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. Canariensis is the useful canary seed:  it may be propagated from seed on any soil.  Height, 1-1/2 ft.

Philadelphus.—­Among the best of our flowering shrubs, producing a wealth of sweetly-scented flowers.  For cultivation, see “Syringa.”

Philesia.—­An American evergreen shrub which grows best in peat, but will thrive in any light soil.  It should occupy a cool position, but be well sheltered from winds.  It is increased by suckers.  Flowers in June.  Height, 4 ft.

Phillyrea.—­This effective border evergreen will grow in any ordinary garden soil, and may be increased either by layers or cuttings.  It has dark green shining leaves, and is quite hardy.  Height, 6 ft.