Although plants will obviously grow in ordinary soil, in the greenhouse it is usual to propagate them in compost – if for no better reason than they can usually be assumed to be very largely free of pests and disease. Composts in general should have a range of different sized particles to make them relatively light and well aerated, provide an appropriate level of minerals and nutrients for the plant, retain moisture, but still be free-draining and provide good anchorage for the roots.
Old-time gardeners and nurserymen would make up their own favourite mixes – often to jealously guarded personal recipes, honed over years of experience – but fortunately a wide variety of composts are readily available to today’s greenhouse grower. So, while it is certainly possible to produce your own, unless it is a particular personal interest, you are better off buying it ready-made, not least because a considerable amount of careful research goes into getting the characteristics of each commercial compost just right. In this respect, it would be impossible to discuss composts without at least a passing mention of the work done at the John Innes Horticultural Research Institute in the 1930s, which gave the world standardised seed and potting composts for the first time.
Seed composts need to be low in nutrients partly because some minerals can be harmful to newly germinated plants and partly since they come with enough nourishment built-in to support their growth to the point at which the first pair of true leaves have appeared. Beyond this point, the seedlings will need to be potted-on into a nutrient-rich mix.
Proprietary brands of seed compost are formulated to have a fine texture and high water-holding capacity, typically containing peat – or a peat substitute such as coir fibre – sand and sterilised loam.
When the time comes to transplant the seedlings, they need a growing medium which can sustain their steady growth and the right balance between quick- and slow-release nutrients. Different plants have differing needs in this respect and a range of factors will influence the choice of compost at this stage, including the kind of plant, its particular stage of growth and how long it is likely to be sitting in the same pot. To get the best results from transplanted seedlings requires a little bit of research on the part of the grower to ensure as close a match as possible.
Potting composts can be loam-based or soil-less and this distinction has some bearing on their character and use. Generally speaking, although soil-less composts hold moisture well and are very well-aerated, they tend to lose their nutrients quite quickly, making them only suitable for plants which will be in the pot for a short time – unless the grower goes in for regular supplementary feeding.
By contrast, loam-based mixes offer a steady supply of nutrients over the long term, making them ideal for slower-growing plants which will take time to outgrow their containers. Both loam and soil-less potting composts drain freely, an essential characteristic to avoid the possibility of water-logging the roots. Although potting composts are generally only used when potting-on established plants, they can also be used to propagate woody plants and grow on root cuttings.
Picking the appropriate growing medium is crucial to the success of any attempt at plant propagation. While there are some very good multi-purpose mixes around, it is important to realise that not all composts are the same and sometimes, to get the best results it is necessary to use a specialised type, which is tailor-made for the job. After all, you might as well give your seeds – and yourself – the best chance.