosynthesise they can store food away rapidly.

Check around the edge of the plastic to ensure that bits of rhizome or stolon are not growing out from under the plastic or up from the ground (they may transfer starch back to the covered rhizomes).

This technique may work well with other weeds too, but is a very useful strategy for stoloniferous or rhizomatous grasses.

In cool or cloudy conditions, where the sun is not strong enough to damage etiolated growth, remove the plastic, mow away all the etiolated growth, and replace the plastic. In this case the technique works by starch exhaustion only, rather than a combination of exhaustion and burning.

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