Half-hardy plants require artificial heat to germinate their seed, and
must be gradually introduced into the open. They may be sown during
March and April in frames or a greenhouse, when many will bloom the
first season. If sown between May and the end of August they will
flower the following spring and summer. They require protection during
winter, such as is afforded by a cold pit, frame, or greenhouse, or
the covering of a mat or litter. Tender perennials may be sown as
directed above, but the plants should be kept constantly under glass.
Some perennials, such as Pinks, Carnations, Saxifrages, etc., do
not die down, but retain their leaves. These are called evergreen
perennials.
Pergularia.--Very fragrant twining plants, suitable for trellis-work,
arbours, etc. A rich soil suits them best. They are easily increased
by cuttings sown in sand under glass. They flower at midsummer.
Height, 8 ft. to 12 ft.
Perilla Nankinensis.--A plant of little merit, except for its foliage,
which is of a rich bronze purple. It bears a cream-coloured flower in
July. It may be raised in the same manner as other half-hardy annuals,
and prefers a light, loamy soil. Height, 1 1/2 ft.
Periploca Graeca.--A hardy, deciduous, twining shrub, which will grow
in any soil, and may be increased by layers or by cuttings placed
under glass. It flowers in July. Height, 10 ft.
Periwinkle.--See "Vinca."
Pernettya.--An American evergreen shrub, which, like all of its class,
thrives best in sandy peat; it delights in partial shade, and a moist
but well-drained position. It is increased by layers in September,
which should not be disturbed for a year. It is a good plan to mulch
the roots with leaf-mould or well-rotted manure. Height, 5 ft.
Petunias.--These ornamental half-hardy perennials prefer a mixture of
sandy loam and vegetable mould, but will grow in any rich, light
soil. Seeds sown in March or April, at a temperature of from 65 to 75
degrees, make fine bedding plants for a summer or autumn display.
As the seeds are very minute, they should be covered merely with a
dusting of the finest of soil. Moisture is best supplied by standing
the pots up to the rims in water. Pot off singly, harden off, and
plant out at the end of May. May also easily be raised from cuttings,
which will strike at any season in heat, but care must be taken that
they do not damp off. They flower in July and August. Height, 1-1/2
ft. to 2 ft.
Phacelia Campanularia.--A superb, rich blue, hardy annual. It will
grow in any soil, and is easily raised from seed sown in spring.
Flowers are borne in June. Height, 1-1/2 ft.
Phalaris.--P. Arundinacea is the well-known perennial Ribbon Grass;
it is easily grown from seed, and the root allows division. P.