How To Improve Seedling Trays

As mentioned in one of my previous articles (see FW, 24 July), polystyrene seedling trays are made by heating small polystyrene beads in a mould; they expand to fill the mould and thereby form the tray. The more pellets that are placed in the mould, the denser and stronger the seedling tray.

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In cheaper trays, there tend to be slightly bigger gaps between the expanded pellets, and fine plant roots can enter these, making it difficult to pull out the plants.

Damping off organisms can also lurk in these small spaces, and the growing plants can become infected and die. One of the Pythium species (fungus-type pathogens) is usually the main culprit, but it’s not the only one.

I have used horse manure compost as a medium for 23 years and have never had damping off problems. This may be because the medium is a rich source of beneficial organisms, which suppress development of pathogens. I also never get powdery mildew on Brassica seedlings.