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Winter Sowings


Potato Planting


All about Asparagus

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Controlling slugs

DIY Potting Mixes

To dig or not to dig?

Home grown workshops


DIY Potting Mixes

Making your own sowing and potting compost mixes saves money, plus you can vary the recipe to suit your purpose and make up the amount you require at any time.

Firstly, you should decide how rich a mix your plant will require. A seed has enough nutrient stored in it to get started, so you could in theory sow in leafmould or coir which have very low nutrient levels. However you would need to prick out and transplant the seedlings almost immediately to prevent them starving, so it’s best to start with at least some nutrient in the compost mix. As the seedling grows, it will need more and more feed, which is why we move seedlings into larger pots with a stronger, ie richer, mix. As a general rule, increase the size of the pot only gradually. The roots should nearly fill the pot before moving into a larger one.

Plants that need good drainage when they are “grown-up” also need good drainage when they are seedlings. Large seeds that need extra warmth to germinate also need free draining compost; cucumber and courgette seeds are especially vulnerable to rotting.

potted seedlingsYou will need to bulk up your home-made compost with material like leafmould or coir, the proportions depending on how rich the mix is to be. These not only eke out your precious home-made compost they also create more air spaces within it. So what to use with the compost?

Peat was once used, but those of us concerned about the welfare of the environment will always use an alternative. Peat should simply not be used.

Leafmould is ideal, but it can be difficult to make or have access to large enough quantities. It is low in nutrient, but retains water well. It also contains micro organisms that help fight plant diseases.
To make leafmould with added nutrient, mix comfrey leaves in a bucket with 2 year old leafmould, alternating ingredients every 10cm. After 2 or 3 months, the nutrient from the comfrey will have mixed in with the leafmould. Alternatively, if you have a leafmould collecting bay, simply mix leaves and comfrey as you fill it up.
Coir, a by product of the coconut industry, is quite readily available, and is a good bulking agent. It is not suitable for ericaceous plants, but encourages good root growth in other plants. Care needs to be taken not to overwater: Coir often looks dry on the surface even when there is plenty moisture beneath, and can become waterlogged. On the other hand it is difficult to rewet if it dries out completely so a moisture gauge, readily available at garden centres, will keep you right.
Sieved garden loam (soil) is often a valuable addition to a potting mix but only use material that is free of weed roots and seeds. It can be tempting to use mole hill soil, but this is often riddled with grass and weed seeds and it can be very difficult to distinguish between the seedlings you want and those that have ‘arrived’. But if you persevere you will learn to recognise the cotyledon leaves of your common weeds.
Perlite, coarse sand and composted bark are useful in mixes needing good drainage.
Composted municipal green waste will provide you with a good bulking agent, and can be used instead of leafmould or coir in the mixes below. It is often described as a soil conditioner and will not be strong in nutrients.

Making a Potting Mix
The quickest and most satisfactory way to mix the ingredients is to use a Rotasieve. This is available mail order from Harrod Horticultural. We have used it for several years and always recommend it to students on workshops. Make sure, when putting the compost through the sieve that it is not too wet and sticky. Here are mixes that can be used at different stages of a plant’s growth. Ones marked * are specially recommended.

Seed sowing mixes
Leafmould alone or Coir alone - seedlings need to be transplanted quickly
Leafmould + loam 1:1 - needs careful watering, not suitable for very small seed
*Loam + leafmould + garden compost 1:1:1 - a good, general mix
Leafmould + wormcast 3:1 - a rich mix

Potting mixes
Loam + leafmould or coir 1:1 - will not sustain plants very long
Loam + rotted manure + leafmould 3:1:1 - very rich mix
*Loam + garden compost + coir or leafmould 1:2:1 - good, general mix.

Other ingredients, like sharp sand or perlite, may need to be added to the above mixes, when an especially free draining compost is required, for example for cuttings.

When permanently growing plants like tomatoes and peppers in containers, you will need a much stronger compost mix: 3 parts compost + 1 part loam or coir or leafmould. This will keep the plant in good stead till fruiting time, when an additional liquid feed will be needed. You can also top up the nutrient levels with wormcast.