Greenhouse Annual Plants

With very little in the way of facilities it is possible to grow a wide range of annual plants in pots to give exciting and colourful displays for the greenhouse conservatory and house. Although sown at different times of the year the basic growing principle for most of them is the same.

If you have noticed enormous plants of Schizanthus (Poor Man’s Orchid) which are usually in flower during early summer you may well have wondered how such size was obtained. In fact a lot of annuals associated normally with spring sowings can be sown in autumn to produce much larger, sometimes earlier, plants. This is true of Agmstemma (Corn Cockle), Mimulus (Monkey Flower), Calendula, Verbascum and Linatia (Toadflax) which are all hardy annuals but will make exceptionally good pot plants.

The method is to sow them from the end of August to October and as soon as they have germinated grow them as cold as possible without allowing them to become frosted. A cold frame is excellent for the early stages and it is usually possible to leave them out there until Christmas, provided lights are left on whenever it is likely to rain. Good ventilation is essential but during frosty nights the vents should be closed and some insulation such as hessian thrown over the lights. At the onset of long periods of very cold freezing weather they should be brought into the cool house. Always provide as much ventilation as temperatures will allow.

It is very important with these annuals that they are not allowed to become potbound in the early stages as this restricts the amount of growth they will make before they flower. Even though with most other plants you are tending to keep them if anything slightly on the potbound side during winter this should never be allowed to happen. As soon as the seedling has been pricked out and filled its space with roots it should be potted. As soon as the roots begin to find the edge of the pot it should be potted on until the final pot is reached. A compost containing loam is easier to manage during winter conditions especially when watering, which if over indulged in can kill the plants. It is essential that plants are spaced out from each other so that their leaves are not quite touching. I know it is tempting to pack as many as you can into the space but being strict with yourself will reap rewards in healthy well-grown plants.

Feed plants with a high potash feed whenever they are established in a particular pot. Avoid high nitrogen feeds as this will make them too lush at a time when they should be making compact growth which is more able to withstand the cool temperatures. The plants will come into flower during the following spring and early summer.

There are F1 forcing Antirrhinums which are sown in October. I like to prick them out into small pots and then pot them three to an 18 or 20 cm (7 or 8 in) pot. They will flower in May and June up to an astonishing 1.5 m (5 ft) high and, although not suitable for a small greenhouse, are invaluable for backing big displays.

Selected Danish stocks can be sown throughout the winter but remember that only the pale yellow seedlings will give double flowers. To make the difference clear you need to place the seedlings in a temperature of 4—7°C (40-45°F) for a few days immediately after germination.

Annuals sown in the spring need much the same sort of care. Hardy annuals will need to be grown cool but give warm conditions to anything described as a half hardy or greenhouse annual. Tropaeolum peregrinum (Canary Creeper) is a hardy annual well worth trying under glass. Grow three plants to a large plot and provide some tall canes for them to twine up. Although they will grow outside bad weather sometimes results in only an average display whereas their pretty little yellow flowers will brighten up a greenhouse or out on the patio and will give height to large displays. I sowed some half hardy Celosia one April and, having pricked them out into small containers, did not have time to pot them on at the critical stage when their roots began to fill the space. As a result they all came into flower as miniature plants 8 cm (3 in) high. That was a very useful lesson. I have also found that they do much better in a peat-based compost.

Browallia are sometimes described as greenhouse perennials but are much better grown as annuals as they are not worth keeping after they have flowered. Extremely easy to grow they can either be sown during spring to flower in summer or during late summer for winter and spring flowering. As they are not hardy they will need a minimum growing temperature of 13°C (55°F) to do well.

Eustoma russellianum, sometimes called Lisianthus, is the Prairie Gentian and is another perennial grown as an annual. This is something of a challenge to grow as it is by no means easy. Light is needed for germination but the real skill lies in correct watering throughout its growth. Endeavours will be rewarded in summer with the beautiful flowers which are bluish-purple in the species but strains of pink and white are available. Grow three plants to a pot from seedling stage to give a fine display.

Cineraria can be sown during the summer and grown on cool to flower the following winter. It is a good idea to stagger the sowings from one packet at six week intervals between April and August which will give a longer flowering period. Calceolaria are also sown during summer between May and July and grown cool will flower the following spring. These are described by seed firms as greenhouse biennials but the same rationale should be used as for growing the annuals especially as to coolness and good ventilation.

28. June 2013 by admin
Categories: Tips and Advice | Comments Off on Greenhouse Annual Plants


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