Cultivation Of Hostas

The art of growing good hostas is to study their needs and then supply them. In general terms hostas need fertile soil, heavier rather than lighter and just a little on the acid side of neutral, plus some shade and shelter from wind. To get the very best from a wide range of hostas, however, it is necessary to look more closely at individual requirements as hostas come from a surprisingly wide variety of habitats and many are more demanding than is generally recognized.

Soil

Most hostas grow best in rich, friable loam with a pH of about 6. Hostas will grow perfectly well in alkaline soils, but seldom look happy on shallow, chalky soils, which tend to cause the leaves to exhibit chlorotic patches, while the glaucous blues take on a muddy tinge. The soil for hostas should be moist but well-drained, and this can usually be best achieved by mixing into the soil at planting time an abundance of organic matter, and then adding more organic matter in the form of a mulch regularly over the years.

Large hostas, such as H. 'Great Expectations', will grow well on heavy clay soils, which are rich in plant nutrients, but can be slower to establish themselves and so take longer to reach maturity. They will become established much more quickly if copious and equal quantities of coarse grit and garden compost or well-rotted farmyard manure are worked into the ground, not only where the hosta is to be planted, but for some distance around it, to allow the roots at maturity to spread out beyond the hosta's own leaf mound. The soil can then be maintained in good heart by mulching regularly with more grit and organic matter.

Sandy soils present quite different problems, though the solution is curiously similar. Hostas generally develop good root systems on sandy soils, but they only do this because they have to quest far and wide for nourishment and moisture. Sandy soils are poor soils, and hostas generally look thin and miserable on them unless steps are taken to remedy this. Because sandy soils are free-draining any nutrients that are applied to them are quickly flushed out of the soil by rain and by watering. Hostas will grow very much better in these conditions if plenty of organic matter in the form of farmyard manure or good garden compost is added to the soil at planting time, and the plants are regularly mulched in spring and autumn with the same materials. It may also be necessary to add extra nutrients, and to apply foliar feeds during the spring and early summer months.

Such treatment is not recommended for the smallest hostas, such as H. venusta, H. 'Shining Tot' or H. 'Suzuki Thumbnail'. Mulches are soon scattered by birds and these very small hostas can be completely suffocated by having coarse mulching materials flicked on top of them. Nor are they generally robust enough to compete with other hostas or the normal flora of perennial or woodland borders. The walls of the raised beds can be made from peat blocks, large decaying tree trunks, railroad ties or bricks. Since such beds need to be sited in shade, these materials will soon develop a covering of moss into which some of the small hostas will run or seed themselves. On the surface of the beds the moss may grow with sufficient vigor to suppress the hostas, but it can easily be deterred by applying a weak solution of ferrous oxide.

Planting

The single factor that most affects whether a hosta (or any other garden plant) will flourish or languish is how well its planting site is prepared. Obviously the size to which the hosta will grow determines the size of the planting hole, but for large hostas such as H. 'Sum and Substance', H. fluctuans 'Sagae', H. montana and its forms and H. sieboldiana and its forms, the planting hole should be as large as for a shrub - a hole should be dug 91cm (3ft) across and 45cm (18in) deep, and the earth at the bottom of the hole broken up. The bottom half of the hole should be filled with alternating 7.5cm (3in) layers of well-rotted farmyard manure or garden compost and soil, with some grit added to the soil if it is heavy, while the top half should be filled with a friable mixture of the same materials, each layer and the final filling being firmly trodden down with the heel. Ideally the planting holes should be prepared a month or more before planting as freshly worked soil tends to drop somewhat in level. Smaller hostas need proportionately smaller planting holes, but they should never really be less than 45cm (18in) across and 23cm (9in) deep. For dwarf hostas the provision of a suitable growing medium is more important than the preparation of the planting hole.

Hostas may be planted either in the spring just as the new shoots are emerging, which enables them to establish themselves quickly as the soil warms up, or in late summer or early autumn while the soil is still warm from the summer's heat, which again enables them to establish themselves quickly. If necessary, they can even be moved in high summer in full leaf, provided the leaves are then cut down and the plants adequately and persistently watered until they are re-established; they will produce a new flush of leaves which the root system will be able to support. They should never be planted in the dead of winter, when the roots are inactive, since excessive rain or frost is likely to cause any damaged roots to rot. One great advantage of planting hostas while they are visible above the ground is that it enables the gardener to get their placing and spacing right.

When planting hostas that have been grown in pots it will usually be found that the roots have become enmeshed at the bottom of the pot. Such roots should be teased out before planting: it is not a tragedy if in the process much of the compost in which the plant was growing is lost. When planting open-ground hostas it is prudent -to remove the old soil so that the roots can make a new start in fresh soil. Any dead or damaged roots should be removed with a sharp knife or secateurs (pruning shears). Once the hosta has been placed in its hole the soil should be replaced in layers, and each layer firmed before the next is added. Once planted the hosta should be well watered in, and particular attention paid to its watering over the following months, especially if planting in the spring. After an initial soaking, a hose very slowly trickling water into the ground beside the hosta for several days is more effective than the occasional swamping with water. After that it is necessary just to keep the ground moist. Large hostas need at least 4.5 liters of water a day until properly established. A thick layer of organic mulching material should then be placed in a doughnut-like ring around the plant.

Mulching and feeding

Hostas are generally described as gross feeders, though this is true only of the larger sorts. For any smaller than H. sieboldii the provision of the right growing medium and adequate watering is more important. Most other hostas will respond well to liberal applications of manures and fertilizers.

In this green age organic feeds such as farmyard manure, garden compost or leaf-mould are prized above artificial fertilizers. In reality the nutritional content of these organic materials is often disappointingly low and unbalanced, and varies from batch to batch. Moreover, such organic materials are usually full of weed seeds. What organic materials do contribute to the growth of hostas is humus, which helps to keep soil moisture levels even and provides a good carrier for other nutrients. With artificial fertilizers, and indeed with most packaged, powdered or pelleted plant foods, the nutritional content is known and the gardener can choose whether to apply a high nitrogen or high potash feed.

Hostas make enormous demands on the available soil nitrogen and will not achieve their sumptuous best unless the supply is adequate. This is especially the case with the larger-leaved ones. Most organic mulches, while they may in the longer term provide some nitrogen, cause short-term nitrogen deficiencies. What happens is that vital nitrogen-converting organisms are attracted from the soil into the mulch, where they play a vital role in breaking it down. For this reason high-nitrogen fertilizers should be used whenever a mulch is applied, and this is especially important in spring. Nitrogen is most vital in spring when hostas are making optimum growth, and a quick-acting high-nitrogen fertilizer such as dried blood or pelleted chicken manure may be necessary. However, excessive and unbalanced applications of high-nitrogen fertilizers will tend to encourage hostas to produce lush, soft, sappy foliage which is not only easily damaged by wind and weather but utterly delectable to slugs and snails. To prevent this from happening the nitrogen should always be balanced by potassium (potash), which helps to produce firm, crisp leaves. Since hostas also need phosphorus for the development of good roots, it is usually best to use a balanced feed. The various elements in fertilizers are expressed as N for nitrogen, P for potash and K for phosphorus. In high-nitrogen fertilizers there may be twice as much N as P and K, while in balanced fertilizers N, P and K are present in equal amounts. Most fertilizers also contain small quantities of trace elements that are essential to plant growth.

Fertilizers can be applied as powder or pellets in a scattered ring round the hosta, as a liquid feed through a can to the root area, or as a foliar feed, applied via a can or through a dilutor attached to a hose. Foliar feeds are particularly appropriate for hostas as they enjoy the extra water this gives them, and it also helps to make the leaves dark and glossy.

Hostas should be fed from early spring until mid-summer. Any feed applied after that, even if it is a balanced feed, will tend to promote soft, sappy growth vulnerable not only to slugs and snails but to disease.

Watering

The leaves of growing plants remain turgid as a result of the pressure of the water flowing through them. This is caused by the surface of the leaves giving off water vapor, creating a partial vacuum that sucks more water into the plant through the roots in an attempt to replace the water that has been lost. It is this process, which is continuous in daylight, that keeps the leaves plumped up and turgid. Plainly, any plant with leaves as large as those of H. 'Snowden' or H. 'Sum and Substance' must give off a lot of water vapor, and must, therefore, need plenty of water to sustain the process. Their leaf size is totally dependent upon their receiving adequate moisture which also accentuates the seer suckering effect. If there is insufficient water available to them, in a drought for example, or when they have been picked, they lose their turgidity, become limp and wither. It is for this reason that the watering of hostas is so essential to their success.

It seems only natural to think that the best way to water plants is by whatever means most nearly resembles rain, but this may not in fact be the best way to water hostas. The main problem is that the droplets falling repeatedly on the leaves of blue-leaved hostas can damage the glaucous bloom for which they are treasured. The use of fine mist sprays avoids this problem. A further problem is that if water is applied overhead and the leaves are then exposed to sunlight, the droplets may focus the sun's rays on to the leaf surface and burn it, causing disfiguring brown and yellow marks. The most effective way of watering hostas is to apply the water directly to the area where the roots are either by can or hose, or, in the case of an extensive planting, by seep hose. In the watering of hostas, the aim is to maintain the soil around them at an evenly moist level and to avoid alternating extremes of wetness and dryness.

Watering should always be done as early as possible in the morning, before the sun's rays get hot. The hostas then have all day to make use of the water, which can only be absorbed in daylight. The problem with watering in the evening is that the moisture tends to attract slugs and snails, which enjoy wet environments. The same principles apply to hostas being grown in pots and tubs, which are more vulnerable to drying out than hostas growing in the ground.

Shade

Hostas are often thought of as shade-loving plants, and while this is true of the great majority it is by no means true of all. Given adequate moisture, hostas such as H. 'On Stage' with much yellow and little green in their leaves are a far better color in some direct sunlight. It is also generally the case that while the leaves may look their best in shade, hostas flower better in sunlight.

In the wild, hostas grow in a variety of conditions that afford them shade. Some grow in open cryptomeria forest, others, such as H. longissima, in high-altitude water meadows where shade is cast by taller growing grasses (frequently miscanthus), while others grow on mountains where they are almost always shrouded in cloud. A few grow on rocks or in crevices beside mountain streams where water is always available to them and so the need for shade is compensated for by the availability of the water. However, these conditions are by no means easy to replicate in Western gardens, which is why hostas are usually grown in the shade.

Shade is more than merely the absence of direct sun. It is a modification of the microclimate, bringing cooler, moister air, often retaining more moisture in the soil if there are not too many competing tree roots.

Hostas grow best in light, dappled shade, such as that cast by a high tree canopy. The problem with trees is that when they are young they cast too little shade, and when they are older they cast too much. It is easier to create just the right amount of shade artificially, by building a pergola for example.

Most of us have to make do with whatever shade exists in our garden, but where there is an opportunity to create it a greater variety of shade density can be offered by using three trees rather than one. If the trees are planted in an equilateral triangle whose hypoteneuse is a line running east-west and whose tip lies north of the line (in the northern hemisphere), the most dense shade will occur within the planting triangle itself. The area to the south of the hypoteneuse will be entirely in the sun, but the areas to the north-east and north-west of the triangle will be in light shade for about half the day.

While trees may seem to provide an elegant and seemingly natural setting for hostas, it should be borne in mind that they compete for the same moisture and nutrients, making the need for adequate feeding and watering all the more essential. Some trees compete with hostas more directly than others. Birches and cherries, for example, are surface-rooting, and such trees invade the space where hosta roots should be, depriving them of goodness. Beeches are also surface-rooting, but oaks are deep-rooted, as are most maples. Trees such as catalpas which come into leaf very late allow hostas beneath to put on growth before the protective shade of their leaves arrives.

Shelter

When hostas are well-sited they have a sumptuous, well-fed look. Sometimes, though, hostas seem decidedly miserable, their leaves not as large as they should be, not as turgid, even a little limp, and very often this is due not so much to a lack of feeding or watering as to a lack of shelter, when the leaves are exposed to too much wind. Wind not only damages the leaves of hostas by its sheer physical force, knocking and bruising them, but also desiccates them, in extreme cases causing scorching. Wind blowing across leaves dries them just as it dries laundry on the line, and this puts pressure on the plants' roots to supply more water to keep the leaf turgid. In some conditions the roots may not be able to do this and loss of turgidity results, causing the leaves to look tired and withered.

Hostas should never be grown in sites exposed to wind unless steps are taken to reduce its force. In the long term this may mean growing hedges or belts of trees or shrubs, but in the short term fences can be used to reduce or deflect the force of the winds. Modern flexible plastic wind-breaking materials can also be used as a temporary measure. The great advantage of these modern materials, as with hedges and some kinds of fencing, is that being semi-permeable they reduce the force of the wind rather than causing it to go up and over, which often creates turbulence that can be as damaging as the winds it was there to divert.