These are usually sown in April-June one year to flower in spring or early summer the next. Many plants described as biennials are really perennials but for best effect are treated as biennials.
Advantages. They flower at a time when not a lot else is happening in the flower garden.
They can be used to replace summer bedding plants so the ground is not bare all winter.
Some are reasonably shade tolerant.
Disadvantages. Because many have a long flowering season they tend to run over into the time when you might want to be putting out your summer bedding plants.
They require three lots of handling before they are finally installed in their permanent position.
Space is needed for nursery beds to bring the plants to the final transplanting stage.
Obtainable as seed from the usual sources of supply, or as transplants from nurseries, garden centres and ‘garden-gate sales’ (if you are buying from the latter, make sure you buy healthy
Plants or you could infect your soil with diseases you do not already have).
Uses. For mass spring and early summer bedding schemes, sometimes underplanted with bulbs.
As blocks of planting amongst permanent subjects e.g., in shrubberies and herbaceous borders.
In tubs, planters, window boxes and other containers.
Method of cultivation
Sow the seed in nursery beds (in the vegetable garden, for instance) in drills, V2. in. (15 mm) deep, in late April to June.
Either thin out in the drills until the correct spacing recommended for the type is reached, or plant the seedlings out to provide space between.
Some biennials (e.g., wallflowers) benefit from having the tops pinched out to encourage busy growth. The instructions on the seed packet will usually tell you where this is appropriate.
Plant out the well grown plants in their permanent positions in autumn (September to November). This is the stage at which you will buy them when you are not raising your own plants. Some growers are now selling biennials in individual container-grown packs to obviate root disturbance, though I have not found any improvement over the bare-root ones by the end of winter. Do make sure the plants you buy are bushy and well shaped, and have not been spoilt by overcrowding in the nursery bed.
After-care. Add a slow-acting fertilizer such as bone-meal while preparing the site before planting, and give a top dressing of quick-acting balanced feed in the spring. Otherwise the plants require little attention. If they are big and bushy, check from time to time throughout the winter to make sure they have not rocked loose in the wind, and re-firm them if necessary.