A hardyis a temporary visitor. It grows from and then and dies all in a single season. The are sown in the open in spring.
A half hardy annual (half hardy annual) also grows from, flowering and dying all in a single season. It is another temporary visitor, used for summer bedding. It is not as as a hardy annual – the must be raised indoors in spring and planted out when the danger of frost is past.
A hardy(HB) grows from seed, producing and in the first season and flowering in the next. After flowering the plant dies. It is a temporary visitor used for spring bedding and for filling gaps in the border. The are sown in the open in summer.
A rockery perennial (RP) will live in the garden for years, thedying down each winter and new shoots appearing in spring. It is a permanent resident – at home, of course, in the rock garden but also grown in the alpine house, sink garden and front of the small border.Hollyhock, Sweet William, Foxglove and Wallflower quickly decline if grown as permanent residents in the border. It is much better to grow them as . Some half (Antirrhinum, Petunia and ) produce a poor floral if planted out in spring for a second year – they are therefore grown as half hardy .
The dividing line between the rockery perennial and border perennial is an indistinct one. It is based on height – herbaceous perennials which grow less than 1 ft high are generally called rockery perennials and those over 1 ft high are border perennials, but there are exceptions.
Popular biennials best to grow
Campanula medium (Canterbury bell). Height 38 cm-1.2 m (15 in-4 ft). A magnificent spectacle, with subtle shades of pink, lavender and purple. The taller varieties may need supporting but there are also some most attractive short varieties.
Cheiranthtli x allionii (Siberian wallflower). Height 30-45 cm (1-1.5 ft). Excellent for spring bedding, being more compact than ordinary wallflowers and sometimes a little earlier. Bright orange is the usual colour, but there are variations.
From the original deep blue colour, with pink, white and other shades of blue taking its place.
Perennials treated as biennials
polyantha (polyanthus). Height 23 cm (9 in). Few plants can rival the polyanthus for colour and length of flowering. The blooms are carried on trusses well clear of the leaves and come in many shades of yellow, blue, pink, red and white. They will grow in sun or partial shade and need moisture-retentive soil.
‘Blue Tit’. Height 15 cm (6 in). A splendid little plant that will flower for months on end if consistently deadheaded. Best propagated from , which strike very easily.
These plants develop over two seasons, from thestage to the production of ripe seeds. Growth is made during the first year; they flower, fruit and die in the second.
A problem with sweet williams and Canterbury bells is that they may not flower until June or even early July, so bear this in mind when planning your beds. It is a good idea to have a spare box ofhandy for filling up the gaps left when they have finished flowering.