Too mulch of a good thing? We weigh up the pros and cons of mulching

Autumn is the traditional time for our Cambridge gardens to be lavished with mulch – but what exactly is it, why do you need it and can you over do it? Abbotswood investigates…

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1. Compost and coverings: the types of mulch to use

Mulches are basically loose coverings of material laid over cultivated soil. They can be made from biodegradable bark or well-rotted compost, or non-biodegradable materials such as plastic sheets, pebbles or membrane. We also use composted woodchip (a by-product from tree surgery) in our Cambridge gardens, which, after a year or so of composting, is great as a covering for naturalistic woodland style paths. It looks particularly good around trees when a circle is cut in the grass and the mulch is placed at the centre. Another source of reliable weed-free mulch is the composted green waste from wheelie bins, which is very fertile and sets off plants nicely.

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2. When’s best to lay to rest

Mulching tends to happen in the spring or autumn. In spring, it’s an excellent way to retain moisture before the summer sun dries the soil out. It’s best timed just before the major flush of new growth from perennials, however: too early, and you have another generation of weeds to remove. Mulches applied in autumn help lock in the summer warmth and moisture in time for winter. We’ve found mulching at the beginning of winter is also an excellent way to get mulch to break down more quickly, with the weather and the worms doing the hard work for you.

3. Mulching merits

Organic mulches help retain moisture, lock in nutrients and encourage beneficial insets. They can also help suppress weeds, protect plant roots from frost and can even deter pests. We’ve found wood chip are an efficient weed suppressant for larger, less formal areas, as well as newly planted hedge lines. We often use compost from our compost bins as a mulch too and although not as highly fertile as, say, manure, it adds valuable humus, which increases the soil’s water holding capacity (a very important consideration in Cambridgeshire). Research has shown a 1% increase in humus, increases water retention by 160,000 litres/ha. This translates to more than 104,400 gallons per acre.

4. When mulching goes wrong

  • Laid too close to plants, mulch can soften stems and allow disease to set in. Always ensure there is a good gap between stems and trunks and your soil conditioner.
  • Mulching can be a wonderful way to spread weed seeds! With the best will in the world, a compost heap rarely gets hot enough to kill perennial weeds, and small amounts of seed inevitably get blown in too.
  • It’s easy to cover the soil and lose too much, or all, of your free seeding plants such as aquilegia, cosmos and calendula. We find this a real disadvantage in areas where we are trying to create a naturalistic country style garden. In the same way, it’s also possible to bury smaller bulbs such as snowdrops so they either disappear or come up blind. Aim for around 5cm of mulch and replace only when the first layer has rotted away. It’s easy to form a hard crust of thick mulch that seedlings and bulbs cannot penetrate.
  • Overly fertile mulch can also produce sappy, vegetative growth that becomes a magnet for pests and diseases. This is a particular issue with manures and composted green waste – which can also have a strong odour. If you are aware that the mulch is strongly fertile, try not to over do it. A thin layer will suffice.
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