The benefits of mulches are numerous. They reduce temperature fluctuations in the soil, reduce water loss through evaporation and protect the soil surface.

Mulches in organic growing

In organic systems, they also play a key role in weed control, where hand-weeding would otherwise add substantially to the cost of growing a crop. Managing larger areas of plants that are not competitive, especially onions, can take a lot of time hand weeding.

The main drawback is that many mulches are made of plastic. Although some of the tougher woven plastics have a longer life of at least a few years, when these start to break down, they often deposit small fibres into the soil.

Testing alternative materials

Therefore we wanted to test some alternatives. Garden Organic tested a standard woven plastic ‘weed fabric’, a biodegradable starch-based plastic mulch, and a paper mulch at 72 of its members’ sites. A crop of onions was grown with the sets planted through the mulch, and the growth of the crop, weeds, yields and pests and diseases were monitored. They were assessed for weed growth once a month, and were weeded just after assessment and then weeds were allowed to grow back again before the next assessment.

How well did the mulches control weeds?

Generally, all the mulches reduced the weed ground cover from 20% to just 5%. The main weeds were annual meadow grass, chickweed and fat hen in the unmulched control plots. Bindweed was the most common weed in the mulched plots, as it was able to grow through the holes for the onions and start to cover the mulch.

Did the mulches affect yields?

Mulching increased yields by around 10%, partly through decreasing weed competition and also through retaining moisture. It also reduced the amount of time weeding from 45 minutes to 10 minutes throughout the season. It was a very dry summer and, in a wetter year, we may have expected to need to weed the unmulched control plots for longer.

Did the mulches affect pests and diseases?

The mulches resulted in a slight increase in the numbers of slugs, but this increase was only small. Again, as a result of the dry weather, slug numbers were generally low, so it would be interesting to observe this effect under wetter conditions.

How well did the mulches break down?

The paper mulch was starting to break down into large pieces by the time it came to harvest, whereas the biodegradable plastic was just starting to break down at the edges. Some people were concerned over the long time that this material takes to break down, which is stated as 36 months by the manufacturer. There is also the worry over the lack of knowledge of the impact that these bioplastics have when they break down in the soil.

How did the price compare?

Overall, the paper mulch was the most popular to use, as it was easy to lay, and could be seen to be breaking down by the end of the season. It is also one of the most expensive to use, having an annual cost of £1.60 – £2.40 per square metre. Weed fabric has a similar cost, but this is spread over a number of years of the lifetime of the product.

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