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Staggered planting allows for continual harvest and transplant of vegetables. It is best to have an excess of plants ready to go into the system, as waiting for seedlings to be ready for transplanting is a source of production delay. Crop scheduling is covered in more detail in Chapter 7.
Collecting seeds from growing plants is an important cost-saving and sustainable strategy, except when F1 hybrid plants are being grown (see below). Seed should only be collected from mature plants, as young plant seeds will not germinate, and old plants will have already dispersed their seeds. Collecting seeds from a number of different plants will help to retain genetic diversity and healthy plants. There are two major categories of seeds: dry seed pods and wet seed pods. Dry seed pods include basil, lettuce and broccoli. Seeds from basil can be harvested throughout the growing season, while lettuce and broccoli can only be harvested after the plant is fully mature and no longer usable as a vegetable. The seed heads should be cut from the plant and stored in a large paper bag for 3–5 days in a cool, dark place, and then lightly shaken to release the seeds. After passing the contents of the bag through a sieve, the seeds should be placed in a paper bag for storage (Somerville et al. 2014a).
Wet seed pods include cucumbers, tomatoes and peppers. The seeds develop inside the fruit, usually coated in a gel sac which prohibits seed germination. When the fruits are ready to harvest, which is usually indicated by a strong and vibrant colour, the fruit should be removed from the plant and the seeds collected using a spoon. Once the gel has been washed off using water and a smooth cloth, the seeds should be laid out to dry in the shade, and turned occasionally, before being stored in a paper bag (Somerville et al. 2014a).
Most commercial vegetable transplants are produced from F1 hybrid seeds, which are created by means of controlled pollination of two genetically distinct parent plants. F1 seed is preferred because most of the plants will have the same characteristics and produce the same quality and quantity of fruit. F1 seeds also produce plants with larger and more vigorous flowers and fruit. Hybrids are therefore more robust and better able to overcome adverse growing conditions. Seeds saved from F1 hybrid plants, however, will not produce plants that are true to the parent type (Rorabaugh 2015).
Seeds can be planted in polystyrene propagation trays filled with growing media such as rockwool, vermiculite or perlite. For commercial growing, seeds are usually started in rockwool or coir germination blocks, which are two and a half centimetre starter cubes with a small hole in the top of each cube into which the seed is placed. The starter cubes can then be transplanted into larger blocks that have a 2.5 cm hole for the starter cube to fit into, thereby minimising root disturbance (Rorabaugh 2015).
The propagation trays need to allow adequate distance between the seedlings in order to favour good growth without competition for light. The trays should be put in a shaded area and the seedlings should be watered each day. Too much water increases the threat of fungal infections. After germination and sprouting, and when the first leaves appear, the seedlings can be hardened off by placing them in increasingly intense sunlight for a few hours each day. The seedlings need to be grown on for at least two weeks after the appearance of the first leaf in order to ensure adequate root growth. They can be fertilized once a week with a gentle organic fertilizer high in phosphorous in order to strengthen their roots (Somerville et al. 2014c).
The seedlings should be transplanted into the system when adequate growth has been achieved and the plants are sufficiently strong. Transplanting seedlings in the middle of the day should be avoided, because plant roots are extremely sensitive to direct sunlight, and the leaves can face water stress due to the new growing conditions. It is recommended to plant at dusk so that the young seedlings have a night to acclimatize to their new environment (Somerville et al. 2014c).
Figure 10: Seedlings germinating in rockwool starter cubes https://commons.wikimedia.org/wiki/Category:Hydroponics#/media/File:Hydroponic_Farming.jpg
The transplants need to be supported in a net cup containing 3-4 centimetres of gravel or growing medium, and the rest of the net cup should be filled with a mixture of gravel and moisture-retaining medium. The medium helps to retain water because the young plant roots only barely touch the water flow in the grow pipe. After one week, the roots should have extended out through the net cup and into the pipe, and will have full access to the water flowing along the bottom. The planting holes in the grow pipe should match the size of the net cups, and there should be adequate space between the centre of each plant hole to accommodate the cultivated plants (Somerville et al. 2014b)
Figure 11: Net cup used for planting in an NFT system https://commons.wikimedia.org/wiki/Category:Hydroponics#/media/File:2009-03-30_Lettuce_roots.jpg
Figure 12: Transplanting onion plugs to a DWC system https://commons.wikimedia.org/wiki/Category:Hydroponics#/media/File:Hydroponic_onions_nasa.jpg
Cuttings are portions of the stem, root, leaf or leaf bud removed from a ‘parent plant’. These portions are then induced to form roots and shoots by chemical, mechanical and/or environmental means. The resulting plants will be clones of the parent plant with exactly the same genetic makeup. For example, tomato plant suckers can be removed, the severed ends placed in water, and roots will form within a few days to a week. The parent plant stock material must be free of disease and pests, and the material selected for cuttings needs to be in the proper physiological state so that roots and shoots develop readily. Transplants from cuttings can be grown by using an aggregate medium in plug trays. Rockwool is also a suitable medium for rooting cuttings. All but the uppermost 4-5 leaves should be removed to reduce water loss. Because the cuttings initially have no roots, misting is typically used in greenhouses to maintain a humid environment and reduce water loss while the roots are forming (Rorabaugh 2015).
In some species, root development is promoted by the auxin hormone being naturally present in the cutting. Other species need to be treated with a rooting compound – a preparation of synthetic auxin. The use of ‘bottom heat’ provided by means of electric cables, electric mats, or hot water tubes running beneath the beds or trays containing the cuttings, will also hasten the development of roots. No nutrients are added to the water until the roots have formed. Cutting production of vegetable crops is very labour-intensive, which is why seeds are usually used instead (Rorabaugh 2015).
Grafting is a technique for connecting two previously separate plant parts such that the resulting plant will live and grow as one. The ‘stock’ is the lower part of the graft including the roots, while the ‘scion’ is the upper part of the graft including the shoot and dormant buds from which new stems, leaves, etc., will grow. Grafting is widely used in commercial tomato transplant production. While it is very labour intensive, there are several reasons for using it, such as maintaining clones that cannot be easily maintained by other asexual methods, and creating specialized growth forms. Professional hydroponic vegetable growers are also now using grafted plants, not just for pathogen protection, but also to increase yields of many greenhouse vegetable crops, including tomatoes, with high- powered, vegetative root stocks that can support two heads. The root stock and scion must be compatible (usually the same family or genus), and both must be in the proper physiological stage to promote the fusion of the two parts into one (Rorabaugh 2015).
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