Plants which continue to live for a number of years irrespective of whether they flower or not, have a comparatively soft, as distinct from woody, growth, and are hardy enough to be grown outdoors all the year round.
Soil and Site Preparation
There are hardy herbaceous perennials suitable for every imaginable soil and situation from light sands to heavy clays, and from full sun to dense shade. Many are extremely adaptable. Ground should be prepared by thorough digging and all weeds of a perennial nature should be removed, as little further cultivation will be possible for some years.
For the same reason, if soil is believed to be poor, organic manure, compost, or slow acting fertilizers, such as bonemeal, basic slag, or hoof and horn meal should be worked in prior to planting.
Planting Guidelines
Planting hardy herbaceous perennials. All can be planted in the spring, from early March to the end of April. Many kinds can also be planted in late September and October, especially if the soil is warm and well drained. It is not advisable to transplant Scabiosa caucasica, Aster amellus, pyrethrums, or Chrysanthemum leucanthemum in the autumn. Bearded (German) irises can, in addition, be transplanted immediately after flowering, at the end of June. Planting should be done with a trowel or spade, not a dibber.
Holes must be wide enough to allow roots to be spread out naturally; deep enough to permit crowns to be just covered with soil. The rhizomes of flag irises should be barely covered ; later they will work out on top, their natural position. The soil mark on the plant, or the difference in color (green above ground, whitish below), usually gives a reliable guide to planting depth. Distance will vary according to habit and height, ranging from 6 in. for low-growing plants to 3 ft. for the tallest varieties.
Plant very firmly. Water in freely if the soil is dry, and continue to water until growth recommences.
Propagation Methods
Almost all hardy herbaceous perennials can be increased by division, i.e. by pulling the roots and crowns to pieces, either by hand or with the aid of pointed sticks or small forks thrust back to back through the plant and levered apart. A knife may be required to cut through hard pieces, but should be used as little as possible.
When dividing old plants the younger, outside portions are to be preferred; old woody centers should be discarded unless stock is short. Each piece of growth must have roots attached. It is no use planting shoots without roots or vice versa. Division can be done at the usual planting season, but in general the best time is spring, when growth is just beginning.
Some herbaceous perennials can be increased by cuttings, e.g. anthemis, delphiniums, lupins, Scabiosa caucasica, and Coreopsis grandiflora. These are prepared from firm young shoots 3-5 in in length taken in spring as growth begins. They should be severed as low down as possible. Trim just below a joint, remove lower leaves and insert firmly in sandy soil in a frame or under a hand light.
Water moderately and shade from strong sunshine until growth recommences, when discontinue shading and gradually increase ventilation. Rooted cuttings may be planted out in June or July and should be watered freely until established.
A few kinds, e.g. anchusas, gaillardias, oriental poppies, phloxes, romneyas, and verbascums can be increased by root cuttings. These are taken at any time during the winter. The thicker roots are cut into pieces 1-2 in. in length, strewn thinly on ordinary potting compost in well-drained pots or boxes, and covered with in. of the same material.
An alternative method is to dibble them in 1 in. apart right way up with the tops just beneath soil level. They are placed in an unheated greenhouse or frame and watered moderately. Shoots will be formed in the spring and the plants can be planted out in June or July.
Most herbaceous perennials can be increased by seed, but seedlings of highly developed garden races (e.g. delphiniums, phloxes, lupins) are liable to vary considerably from their parents. Seed may be sown in a greenhouse or frame in March or outdoors in May or early June. Seed beds should be as fine as possible with extra peat and sand if needed. If sown in frames, prick off as soon as possible and harden off for planting out during July or August.
If sown outdoors, prick off into another similar bed, placing the seedlings 3 or 4 in. apart, in rows 6 or 8 in. apart, so that they can grow into sturdy plants for removal to their permanent flowering quarters the following spring. Germination of some perennials is extremely slow and such are best sown in pots, pans or boxes. These should not be discarded as failures for two or three years.
The double-flowered forms of Gypsophila paniculata are propagated by grafting young shoots in spring or early summer on to seedling roots of the single gypsophila. The shoot is cut to form a wedge, the root is slit vertically, the wedge is inserted in this, bound in position and the whole potted and placed in a propagating frame.
Most perennials are improved by division or renewal from cuttings every four or five years, some even more frequently, but this does not apply to paeonies or Christmas roses (helleborus), which resent root disturbance.
Seasonal Care and Maintenance
Better results are often obtained by thinning the shoots in the spring. This is particularly true with delphiniums and Michelmas daisies. Thinning should be done early and the best shoots only retained. The number will depend upon the age of the plant and the purpose for which flowers are required. Thus with young delphiniums needed for exhibition, only one shoot will be kept per plant; two year old plants might carry two or three shoots; three-year-old, five or six shoots.
Staking, Most tall herbaceous perennials require staking. So far as possible, allow one stake to each main growth ; thrust into the ground firmly near the base of the plant and allow the stakes to lean outwards at the top like a shuttlecock to open growth rather than crowd it together.
Plants of medium height that are liable to flop about, e.g. gaillardias, erigerons, etc., are most readily supported by thrusting bushy twigs into the ground round them while still young; they will grow up through the twigs and find. their own support.
Cultural Routine. Most herbaceous perennials will benefit from annual feeding. This may be done by spreading animal manure or compost around them in April or May, and by sprinkling a good compound fertilizer around them occasionally during the spring or summer and hoeing in. Watering of established plants is usually unnecessary unless the weather is exceptionally dry, or special results, e.g. exhibition flowers, are required.
When flowers fade they should be cut off, but leaves should not be removed unnecessarily until the autumn, when growth dies down. Then all shoots can be removed an inch or so above ground level, except from plants with evergreen leaves, e.g. kniphofias, bearded irises, etc., which must be allowed to retain all healthy foliage. Dead or diseased leaves may be removed at any time.
Winter Protection Tips
A few hardy perennials require some winter protection, especially if the soil is wet and heavy. The two most important are Gunnera manicata and all kinds of eremurus. The first can be protected by placing a low tunnel mesh wire netting over the crowns, after the dead leaves have been removed in November, piling dead leaves, straw or bracken on top and pegging more netting on top to keep in position. The leaves, etc., must not be heaped directly on the crowns, or they may cause decay.
Eremuri can be protected by heaping sharp sand or boiler ashes over the crowns in November. This can be left in position till the shoots grow through the following spring. Christmas roses (helleborus) may be protected in November and December with cloches or frames, but solely to protect the blooms from mud splashes, not because the plants lack hardiness.
