Many plant species are propagated in glasshouses. A few principles are described here to help ensure success. Seed trays should be thoroughly cleaned to prevent the occurrence of diseases such as ' damping off ' (see p246). Fresh growing medium should be used for these tiny plants that have little resistance to disease. Compost low in soluble fertilizer is less likely to scorch young plants. Compost should be firmed down in containers to provide closer contact with the developing root system.
With very small seed there is a danger that too many seeds are sown together, with the result that the seedlings intertwine and are hard to separate. This problem can often be avoided by diluting batches of very small seed with some fine sand before sowing. Small seed samples need to be sown on the surface of the compost and then covered with only a fine sprinkling of compost. In this way, their limited food reserves are not overtaxed as they struggle through the compost to reach the light.
Water quality is important with young plants. Mains water is recommended, as it will be free from diseases. Water butts and reservoirs need particular scrutiny to avoid problems. Water that has been left to reach the ambient temperature of the glasshouse is less likely to harm seedlings. The compost in seed trays should be kept permanently moist (but not waterlogged), as seedling roots dry out easily. Glass or plastic covers placed over seed trays will help prevent moisture loss, and these can be removed when root establishment has occurred and seedlings are pushing against the covers.
As soon as seedlings have expanded their cotyledon leaves, they should be carefully transferred ('pricked off') from the seed tray and placed in another tray filled with compost having higher levels of fertility. The seedlings should be spaced at approximately 2.5 cm intervals, thus providing a root volume for increased growth. Later, plants will be transferred to pots ('potted-on') to allow for further growth.
Plants growing in glasshouses are tender. The cuticle covering leaves and stems is very thin. Growth is rapid and the stem's mechanical strength is likely to be dependant on tissues such as collenchyma and parenchyma rather than the sturdier xylem vessels (see p92). When a plant is transferred from a glasshouse to cooler, windier outside conditions (for example in spring), it may become stressed, lose leaves and stop growing. It is advisable to 'harden off' plants before this stressful exposure. Reducing heat and increasing ventilation in the glasshouse are two ways of achieving this aim. Traditionally plants were moved out into cold frames to gradually expose them to the conditions into which they are to be planted. Moving plants out during the day and back inside overnight for a number of weeks, is another strategy.
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