This is a valuable division of garden plants, for by the biennial habit of storing up in the first season a reserve which is expended wholly in the second season, a much greater quantity of blossom is possible than with either the annual or perennial habit of growth.
It is also notable that many of our important vegetable crops are in this category; though where these are concerned (e.g. cabbage, beet, turnip, etc.) the plant is not permitted to flower and seed but the food stored up for these functions is taken for culinary use.
Where a new garden is being made annual and biennial plants will be of great service, for it generally takes from three to five years to achieve a garden furnished satisfactorily with perennials and shrubs; and even then biennials will still be needed.
The chief drawback to the cultivation of biennials is the space which must be given to them in the reserve garden or frame, since they are not moved into their final stations until they are large healthy plants.
The term biennial is not used too strictly by the gardener and some short-lived perennials, some monocarpic plants and, certain annuals also are sometimes given biennial treatment.
Cultivation Biennials may be sown in spring in a frame or cold greenhouse or outdoors from May onwards in beds of fine weed-free soil.
If seeds are sown in drills instead of broadcast it will be easier to keep them free from weeds by running the hoe between the rows from time to time.
After a severe thinning seedlings should be pushed on with adequate feeding until by October they will be large, leafy plants, which may then be put into their final stations in the flower border. If the weather is dry when the time comes to transplant give the bed a thorough watering. Careful lifting, using a trowel, will minimise root disturbance, and subsequent checks to growth.
If it is intended to treat hardy annuals as biennials (excellent results in mild areas) the only way in which the operation differs from that described above is in the time of the seed sowing, which should be at the end of the summer or even in early autumn, but do not sow too early or the plants will flower in their first year. Given this biennial treatment annuals will make much larger plants than when grown in the normal way and this must be allowed for when they are planted out in their final positions.
Half-hardy biennials will need over-wintering in frost-free conditions in a frame or a cold greenhouse. They are not an important section and one may well do without them, devoting precious greenhouse space to other things.
The following are biennials, or are often treated as biennial: adlumia, althaea (hollyhock), antirrhinum (snapdragon), Campanula medium (Canterbury bell), cheiranthus (wallflower), cnicus (fishbone thistle), Dianthus barbatus (sweet william), Digitalis purpurea (foxglove), Erysimum arkansanum, Hedysarum coronarium (French honeysuckle), Humea elegans (half-hardy), hunnemannia, lunaria (honesty), matthiola (Brompton, Nice and intermediate stock), some meconopsis, myosotis, Oenothera biennis (evening primrose), onopordon (cotton thistle), Papaver nudicaule (Iceland poppy), verbascum (mullein).