There is the perception that seed tray cavities should have angles to prevent the seedlings from becoming root bound, but I haven’t experienced this. Whether the cavities are round or pyramid-shaped, I have found that when transplanted, the seedling roots grow immediately at a right angle into the soil.

The material that the trays are constructed from does make a difference, however. Plastic trays, being smoother, makes for easier removal of the plugs as well as cleaning and sterilisation.

However, they are usually more expensive if they are ridged. Those made from thinner plastic are floppy and more difficult to handle, but can still be very effective in many circumstances.

Polystyrene trays come in various densities. The denser the tray, the more expensive it is, as they contain more material. The denser ones are also far sturdier, so breakages are kept to a minimum, and they will last longer.

Polystyrene trays are made by adding a certain mass of beads into a mould that is then heated, which causes the beads to expand into the shape of the mould.

The more beads used, the greater the density of the tray and the lower the chance of roots and pathogens getting into the small gaps between the swollen beads.

The lighter trays are popular with nurseries that sell seedlings in the trays, as this lowers the cost for the buyer.

Size of cavities

The size of the cavities is also an important consideration, as trays with smaller cavities allow more seedlings to be produced in the nursery area.

However, this does not benefit the farmer, as the root system remains smaller inside a smaller cavity.

As to whether one can get away with using small cavities, it all depends on the species of plant grown. Lettuce seedlings, for example, are often grown in smaller cavities, making the seedlings cheaper and leading to just a small delay from planting to harvest.

Trays for tomatoes

Tomato seedlings benefit from being grown in larger cavities, such as in trays with 128 cavities, as the time lapse between planting and harvesting can be shorter, with significant benefits, and, importantly, the fruit start setting lower down on the transplants.

Plants adapt to their environment in the wild; tomatoes will use all their energy for vegetative growth to enable them to outgrow the competition and then have more sunlight and start setting fruit. It is an inbred survival mechanism.

Modern tomato varieties have inherited this instinct, so seedlings with more room and sunlight will start to bear fruit earlier, providing the farmer with financial benefits.

It can also benefit the nursery by having the plants reach a suitable size sooner, thus increasing the output, but this requires more growing medium.

I had a farmer client who was aware of this principle. He used to sow in 200-cavity trays and then transplant into small plastic nursery bags to expose the seedlings to even more light. This made a substantial difference to the yield from the lower parts of the plants. He reused the bags from year to year.

Clearly, the choice of seedling trays requires a lot of thought around suitability for the purpose, and it’s a big investment for anyone growing seedlings.

Bill Kerr is a vegetable specialist and breeder.