The Aster are hardy herbaceous perennials and half-hardy annuals.
The perennial asters may conveniently be considered under four distinct headings.
Dwarf early-flowering kinds derived from such species as Aster Amellus and Aster Thomsoni.
The taller, later-flowering varieties with comparatively large blooms, which have been raised from the North American species Aster Novi Belgii and Aster Novse Anglias, and together with the next group are popularly known as Michaelmas daisies.
The late-flowering kinds with narrow foliage and ele¬gant habit and large panicles of starry flowers have their origin in such species as Aster ericoides, Aster cordifolius, diffusus, and Trade-scanti.
There are also miscellaneous low growing kinds most suited to the rock garden or the extreme front of the herbaceous border.
The annual asters are derived from a Chinese plant botanically known as Callis-tephus hortensis. Numerous distinct types have been evolved by hybridisation and selection, but all have similar cultural re¬quirements.
All the perennial asters are extremely hardy and will grow in any ordinary garden soil, though in the case of the choicest alpine species it is advisable to add a liberal quantity of sand and a little leaf-mould or peat moss litter if the natural soil is of a heavy, wet, or sticky nature. For the other kinds the only preparation necessary is good deep digging and the incorporation of a little well-rotted manure into the lower spit. The taller kinds, such as hybrids of Aster Novi Belgii, cordifolius, etc., should be staked early with long bamboo canes. Even some of the dwarf hybrids of Aster Amellus and Aster Thomsoni will pay for some support, which may be provided in the form of short, brushy sticks. Clumps increase in size very rapidly and should be divided every two or three years. If very fine blooms are desired, it is advisable to thin the growths in spring, leaving only three or four to each plant according to its strength. The most suitable planting seasons for most kinds are October, March, and early April, but varieties of Aster Amellus should only be planted in spring and never in autumn.
Annual asters will not thrive on a starvation diet. The soil for them must be deeply dug and well enriched during the winter months. They are best suited by an open sunny situa¬tion, but partial shade is not an insurmount¬able handicap. Great care must be exercised in planting to avoid burying the plants deeply. This is a frequent cause of collar rot. During the summer the plants must not be allowed to suffer from drought. Copious waterings should be given as soon as re¬quired. After the flower buds begin to swell, feeding with weak solutions of natural manure will also prove beneficial. Exhibitors must also practise rigorous thinning and dis-budding to produce blooms of the highest quality. Wilt disease is sometimes very troublesome, but this difficulty may be avoided by planting one of the wilt-resistant strains.
The perennial asters are all most easily increased by division at planting time, though a more satisfactory method is by cuttings of firm young growth 3 or 4 inches in length, which will root readily in sandy soil in a cold frame in spring. Seed sown in a cold frame in March also offers a possible means of increasing the species, but named varieties will not breed true in this way.
Sowings of annual asters can be made under glass in pans or boxes of a light compost in March. A gentle heat of 55 degrees to 65 degrees is a great aid to rapid germination, but on no account must the young plants be subjected to sudden fluctuations after they are seen appearing through the soil. As soon as they have formed their first rough leaves prick out the plants 3 inches apart each way in boxes or in a made-up bed in a warm frame. For the first few days the plants should be kept fairly close, but after they recover from the check a start must be made with the gradual process of hardening off in prepara¬tion for planting about the end of May. In most parts of the country splendid results can also be obtained by treating the plants as hardy an¬nuals and sowing where they are to bloom in mid-April, thinning out the seedlings at a later stage to 1 foot apart each way. This method is practised with great success by many commercial growers.