How to Grow Biennials

Biennials are plants that complete their life cycle in two growing seasons. They produce rosettes of leaves and sometimes the stems the first year, but do not flower until the second, after which they die.

If given an early start, some biennials will flower the year they are sown. This can be a disadvantage, as they will then bloom later than they ordinarily would and at a time when temperatures may be too high for them to last well. This is particularly true of pansies, which produce sturdier plants and flower more prolifically when the weather is cool.

Certain biennials, such as honesty, tend to behave like perennials because they perpetuate themselves by self-sowing each year.

Also generally included among biennials are some plants that are actually perennials but are usually grown as biennials. This is because after producing vigorous growth and plenty of blooms in their second year, these particular plants deteriorate to such an extent that they are not worth retaining. Examples include sweet William, forget-me-nots, and wallflower.

Biennials are usually sown in late spring or early summer. Sow directly outdoors, as you do with annuals, a lightly shaded area. Seeds can be sown in rows or broadcast. (If preferred, sow in a flat or in a large pot.) You should water the seeds with a fine sprinkler. To keep them from drying out, you may find it helpful to cover the seeded area with a piece of burlap topped with a sheet of plastic.

After a week inspect daily for germination. Remove the covering as soon as the first seeds sprout; otherwise the seedlings will become spindly or die.

When the seedlings are well developed, lift them carefully and transplant them to a sunny or lightly shaded nursery bed.

If more convenient, plants can be raised in an unheated greenhouse or cold frame and then set out into flats or beds. Sow them at the same time as when seeding directly outdoors, and take care not to allow the temperature to rise above 70 degrees F. Some form of shading may be needed.

In cold regions, it is essential that biennial plants be set in their flowering positions in time to become established — six to eight weeks before the first hard frost. This includes those that have been raised in a greenhouse or cold frame.

Before final planting, clear the bed, dig over the soil, and work in well-rotted manure or compost, together with some bone meal or superphosphate at the rate of 3-5 pounds per 100 square feet.

Lift the plants from the site to which they have been transplanted, easing them out with a trowel and leaving as many roots as possible attached to each plant. If the soil is very dry, water it first to ease the lifting and to minimize the possibility of root damage.

After lifting, put the plants in their permanent positions as soon as possible, before their roots dry out. Firm the soil around the plants and water it.

In cold areas, plants put out in exposed positions will need winter protection. Lay a few evergreen branches over them after the soil freezes, and then remove them gradually in the spring. An alternative is to keep plants in a cold frame during the winter.

Although biennials are usually transplanted at least once, they can be sown in their permanent positions and later thinned.

Some so-called biennials, most notably sweet William and foxglove, will often survive an extra season if the stems are cut back to the basal rosette leaves immediately after flowering.

  • Republished from:

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