This is tough to answer because seeds need such different conditions to germinate. Some need to be stratified (subject to cold), scarified (making the seed coat permeable to water), light, treated with smoke or just kept warm. So I will stick with generalities and commonly sown seeds such as annuals.

Whatever the seeds, all need moisture, the correct temperature, and air (oxygen) to germinate, and then some nutrients for continued growth and the compost needs to supply these. Because we often sow the seeds before ideal sowing time, to get plants into bloom or cropping early, we also need to be aware of possible problems such as damping off. Because of this, it is important that we use sterile compost and containers. The containers must have holes in the base because the compost must not be waterlogged.

Whatever compost you use it must be fresh. Don’t use old bags that have been outside all winter or left open. Nutrients may be washed out, fungal spores may be in the compost, and the structure may have deteriorated.

When it comes to the type of compost, there are so many factors to consider; both what is best for the seeds and for the environment. I know I will incur wrath but peat-based composts are still reliable, consistent and they work. Peat-alternative composts vary from OK to diabolical. Most are made of recycled materials and some are OK for potting but they are very fibrous (high wood content) which can make watering difficult because they drain too easily. Coir is an option but I find that water does not move vertically as well in coir as in peat and the top can be dry while just below the compost is soaking. And long whiskery fibres in coir composts can make seed sowing tricky. I also question the carbon footprint of coir. A good, local, recycled compost is the best answer, if you can find it. I think we need to be flexible and buy local to be sustainable.

The alternative is a John Innes type compost but these are based on loam, stripped off fields, and they contain peat too. The big issue is that the quality of loam is too often not good and I don’t like sowing annuals in it. I often mix John Innes with multipurpose and there is nothing wrong with this. The important thing is that the compost drains well and is clean (sterile – or at least not full of pests and weeds).

Garden soil and garden compost are not suitable because of all the weed seeds, pests and everything else. In theory you could sterilise the compost in a microwave but the thought of woodlice, worms and springtails being steamed alive is more than I can bear. The price of buying a little compost is small compared to the price of seeds.

I tend to use a compost that I know is reliable (whether or not it has peat content) for seed sowing. I don’t actually need much of this compost. A recycled compost is then OK for growing on. I often mix in some perlite or vermiculite to improve drainage. I cover most seeds with these too because they allow light to reach the seeds, which is necessary for some seeds.

compost for seed sowing