There are three main facilities normally used for raising seedlings in a nursery. The choice of a particular one will depend on the available resources and prevailing environmental conditions.
- Greenhouses – environment fully controlled
- Nethouse – environment partially modified
- Open field – where climatic conditions are normally favourable for the crops grown.
Media for raising seedlings
Soil is the major medium for germinating seeds and growing seedlings, although it is not the best. There are artificial media made of perlite, vermiculite and peat moss, which are used as soil substitutes. For best results, a growth/rooting medium should posses the following qualities:
- Sufficiently firm enough and dense to hold seeds in place during germination.
- sufficiently porous to let excess water drain away
- Have a high water holding capacity.
- Free from weed seeds, nematodes and other pathogens.
- High cation exchange capacity so that it can provide nutrients
- Able to withstand sterilization treatment without being altered.
- not be toxic to plants
Since it may not be possible for one medium to have all these characteristics, different media are normally mixed together to obtain a ‘near’ ideal mixture. Materials found in the locality may also be used. Some media may be contaminated, therefore it may be necessary to sterilize them before they are used, to kill weed seeds, insects and numerous soil pathogens. This treatment includes the use of heat and/or chemicals. Steam sterilization and solarization are used in the heat treatment to kill all pathogens in the media. One of the advantages of this treatment is that the media can be used sooner after treatment and the treatment can be applied to the dry, wet or even cold media.
There is, however, a risk of the media structure or composition being modified by the heat. Chemical treatment which is also referred to as fumigation involves the use of insecticides which may be specific to certain pathogens or broad spectrum (controls a wide range of pests). These chemicals should be handled with caution as most of them are toxic to both humans and animals. Some of the commonly used chemicals are, chloropicrim and vapam.
The seedbed or seedbox should be watered carefully with a fine stream of water. After the plants are well established, watering should be done thoroughly but not too often. It is advisable to irrigate seedlings in the morning and not in the afternoon as the latter leaves the soil surface moist overnight, a condition favouring damping off.
Shading should be done to protect the young seedlings from high heat intensity in sunny areas and also from from heavy rain. Shade can be provided by polythene nets or even grass. The shade should be removed some days before transplanting to allow the seedlings to acclimatize to field conditions.
This is a way of regulating plant density in rows and in holes. During thinning, weak, diseased plants are pulled out to allow healthy seedlings to grow well. It is normally done when seedlings have formed a few true leaves.
Insect pest and disease control: This is a continuous process from seedling emergence to transplanting. It is normally done by physical means but chemicals can also be used if the need arises.
This is done by physical means when weeds emerge.
Transplants must be ‘hardened-off’ so that they can withstand the transition from a relatively sheltered and protected environment to a sometimes harsh open situation. Generally, hardening is imposed from about 1 to 2 weeks prior to transplanting seedlings, by gradually exposing them to higher (or lower) temperature and the higher light intensity prevailing in the field. It should, however, not involve any treatment that may reduce the rate of photosynthesis, such as nutrient stress. Care should be taken not to over-harden plants, as this may delay maturity and in some instances even reduce crop yields.
This refers to the operation of lifting the seedlings from the seedbeds or containers and transferring them to the field where they will grow and mature. The main aim during transplanting should be to interrupt growth as little as possible, and if the operation is not carried out properly it can severely check growth or in extreme cases cause death of transplants. Most vegetable seedlings are ready to be moved 4-8 weeks after sowing. The seedbed should be given a thorough soaking about 6-12 hours before the plants are moved to ensure that they are fully turgid and that the roots retain plenty of soil when the plants are lifted. The main field should also be irrigated at the same time so that the planting holes can be opened up easily and the plants easily ‘firmed’. The best time to carry out transplanting is in the late afternoon or early evening, as this allows the plants some time to get partially reestablished before having to face heat stress during the day. A cool cloudy weather is ideal for transplanting. It is always wise to raise about 30% more seedlings than are actually required so that the weak ones can be discarded and any casualties replaced. The adaptability of vegetables to transplanting varies widely between crops. Transplanting success depends on how rapidly a plant is able to regenerate those parts of the root system that were lost/damaged during transplanting.