Nursery Techniques - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 38

2.9 Sowing Seeds in Polypots and Beds

Before sowing seeds of any species, proper time or season should be checked. Early
spring and early autumn are the two sowing seasons in most tropical regions. Even
so, the exact time of sowing will depend upon the time of outplanting, desired age
of plants at the time of outplanting, availability of fresh seeds and their viability, and
other such factors.
Seed treatment may be required for certain species, whereas many species do not
require any seed treatment. Appropriately treated seeds will germinate faster and in
better percentage. Most seeds must be sown immediately after treatment, although
certain pretreated seeds can be dried up and stored for eventual sowing. Care should
be taken to ensure that seeds are not damaged during treatment, especially in case of
treatment with boiled water or acid.
The polypots should be thoroughly drenched in water by flood irrigation or a
sustained run of sprinklers. Water should be allowed to drain out for at least a period
of 8 h. The seeds then should be sown by hand picking and dibbling at an appropriate
depth inside the soil. This depth is roughly determined by the size of the seed, but
certain seeds will germinate faster if left close to the surface. On the average, the soil
cover on the top of the seeds should be equal to the shorter dimension of the seed
itself.
Sowing in seedbeds and mother beds should follow techniques specific to the seed
and the type of bed. Seedbeds are meant for very small seeds—in fact, too small to
isolate individually—and such seeds should be homogeneously broadcast over the
bed after mixing with soil to space them out. A thin layer of soil mix—usually the
same as the potting mix—should be spread over the seeds. Mother beds are used for
raising plants in situ. Seeds should be sown at desired spacing in regular drills. Beds
should be irrigated thoroughly and allowed some time for drainage before sowing.
In most cases, resowing of seeds after allowing average germination time is nec-
essary, because not all seeds will germinate. Seeds may be resown in polypots and
beds where germination fails, but out of the resown seeds, again some seeds will not
germinate. Sometimes this cycle of resowing can be long, especially when germi-
nation percentage of seeds is poor. One may lose the entire sowing season and yet
have more resowing to do. To avoid such a situation, seeds can be sown both in poly-
pots and in seedbeds at the same time. After average germination period, the failed
polypots can be transplanted with the plants from the seedbeds. This double-edged
method also ensures that all the plants are of the same age and there are no laggards
in terms of age.
Another technique to overcome resowing cycles is to sow multiple seeds in each
polypot. A general scheme is to sow one seed in a polypot if expected germination
percentage is more than 80 %, two seeds if this is 50-80 %, and three seeds if this is
35-50 %. After germination has taken place, the extra seedlings are pricked out and
transplanted to those polypots where germination has failed.