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How to make your own seed compost

Forgot to make leaf mould? You may already have a supply in your own garden. Photograph: Kim Stoddart

Forgot to make leaf mould? You may already have a supply in your own garden. Photograph: Kim Stoddart

Thanks to my local seed swap, bartering pals and my fellow seed-saving editor, Jane; I now have a fine bounty of “plantables” ready and raring to grow for the gardening season ahead. The question is – what am I going to plant them into this year? I assumed mainly garden compost, but I was wrong, and everywhere you look for advice on this subject there’s a different recipe on offer. To make matters worse; most recommend ingredients such as biochar, coir, vermiculite or sharp sand; all of which need to be bought in and are therefore no good for me.

Leaf mould doesn’t cost anything to produce and is supposed to be a key ingredient, but I’ve forgotten to get round to producing any. Oh dear. I really should have seen this coming- I knew leaf mould was important. Annoyed with myself, I start to panic. A decent propagation compost is after all essential:without it, nothing will grow.

Then I think back to whenever I’ve planted seeds directly into the soil before – they have generally always taken. The biggest risk has been whether they’ll get munched by slugs, not that they won’t germinate in the first place. Putting aside all the elaborate, conflicting and frankly confusing recipes for potting mixes that have made my head whirl; I realise I need to understand better what I’m dealing with in order to make it work for me. I happen to know that Ben Raskin, the head of horticulture at the Soil Association has just written a children’s book on compost which seems like a good starting point and just about the appropriate level for me on this subject. I decide to tap him for some ideas.

The issue with seeds, he explains, is that they don’t want too much in the way of nutrients, and bog standard garden compost (on its own) is far too rich for them. You also need the right texture and drainage for seedlings to germinate and grow, which is why ingredients such as coir, leaf mould and vermiculite are recommended in place of peat. Ben suggests wood chippings as another option, which a friendly local tree surgeon can often provide for free. But to use these they have to be well rotted (for at least 12 months) first, so that’s out for this year at least. On the plus side I do have a few molehills around the soft fruit bushes at the back of the garden. Ben points out the fine crumbly nature of the worked soil lends itself well to potting – so that’s something.

A few days later and between rain showers I inspect the moles’ handiwork – it seems they have been rather busy. As I’m walking back with a few buckets of this fine compost ingredient, I spot a bed of hellebores bursting with flower heads. They live in a shady spot surrounded by trees and, it occurs to me, amid a blanket of leaves. Not being a fastidiously tidy gardener, I happen to know that there must be at least four years of rotted down leaves here.

Bingo. It would seem I have lazy gardener’s leaf mould in rather plentiful supply.

Reading into the subject some more, I think the success of home-made seed compost depends on the quality of the ingredients you have to work with. Good compost, while too potent on its own, makes an important addition, as does some of the best topsoil from your garden – and everyone seems to agree that leaf mould provides an essential part of the mix.

I know there’s the issue of germinating weeds, but I’m loath at this point to start pasteurising soil by heating it in the oven. I ask you. I know what the plants I’m trying to grow look like, so it’ll be easy to pick any invaders out by hand.

Based on my positive experiences previously of planting seeds directly into the ground and seeing them flourish, I decide to keep my first batch of seed compost simple. I’ll use an equal(ish) mixture of soil (molehill and top), compost and leaf mould. I think this should do the job. Maybe it pays to be a bit lazy sometimes after all.

Have you tried making your own seed compost in the past, and if so, what combination worked best for you? As always I’d love to hear your experiences and ideas on this subject.

<p>Buying seed compost can be expensive, but what about making your own? Thrifty gardener <strong>Kim Stoddart</strong> finds out how</p>

Compost seed

Homemade garden compost is usually used to enrich and add organic matter to the soil and it can also be used to make seed and potting mixes. I would recommend that all home composters give making homemade seed and potting composts a try.

The requirements of a homemade seed of potting compost are the same as for those provided commercially. It should be:

  • free from pests and pathogens (plant and human)
  • moisture retentive while allowing air circulation
  • of a uniform consistency (sieving may be necessary to achiev this)
  • of a texture that will allow root development and support for the seedling
  • and contain sufficient nutrients.

When making homemade compost mixes only mature i.e. fully decomposed compost should be used and this should be sieved (screened) so that it consists of small particles of a relatively consistent size.

The cheapest shop option for small quantities is a Seedling Potting Riddle this has a 4mm galvanised mesh sieveto provide a compost for planting seed.

A hand operated Rotary Soil Sievis a more expensive option which will create a compost, suitable for for Seed and Potting compost. Dry compost from the bin on the seive and the handle is rotated pushing the material through the 2.5cm/1″ x 1.5cm/0.5″ mesh. Both of the above can be purchased from http://www.harrodhorticultural.com and other suppliers.

While sieves, such as the above, can be purchased it is relatively easy to make a wooden a box frame of a size to fit onto your wheelbarrow. A strong weld mesh base with 6mm holes should be fitted securely to the frame. Remember it will be used quite roughly so make sure that the frame and base are well secured.

Homemade Seed and Potting Compost – Risks

Making homemade seed or potting compost using garden compost direct from the bin or wormery for germinating seeds, potting on young seedlings and rooting plants from stem cuttings risks the introduction of weed seeds, pathogens or organisms that may damage tender plants . The same risks exist if untreated soil is used as part of the compost mix.

It is not necessary to heat treat the garden compost or soil mixtures used for potting on those plants that are almost ready to be transplanted into the garden.

Oven sterilisation of Compost

Sterilising the compost or soil in Pyrex dish or metal roasting tray (covered by foil) in a preheated oven at 160C for an hour is an effective means of sterilising the compost. The dish or tray should be filled to a depth of not more than four inches so that the compost in the centre reaches the required temperature. The tray of sterilised compost should be remove from oven and allowed to cool before use.

Gardening Knowledge at www.gardeningknowhow.com suggest a similar method but using lower temperatures involving baking at 82-93 C. (180-200F) for at least 30 minutes. It is suggested that heating to higher temperatures can result in the production of toxins

Sterilisation is an effective way of eliminating the risk but it means that everything is killed is including beneficial organisms.

Heating the garden compost, or soil, at a lower temperature with remove pathogens without damaging nutrients . Motherearth (www.motherearthnews.com/homesteading-and-livestock) suggest a method for the pasteurization of compost so as to destroy harmful organisms but not the beneficial ones. Compost is placed in an aluminium baking pan (I prefer a Pyrex baking tray) again to a depth of 4 inches. A meat thermometer is placed in the compost at centre of the tray, so that it can be read while the tray is in the oven. Pre-heat the oven to 93C ( 200F) and insert the tray. Bake for 30 minutes once the thermometer records a temperature of 71C (160F). Cool the compost before use.

Sterilizing Compost with a Microwave

Gardening Know How (www.gardeningknowhow.com) suggest a method of sterilising compost or soil using a microwave. About a kilogram of compost is heated in a microwavable container covered by Clingfilm with added ventilation holes.

The compost is heated for about 90 seconds per every kilogram (approx 2 pounds) on full power.

Alternatively a kilogram of moist soil can be microwaved in an open polypropylene bag for 2 to 2 .5 minutes on full power (650 watt oven). The bag is closed and allowed to cool before removing.

A problem common to all methods involving invading the kitchen is that the sterilisation or pasteurisation will give rise to an earthy smell that may not be popular with everyone in the household. An alternative could be used a propane gas Outdoor Camp Oven.

Seed Compost

Seeds contain stored nutrients and will need few additional nutrients to germinate and start to grow , so compost straight from the bin or wormery will be too rich to be used as a seed compost. It will also be of the wrong texture and may not provide adequate drainage.

While Leafmould has a low nutrient level and could be used as initial seed compost the seedlings would need to be pricked out and transplanted almost immediately after they have germinated to provide sufficient nutrients for future growth.

It is therefore better to make a seed compost containing just sufficient nutrients to provide for germination and initial growth. Then, as the seedlings grow, and need more nutrients transplant them into larger pot containing a potting compost with a richer mix of nutrients. Potting onto larger pots, may be required several times, increasing the size of the pot gradually, when the roots nearly fill the existing pot.

In addition to providing nutrients the compost should provide drainage, so that the soil does not become waterlogged and cause the seedling to rot, while retaining sufficient moisture for the plant to grow. It will need to contain air spaces so that soil microbes and the roots will have oxygen while being of an even consistency free from lumps. This favours a light or fine textured mix but it must be sufficiently firm to retain and support the seedling as it grows to a size suitable for potting on. It should also retain its volume in the pot and be free from pest and disease.

Homemade garden compost is usually used to enrich and add organic matter to the soil and it can also be used to make seed and potting mixes. I would recommend that all home composters give making homemade seed and potting composts a try. The requirements of a homemade seed of potting compost are …