The Astilbe are hardy herbaceous and shrubby perennials.
Several species and varieties commonly known as spiraeas are now classed under this heading, including the hybrids with Spirae astilboides, which are very useful for forcing in the cool greenhouse.
In terms of cultivation the astilbes have a universal liking for a deep, moist, rich soil. Astilbe Davidi and Astilbe rivularis in particular are essentially moisture-loving plants. Planting can be carried out in either autumn or spring. Care should be taken to avoid overcrowding the plants, as with most species the foliage is little less decorative than the flowers, and the natural beauty of the plants is only appreciated when they are given ample room in which to develop. For greenhouse culture the roots of the hybrids should be potted as early as possible in the autumn and at once plunged in a bed of ashes out of doors to encourage strong root growth. Within a month or six weeks they can be taken in¬doors and gradually inured to the effects of light, air, and heat. During the growing period an abundance of water is required, and when in flower it is advisable to stand the pots in earthenware saucers to ensure that they do not suffer from lack of moisture at the roots.
Propagation is most easily effected by division of the root stocks in spring or autumn. Large clumps can, if necessary, be divided up into single crowns.
The Aubrietia are hardy trailing rock plants.
The aubrietias are among the most useful rock plants for rapidly covering banks, dry walls and extensive slopes in the rock garden. For this purpose the hybrids are particularly recommended, as they are very hardy, grow rapidly, and are obtainable in a wide range of excellent colours such as purple, crimson, lavender, pink, etc.
For cultivation the aubrietias require a sunny position and a well-drained soil containing plenty of lime. If this is deficient it may be added when preparing the ground in the form of air-slaked lime or chalk or as old mortar rubble. If the ground is naturally of a heavy, wet nature it should be lightened with sand, limestone chippings, etc. Aubrietias may be planted in October, March, or early April if lifted from the open ground. Pot-grown plants can be established at almost any season. Old plants that are getting straggly should be trimmed up with scissors or shears immediately after flowering.
Aubrietias may be increased by cuttings of young growth a couple of inches in length inserted in very sandy soil in a cold frame or shady border outdoors in June and July. Selected shoots must have bright, glossy foliage because dull-leaved shoots are of little value. More mature cuttings will often root outdoors in August or early Sep¬tember. Seed sown in a frame in March or in a shady border in April germinates readily and is a very easy means of producing mixed plants. You need to be very aware that named varieties do not breed true in this way.
