Gardening Q&A: is multipurpose compost suitable for growing small seeds?

Horticultural advice from the experts at the RHS
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Question: I have bought a large bag of "Miracle Gro" multipurpose compost as well as a bag of B&Q own, only to find both are no longer suitable for sowing small seeds. But I have just tried it covering them with sifted matter. Not very successful and now even small weeds are appearing. What do you think of multipurpose compost?

Answer: Multipurpose compost has a wide range of tasks to perform and to be fair, it’s a challenge to be able to offer multipurpose as good compost for seed sowing as well as potting on young plants and growing patio plants in containers for summer.

For best results, germinating seeds need to be well-drained, so the compost should contain up to one third sand or perlite. One of the constituents of multipurpose compost can be recycled landscape waste which is quite rich for germinating seeds and also can contain those unwelcome lumps.

It is best to buy a specific seed and cuttings compost. You can cover small seeds with a thin layer of sieved compost or use vermiculite, which lets light through to seeds and drains well.

The manufacturers may be able to tell you more about their reasons if the recommended uses for their particular products have changed.

Weeds in multipurpose compost are unusual because in manufacture, green waste is heated to high temperatures that kill any lurking weed seeds. After some weeks, compost would, however, collect opportunistic windblown seed.

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