Old seedlings are often discarded due to the belief that much of the yield potential is lost. There is some validity to this, but in my experience the damage is usually less than expected.
I had this experience towards the end of last year while growing organic vegetables. This is a new venture for me; I recently sold my breeding programme and entered a contract to supply a wholesaler on a regular basis.
Our area is subject to rather heavy frosts in winter. One day in late winter last year, I saw that the temperature had dropped to -10°C in a tunnel, but damage to the plants was only moderate, and I thought we were over the worst.
The next frost brought a temperature of only -7°C, but because there was a wind, we ended up with a penetrating black frost. This burnt my winter cover crop of grazing vetch so that it turned white, and it did the same to my broccoli and cauliflower seedlings under plastic cover.
I waited to see if there would be any recovery. Slowly, most of the seedlings started to grow out. I had already planted new batches, but out of curiosity kept these burnt seedlings going.
I then decided to plant some of them next to the new seedlings to see the difference, although they were two months older.
The results were remarkable: about 1% of the broccoli produced smaller heads, but the rest were fine, and the cauliflower plants produced a good crop.
Kept in limbo
Seedling nurseries often find that, due to poor weather or some other circumstance, a client cannot take delivery of an order at the designated time. These plants are often then discarded. However, if they are deprived of nitrogen, they can be kept in limbo, as they don’t grow out too large in the seedling trays.
As soon as they are ready to be planted, they can be given a dose of LAN (one closed handful in a watering can applied to about 2 000 seedlings). To illustrate this, I took a tray of hardened cauliflower plants and applied the nitrogen to half the plants. The results were clear after nine days, as can be seen in the photograph above.
If there is to be any setback with old seedlings, it usually occurs in very early-maturing varieties.
That’s all there is to it. In difficult times, simply withhold nitrogen to keep the plants in limbo. Then stimulate them with LAN before they leave the nursery. If they are already in an active growing state and need to be stopped, flush the nitrate out of the trays with a drenching irrigation.
Seedlings are expensive. Why waste good money by discarding them when it’s unnecessary?
Bill Kerr is a vegetable specialist and breeder.