Polemonium (Jacob’s Ladder).—­Hardy perennial border plants of an ornamental character and of the easiest culture.  Any soil suits them, and they merely require sowing in the open either in spring or autumn.  P. Richardsoni is most commonly met with, its blue flowers being produced in early autumn.  Height, 1-1/2 ft.

Polyanthus.—­Sow the seed late in autumn in well-drained boxes of light, rich mould; cover it very lightly, place under glass, and water sparingly, but give enough to keep the plants moist.  The seed requires no artificial heat to germinate it.  The roots should be divided each year as soon as they have flowered, and fresh soil given.  The single varieties only are florists’ flowers.  The Polyanthus is a species of primrose, grows best in a rather shady position in a loam and peat compost, and produces its flowers in May.  Height, 6 in.

Polygala Chamaesbuxus.—­A hardy evergreen trailing plant requiring a peat soil in which to grow.  It may be increased from seed or by division of the roots.  May is the time at which it blooms.  Height, 6 in.

Polygala Dalmaisiana.—­This showy evergreen shrub needs a greenhouse treatment.  Soil—­three parts peat, one part turfy loam, and a little sand.  It flowers in March.  To increase it, top the shoots, which will cause it to throw out new ones.  Take the new growth off when it is 3 in. long, and place it under glass in a propagating house.  Height, 1 ft.

The hardy annual varieties of Polygala are obtained by seed sown in peat.  These flower at midsummer. (See also “Solomon’s Seal.”)

Polygonatum.—­These pretty herbaceous plants are quite hardy.  The flowers, which are borne in May or June, are mostly white.  Plants succeed best in a rich soil.  They may be raised from seed, or the roots can be divided.  Height, 1ft. to 3 ft.

Polygonum Brunonis (Knotweed).—­This strong-growing creeping perennial plant is not particular as to soil so long as it can enjoy plenty of sunshine.  The shoots root of themselves and must be kept in check, else they will choke other things.  It flowers in August, after which the leaves assume beautiful autumnal tints.  Height, 1 ft.

Pomegranate.—­This requires a deep, loamy soil and a warm, airy situation.  May be propagated by cuttings of the shrubs or the root, putting the cuttings into light, rich soil, or by layers.  The double kinds of Punica, or Pomegranate, should be grafted on to the single ones.  There is a dwarf kind, bearing scarlet flowers in August, which requires heat.

Poppies.—­See “Papaver” and “Stylophorum.”

Portulaca.—­The seeds of the hardy annual species of this genus may be sown in a sheltered open spot in spring.  The half-hardy annuals should be sown thinly in boxes during March and placed in gentle heat.  Harden off and plant out in May, as soon as the weather permits, in a light, dry soil where it can get a good amount of sunshine.  Its brilliant and striking colour admirably adapts it for small beds, edgings, or rock-work; and it will succeed in dry, hot sandy positions where scarcely any other plant would live.  It flowers in June.  Height, 6 in.