The Hosta... An Introduction
Hostas are hardy clump-forming herbaceous perennials springing from short, sometimes stoloniferous rhizomes with fleshy white roots. The leaves are mostly basal, stalked, large and simple, forming a mound. The flowers are large, tubular, funnel- or bell-shaped with usually six spreading lobes, white to dark purple, the stamens bent, resting on the tube, presented in a raceme at the top of a usually unbranched scape.
Most hostas develop into dome-shaped mounds as typified by H. sieboldiana 'Elegans'. Some hostas, however, make low, rather flattened mounds, as with H. 'Resonance', while others are decidedly upright in habit, the petioles being clearly visible as one looks at the hosta, as with H. nigrescens or H. 'Krossa Regal'.
Stoloniferous hostas (those with creeping roots) generally form colonies of interlocking individuals. The colony as a whole may appear flat but the individuals within it will still be dome-shaped, flattened or erect.
Clump and leaf size
Hostas vary in size from minuscule to majestic.
- Clump size
- Dwarf: Less than 10cm ( 4 in )
- Miniature: 13 - 23cm ( 5 - 9 in )
- Small: 25 - 38cm ( 10 - 15 in )
- Medium: 40 - 60cm ( 16 - 24 in )
- Large: 64 - 91cm ( 25 - 36 in )
- Very large: Over 91cm ( 36 in )
- Leaf size
- Tiny: Less than 2.5cm ( 1 in )
- Miniature: 4 - 6.5cm ( 1 1/2 - 2 1/2 in )
- Small: 7.5 - 15cm ( 3 - 6 in )
- Medium: 16.5 - 25cm ( 6 1/2 - 10 in )
- Large: 27 - 35cm ( 10 1/2 - 14 in)
- Very large: Over 36cm ( 14 in )
Hosta leaves can vary as to their overall shape, the shape of the leaf base and the shape of the tip. The overall leaf shape is defined in terms of its length to breadth ratio. The ratio of an oval leaf will be 2:1 or 3:2. A broadly oval leaf will have a ratio of 6:5, but if it gets any wider the ratio becomes 1:1 and the leaf is then defined as round. At the other extreme the leaf may be narrowly oval, with a ratio of 3:1, but if it gets any narrower it becomes elliptical or lance-shaped (6: 1).
Four different types of leaf base are found in hostas: heart-shaped (with two equal, rounded lobes on each side of the stalk or petiole at the point where it enters the leaf blade); truncate (as if cut straight across the base); wedge-shaped (with the sides straight but converging); or attenuate (the sides curved and converging). These categories are not hard and fast but drift gradually from one into the next, so that in practice one finds bases of hosta leaves that are neither heart shaped nor truncate but somewhere in between.
The tip may be either mucronate (coming abruptly to a sharp point), cuspidate (tapering gradually to a sharp point), acute (coming to a point) or obtuse (rounded).
Juvenile and adult leaves
Many hostas have juvenile and adult phases, the leaves produced in the former phase normally being narrower than those in the latter. This is particularly noticeable in tissue-cultured plants. In many hostas the spring leaves also differ from the summer leaves, the more recently produced leaves at the centre of the clump. A few hostas put on a distinct late flush of leaves, those of H. 'August Moon', for example, being longer and smooth, whereas the main leaves are rough and puckered. The second leaf flush of H. 'Undulata' produces leaves which are mottled green rather than crisply variegated.
Leaf substance and margins
The substance, or thickness, of hosta leaves can vary considerably and this is a major factor in determining the extent to which leaves may be eaten by slugs and snails. The thickness or thinness of leaves is mentioned only where they depart substantially from the norm.
The margins of hosta leaves are always entire, which is to say they are never toothed, cut, serrated or lobed. They may, however, vary from flat, as in H. 'Devon Green', to slightly rippled as in H. 'Daybreak' or acutely crimped, goffered or piecrust as in H. pycnophylla.
Leaf plane, surface, and veining
The blade may be wavy or twisted, variations which tend to accompany rippling of the margin. It may also be cupped, as in H. 'Tokudama', convex as in H. 'Brim Cup', or slightly arched as in H. 'Silver Lance'.
Hosta leaves may be smooth or puckered (dimpled) and the puckering may be slight or deep, often occurring on the leaves of mature plants but not on the leaves of young plants.
Hosta leaves exhibit a typical veining pattern called campylodrome, which means that the veins entering the leaf at its base curve outwards as the leaf widens and then curve inwards as the leaf narrows towards the tip. The veins appear impressed when seen from above, but prominent when viewed from beneath. Where the veins are deeply impressed they give a furrowed or corrugated appearance, as in H. 'Green Acres'.
Hosta leaves are always smooth -that is, without hairs -but may be conspicuous in other ways. They, may be matt, or shining as in H. yingeri, but are most often satiny. They may also be covered in a waxy coating and this is invariably the case with blue hostas, their blueness being due to this coating. They may also be frosted or pruinose, having the appearance of being covered in frost or chalk. These effects are usually most pronounced on young leaves, fading as the season advances, or if grown in too much sun.
The reason that the leaves of plants are colored is that they contain pigments carried in bodies called plastids. The dominant plastids throughout the plant kingdom are green ones called chloroplasts, and it is these structures which are mainly responsible for taking the energy from sunlight and converting it into the chemical energy which drives the plants' metabolism. Other colors in plant leaves are caused by other pigments, carried in other plastids. Significantly for hostas, all of these plastids are very, very similar, it taking a change in only a very few atoms to slip from one colour to another, and such changes can and do take place, if only occasionally, when cells copy themselves.
The plastids are not part of the cell nucleus but are contained in the area surrounding it. This means that they have no part in determining the shape of the organism, only its colour. They are, moreover, independent of the nucleus insofar that they have their own DNA, the essential programming code that tells them how to replicate themselves. However, the replication of cells is not like a modern factory line, at the end of which every item is identical. Errors creep in, and it is a simple fact of life throughout the natural world that the more copies that are made the greater the likelihood of error: and the probability of error also increases with the passage of time. Thus the older a plant gets, the more likely it is that such an error will occur. Whether such an error results in a significant new color combination depends on where it occurs in the developing plant or shoot.
The growing tip of a plant is always essentially the same, whether it is an embryo or a new shoot, but different parts of the plant develop from different parts of the growing tip. Three different layers known as L 1, L 2 and L 3 are involved here. The outer layer is the part from which the epidermis or surface of the leaf arises, and this plays a particular part in determining the leaf margin. If colored plastids occur in L 1 this can result in a colored margin. It is from the middle layer, L 2, that the greater part of the leaf arises, and if colored plastids are in this layer the result may be a colored centre to the leaf. However, plastids can also move from layer to layer, plastids originating in L 2, for example, moving to L 1, which is what happens when the streaking in a streaked hosta migrates to the margin. Plastids can also lose their color, for just as the laws of chance allow plastids to change color, so there are mechanisms to correct the error, and when this occurs the plant or leaf is said to revert. All variegation and coloring other than green is essentially unstable, though marginal variegations are usually more stable than central variegations.
The flowers of hostas are borne on elongated stems or scapes which arise directly from the leaf mound at ground level and terminate in a more or less crowded raceme. The scapes are usually round. They are usually solid, but occasionally hollow, and usually simple, but occasionally branched, as with H. tibae. The scape may be upright or, leaning, and straight or bent. The flowers are held away from the scape on short pedicels which hold the flowers in a drooping or horizontal attitude. Each pedicel emerges from a small bract (known as a flower bract) which clasps the stem. These may be so tiny they can scarcely be seen with the naked eye, as in H. sieboldii, or large enough to be quite noticeable, as in H. 'Summer Fragrance'. Often the flower bracts wither after blooming. In a few species and cultivars these flower bracts become so large that they look like leaves, sometimes wrapped around the flower bud as in H. kikutii.
Scapes may however bear a second type of bract called a leafy or foliaceous bract, which is not always easily distinguished from a flower bract. Leafy bracts occur below the flowering part of the scape, and are usually largest nearest the ground. They are mostly outward-facing but some are small and clasp the scape, as in H. 'Opipara'. In several species and hybrids these bracts are conspicuous, as in H. 'Undulata'. If the leaves are variegated, they are usually variegated also.
The flowers of hostas are usually bell-shaped or funnel-shaped with six more or less spreading lobes and six stamens situated below the base of the ovary or attached to the tube, usually longer than the perianth and therefore protruding. The ovary is stalkless, three-celled, and the stigma usually protrudes beyond the stamens.
From a descriptive point of view the part of the flower that matters most is the middle section, between the tube and the lobes, for it is this that determines whether the flower is funnel-shaped or bell-shaped. If it is funnel-shaped the tube expands gradually so that in outline the tube and the lobes form a single continuous curve. If the flowers are bell-shaped it expands abruptly just past the tube. The flowers of some hostas are double, and in a few cases there are flowers which never open, as in H. clausa. H. laevigata and H. yingeri have flowers with very narrow lobes, described as spider-shaped.
The anthers are important diagnostically. In true species they are either yellow or purple: in hybrids they are usually bi-colored. The color can only be identified before the pollen is shed.
The flowers range from quite a deep purple to what appears to be pure white, though in fact even in H. plantaginea the lobes still retain a hint of violet. The majority of hostas have flowers that are more or less lavender.
Originally it was thought that only H. plantaginea was fragrant, and indeed all the fragrant hybrids that have been raised in the West owe their fragrance to H. plantaginea. However, it has recently been discovered that the Japanese grow a small number of fragrant cultivars which possibly owe nothing to H. plantaginea.
After flowering, the ovary will swell into an elongated, many-chambered capsule. The color of the capsules generally follows that of the leaves, being green with green-leaved varieties and glaucous with glaucous varieties. Occasionally they are purple as in H. 'Grand Master', and in variegated varieties they may be variegated.
The capsules open about six weeks after flowering. The seeds are ovoid and nearly flat. If they are fertile they will be black, if sterile they will be of a pale color or nearly white. The production of capsules does not mean that they contain fertile seeds. Many cultivars do not form capsules at all, the scapes withering after flowering.