How to improve seedling trays and what is the ideal sealant

Polystyrene seedling trays are made by heating small polystyrene beads in a mould; they expand to fill the mould and thereby form the tray. In an article on, Bill Kerr advises on how to improve seedling trays. "The more pellets that are placed in the mould, the denser and stronger the seedling tray.

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," he says. 

"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.

Once the pathogen has appeared, the trays must be sterilised, or the problem will keep cropping up and possibly cause major losses.

The most effective way of sterilising trays is in a heat chamber. To save cost and effort, some growers wait until damping off starts before using heat sterilising.

Another solution is to use a commercial dip containing copper; this will also reduce the penetration of roots into the fine gaps in the polystyrene.

If you are a small grower, you can use a simple, home-made steriliser such as bleach mixed with water at a 10% solution, or hydrogen peroxide at 3% concentration. Leave the trays suspended in the solution for 20 minutes, then rinse and dry them." 

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