Using Organic Mulches in the Vegetable Garden

I love the word 'mulch'. Like 'humus', it brings to mind thick, dark, moist soil like that found in natural woodland, rich, fertile, wondrous stuff produced in what is possibly the most perfect process on Earth: an endlessly recyclable system where nothing is produced that isn't eventually reused.

While it's hard to recreate this as a closed system in the average garden, using organic mulch is highly beneficial for your crops, the soil, and the organisms that dwell within it. There are many sustainable mulching materials to choose from (many of which may be cheap or free depending on sources in your area) including straw, hay, home-made compost and leafmold, grass clippings, pine needles, bark, woodchips, sawdust, comfrey leaves and even seaweed (make sure to rinse off salt first).

Here are the main benefits of using organic mulches in the vegetable garden...

Mulches for Weed Suppression

An organic mulch will smother weeds, but unfortunately you do need to do some weeding first! You can hoe off short-lived weeds, but perennials such as ground elder or dandelion have to be dug out root and all. It's really essential to make sure you've got all traces of root out, as it will regrow from the tiniest scrap left behind.

When mulching wider areas between rows of plants it's worth laying down cardboard or newspaper before topping off with compost, leafmold, bark or similar, as this will prevent birds from mixing weed seeds from the soil below with the mulch material as they poke around.

Then, lay your preferred mulch at least 2-3" (5-8cm) deep, and preferably deeper, to exclude light and ensure that any weed seeds below cannot germinate. If you've done your intial weeding thoroughly, any weeds that grow will be rooting into the mulch itself and are easily removed with a hoe while still small.

Mulching for Plant Health

When the weather gets hot, water in the soil evaporates rapidly and it doesn't take long for plants to feel the effects of drought – they may wilt or drop fruit, tomatoes can suffer blossom end rot, and potatoes and other root vegetables refuse to swell. Mulch helps to reduce the rate at which water evaporates, but it's important to make sure that the ground is wet first – either apply the mulch after heavy rain, or water the area thoroughly beforehand.

Mulches help prevent erosion and nutrient leaching caused by downpours, as the water filters into the soil beneath more gradually. Wet soil can result in fruits such as strawberries and squash rotting, so raising them off the ground on a bed of straw prevents this and will also stop mud from splattering fruits.

In the longer term the increased organic matter improves moisture levels for any soil type, helping sandy soils to retain water and creating pores in heavier soils that alleviate waterlogging problems.

Waiting until summer to begin mulching heat-loving crops such as tomatoes and peppers can be beneficial, as mulching too early can slow the rate at which the soil warms up. Dark-colored mulches – compost or leafmold for instance – absorb heat during the day and release it at night, so are a good choice for heat lovers.

Ericaceous plants such as blueberries and cranberries that require a low soil pH and consistent soil moisture benefit from a mulch of pine needles, bark or sawdust. I tend to use sawdust around my blueberries as we process our own firewood and so have a plentiful (and most importantly free!) supply, however some people frown on this as fresh sawdust is prone to knitting together into a water-resistant crust. I deal with this by applying aged sawdust fairly thinly a few times a year, and occasionally agitate it with a cultivator or rake if it looks like it's forming a mat. I've also used pine needles in the past and they are a good option – water filters through them easily and they take a very long time to rot down.

The Drawbacks of Using Organic Mulch

There's no such thing as the perfect cure-all, and organic mulch is no exception. I recently had to remove a grass mulch from around my broccoli when I discovered that voles were using it as the perfect cover to hide in while they ate the leaves and newly-emerging heads of my plants.

Slugs too are inevitably drawn to mulch, so if you have a big slug problem you might want to avoid mulching vulnerable plants. However mulch will also encourage slug predators such as ground beetles, and some people feel that mulches provide an alternative food source for the slugs, which in turn reduces damage to your crops.

Molds and fungi can sometimes grow on mulches, but this is uncommon and can usually be avoided by occasional stirring or raking of the mulch. And do remember that some fungi are beneficial – seeing a mass of thin white threads (mycelium) in mulched soil is not a cause for panic!

Mulches do need to be re-applied occasionally, so some mulching materials will need storage space, but a bin or a sack hidden away behind a shed works well for most.

Despite these drawbacks, the benefits of mulching are too great to ignore. It mimics the natural processes of a forest, where leaves fall to the ground, decay, and are recycled in the soil to nourish and protect the plants above. And, the more you can use materials from your own garden as mulch, the more sustainable it becomes.

By Ann Marie Hendry.