Of course, the choice may not be yours. Increasingly, contract growers — and especially those producing for supermarkets and DIY chains — are required to grow plants to an exacting specification that may include nutritional programmes as well as types of growing media and the colour of pots.
But if the decision is yours, there is a lot to consider. Many formulations are still based on peat but there are also products that include bark, sand, Rockwool, perlite, vermiculite, coconut fibre and recycled botanical waste. It is essential to think about what the plant needs, but price is also an important factor, as is the reliability of the supplier to deliver the right quality in the right quantity at the right time.
Clearly, the substrate should provide support and anchorage, while allowing the roots to develop easily into a strong network. The plant also needs air and water, and for it to obtain these, the growing media needs the correct ratio of pore spaces to hold the ideal amounts of air and water. But the size of the pores is also important: if there are too many very small pores, the substrate will hold on to the water so that the plant is unable to extract what it needs.
Any new substrate you intend to try should be tested for its ability to re-wet. Peat is notoriously reluctant to absorb water once it has dried out, but the problem is overcome in the nursery by using wetting agents. Growers are also finding peat is easier to re-wet when it contains a proportion of compost derived from botanical or green waste.
The physical structure of the compost also needs to be suitable for the method of irrigation and for mechanical processes such as tray- or pot-filling and automatic seeding. The weight of the material will also have a bearing on transport — both to the nursery and internally — and it may affect the choice of container.
Chemical aspects of the substrate are just as important as the physical. Always pay attention to the nutrient analysis — both the form and amount of nutrients. Check that the pH and the nutrients are suitable for the species you grow and consider requesting the addition of specific nutrients where beneficial. Depending on the crop, where and when it is grown, it may also be beneficial to have pesticides incorporated at the point of manufacture.
Any growing medium must be free from pests, diseases, weed seedlings and sharps. It also needs to be consistent — both within the batch and from delivery to delivery. And, of course, you need a supplier who guarantees delivery when you need it.
It should also be delivered in a form that suits you. Many nurseries now use big-bale breakers to take advantage of bulk supplies of media, but if you need lots of different mixes you will probably be handling smaller bales or packs. It is worth noting that some suppliers also give technical advice and prepare prescription mixes to suit specific crops.
With profit margins remaining thin, many growers are tempted to opt for the cheapest substrate in order to keep costs down. But it is important to keep an eye on the other costs. A cheap growing medium may have longer-term cost implications when it comes to storage or handling. Worse still, it may not produce the quality of plant needed to secure a decent return.
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