Raising Annuals in the Greenhouse for the Garden
There are several advantages to raising your own bedding plants in the greenhouse, apart from the obvious one of saving money. Every year leading nurseries offer a splendid range of seeds for popular and less common bedding plants, together with new varieties and introductions, and also F.1 hybrids that have greater vigour than ordinary strains.
When you grow your own plants from seed you have better control of their quality and timing. Bought plants are often damaged through overcrowding in their seed trays, or through having been kept in them too long before you are able to plant them out. Sometimes, also, they have not been hardened off properly.
All these disadvantages can be avoided when you raise your own plants in pots; this helps enormously when bedding out since there is less root damage and the plants grow away faster.
When to sow
Most of the favourite bedding plants can be sown in the greenhouse from about mid to late spring (March to April). Some, however (such as fibrous-rooted begonia and antirrhinum) are slow-growing and -these should be sown as early as late winter (January), otherwise they may not be ready for reasonably early display. On the other hand, avoid unnecessary early sowing, or you will waste greenhouse space and heat; you may also spoil the plants if bedding out has to be delayed because the weather is too cold. Fast growers, like tagetes (marigolds), can be left until the very last.
In some cases it is a good idea to sow seeds from a packet over a period of time, and not all at once — especially when the quantity of seed is generous. Having several batches of seedlings in various stages of development means you will be able to enjoy a long flowering period.
How you sow can be varied to suit the cost, quantity and the size of the seed. Large seeds that are easy to handle (like zinnia) and that may also be expensive (as in the case of F.1 hybrids), should be sown individually in small pots. Finer seed can be sown in a tray or pan; prick out the seedlings into more trays when they are large enough to handle. If seed is cheap and very fine it can be mixed with a little silver sand, to reduce the density of distribution, and sown directly into seed trays. Instead of pricking out you can then thin the seedlings by pulling out the excess and discarding them. This may sound wasteful, but it saves considerably on time — also a valuable commodity.
For germinating seeds use a sterilized seed compost such as the John Innes Seed Compost or one of the many proprietary composts. See that it is nicely moist — but not wet — before sowing. A useful rule is to cover the seed with its own depth of compost. Very fine or dust-like seed, however, should not be covered. Many failures are due to deep sowing, tocompost or (always fatal) to compost that is allowed to dry out after sowing and during germination.
Nowadays plastic seed trays are available; these are clean and easy to use. The standard size is 35 by 25cm (14 by 9 in) but smaller ones are useful as germination trays for the propagator.
Germinating the seeds
After sowing cover the seed containers with glass and then a sheet of brown paper, newspaper or translucent white paper. Or you can slip the tray into a polythene bag, to help to retain moisture.
Some form of propagator will be most helpful for germinating the seeds. For bedding plants high temperatures are not only not necessary, but quite undesirable. Too much heat will force the seedlings and they will become spindly, pale and weak. A temperature range of 7-18°C (45-65°F) is adequate for most parts; the lower temperatures will suit the more hardy plants (like antirrhinums) and the higher will be needed for more tender subjects like zinnias. But if you propose using a propagator for other greenhouse work then get one that can be ‘turned up’ for more warmth when required. Generally an electric propagator is convenient and, if fitted with a thermostat, is economical and allows of easy temperature control. There are also inexpensive, small electric propagators for warming only one or two seed trays, and designs that are heated by paraffin oil lamp. Many people manage to germinate the odd trays of seeds in their homes on the windowsill of a warm room. If you have a warm greenhouse used for warm house plants, you can also use it for seed germination. It will, however, be too warm — and probably too dark — a place for the seedlings to grow on happily.
Germination time may vary from a couple of days to about three weeks depending on the type of seed, and temperature — don’t be tempted to hasten germination by using unnecessarily high temperatures. Make a daily check to see whether germination has occurred. Remove the container’s cover when the first seedlings are through so that more light can penetrate. Exposure to bright sunshine in the early stages is, however, harmful and may result in scorching of the tiny seedlings, so put the container in a shady corner.
Pricking out seedlings
Pricking out should always be done as soon as possible — as soon as the seedlings are big enough to handle easily. In the case of very tiny seedlings, such as lobelia, small groups can be ‘patched out’ since it is impossible to separate them.
When pricking out, be generous with your spacing. Commercial growers often overfill their seed trays; the roots become entangled and suffer from the disturbance caused when the young plants are divided for planting out. After pricking out it is a sensible precaution to water in the seedlings with a Cheshunt compound (used according to maker’s instructions). This will help prevent damping-off disease — a serious menace to seedlings. This fungus disease attacks the stems at compost-surface level and the seedlings then topple over. It occurs mainly where greenhouse hygiene has been neglected and will spread very rapidly unless promptly checked.
Hardening-off and bedding out
All bedding plants must be given a period of gradual acclimatization to the open air before planting out. This is called hardening-off and frames are especially useful for the process. In the greenhouse itself move the seed trays, gradually, to cooler spots. Then about three weeks before you plan to bed them out, move the trays to frames outside. Open the frame lights a little more each few days as the three-week period passes until, finally, they are fully exposed day and night. For the first week or so you may have to close the frames at night depending on weather conditions. Do not bed plants out until all danger of frost has passed.
If you have used peat pots, or pots of similar organic composition, for raising some of your bedding plants, do remember to keep them well watered after bedding out. Plants in these pots are intended to be planted pot and all to avoid root disturbance, but they will not rot down to allow the roots to grow out unless kept quite moist.