Planting the Plantsman’s Garden: 3 — Herbaceous Perennials

[T]here is no satisfying substitute for hardy perennials. Shrubs, annual flowers and bedding plants and the limited range of popular perennials which have been dominant in recent years, cannot match the returns of a well-balanced collection of hardy plants with their entrancing diversity in form and color and the vastly longer period of the year over wich they give pleasure.

Alan Bloom, Hardy Plants of Distinction, 1965

In the preceding article I mentioned a division between, on the one hand, those gardeners who are primarily plant collectors and are more interested in the plants themselves and their health than the total effect of their various associations, and, on the other hand, those who are more concerned with the beauty of a path, a view, and, ultimately, of a garden as a whole. I should like to think of these not as two irreconcilable stances but as two opposed moments, the systole and diastole, of gardening. It is true that when I stand in one of these positions I only dimly “see” the other. Either I am trying to improve a view and am busily planting or transplanting to that end, or I am trying to find the ideal conditions for a plant I can see in my mind’s eye as a magnificent specimen.

These two poles of the gardening activity ought, however, to be self-correcting. Too much concern with planning — color schemes, foliage similarities or contrasts, floral combinations and the plants become no more than material for the garden, like ornaments or paths. But too many specimens and the result is bewildering; views disappear and, worst of all, the plants cease to provide the necessary setting for each other. Planting is always problematic (as time goes on it causes me more, not less, anxiety) and this is one of the contradictions whose solution (or transcendence) I think of as inspiration. I believe that what is finally most distinctive about a garden owes far more to hundreds of these inspired decisions than to the basic idea, however grand, that was its starting point.

Perennial Islands

Two modern English authors, Alan Bloom and Graham Stuart Thomas, have written extensively on perennials and their books have become standard works of reference. As we might expect, both have made valuable suggestions for using these plants in the garden.

Alan Bloom, who owns the world’s largest perennial nursery, has developed the idea of island plantings of herbaceous plants in lawns. This can be an attractive format and has become quite popular in Britain where sizeable lawns are common and easy to maintain. It allows for small, manageable groupings of perennials and for a three-dimensional composition that can be viewed from all sides. Where the idea has been put into practice, as in the extensive grounds at Mr Bloom’s nursery, the lawns are converted into wide, grassy paths that meander among flowering plants.

Since most perennials are considerably less than head-height, it is possible to look across a nearby island to others in the distance. This offers the prospect of going down directly into a garden — a kind of idealized meadow — of flowering plants, which, no longer ranked in borders, become intimately present to us. It is the ideal home for these plants, many of which are native to meadows, and it allows their individuality to be displayed to its fullest extent.

There is, however, a consequence of the idea that is not usually spelled out. Since several islands can be seen in one sweeping view, the serious gardener would want to make the most of this circumstance. He would want to introduce some thematic order into the entire complex of several islands and to intensify the effect of these continuously shifting, kaleidoscopic views. Seen in this light, the format could be still more demanding than that of the grand border, and we can only imagine the strictures Gertrude Jekyll would have placed on it had the idea occurred or appealed to her.

Here on the Pacific coast the comparative rarity of large lawns and the difficulties involved in their maintenance severely limit the application of Alan Bloom’s idea. Usually, our lawns are valued for their open space and those who can afford them are not apt to want them pitted in winter with beds of largely dormant perennials, although that problem could be partially solved by planting their edges with low evergreen plants. Still I think we should keep the island notion in our vocabulary of gardening. Elsewhere I have urged the use of islands of low, mounding shrubs for the hillsides of country gardens in place of lawns, and here I think it is worth contemplating the possibility of beds of perennials among the shrubs that are treated in this way. The versatility of new irrigation systems that allow the watering of separate plots at different times makes such plantings possible, and the combination of a basically evergreen effect with interspersed masses of summer-flowering plants is inviting.

Mixed Plantings: Gardening in Four Layers

We have so far not mentioned one obvious possibility for placing perennials in the garden — that they should be planted here and there to rise above groundcovers, mingle with shrubs, form specimen plantings against fences, and so on. “The place of perennials in the garden,” says Graham Stuart Thomas, “is everywhere, to act as complements to the greater or lesser things about them.” In place of the laborious perennial border, Thomas urges the return to an earlier style of gardening when plants of all kinds were grown together. It is a kind of gardening sometimes called gardening in four layers, with emphasis on blending the plants of the garden by heights. Thomas describes such a garden as one “where there are some trees giving retreats for shade-loving shrubs and plants, and casting their dappling of shade across the lawn; groups of larger shrubs on corners and elsewhere to give shape to the views and to create surprises; and a general mixture throughout of dwarf shrubs, perennials and groundcovers, with bulbs to provide added interest.”

This, too, is an inviting prospect and one can easily imagine cimicifugas growing behind daphnes or some aconites or campanulas arising among dwarf shrubs that have flowered in the spring. In Graham Stuart Thomas’ gardens (he has made three) photographs show the format carried to a kind of perfection. I, too, am fond of mixed borders and think they should comprise a large portion of any plantsman’s garden. However, I have two objections to using perennials only in this way. One is that whereas I can imagine certain plants as ideal components of such borders, others seem less well adapted and are better seen among other herbaceous plants. A far more important objection, however, is that, if at all possible, one does not want to forego those displays of certain combinations of perennials that Miss Jekyll called variously “brilliant” and “gorgeous.”

Gardening in four layers is a useful image, but it is also useful to think in terms of the creation of habitats. Part of a garden, even a small one, should be reminiscent of a woods, another part should be open hillside or a clearing, and, where we can manage it, there should be north slopes, south slopes, dry areas, and bogs. The function of mixed borders would be primarily to blend these areas together. Thus, there are many low groundcover plants and sub-alpines in the garden, but somewhere these coalesce into a true rock garden where the best high alpines can be grown. Similarly, there are numerous herbaceous perennials growing in places that suit them best and in mixed borders, but these in turn become massive displays here and there, like pieces of a meadow of pure flowers.

Three Perennial Bays

With all these possibilities in mind, I should like to take another brief look at the idea of bays of herbaceous plants among shrubs. In a less miscellaneous garden than our own, one where all the principal plantings were specified on a plan and the woody plants were mostly California natives, three bays were left for perennials. The object was to provide summer flowers in an otherwise spring-flowering garden that has a minimum of irrigation in an area of limited water supply. This garden is near the coast and these particular shrubs were planted on its westward side as a windbreak. Originally I had thought to plan a perennial border along the entire length of the plot with the windbreak as a backdrop, but the maintenance that would be required seemed excessive. The alternative of three bays solved not only this problem, but also allowed us to increase the evergreen effect of the entire planting in winter.

After considering an enormous range of possibilities, we finally decided to use very late flowering plants in the center plot and those that start flowering in early summer on the two sides. Since there were many unusual specimen plants in other parts of the garden, we thought we could give this plot over simply to decorative color schemes. In the late-flowering center are the deep blues of Salvia guaranitica and S. azurea subsp. pitcheri, and, in front of these, several plants of Chelone obliqua and a few brilliant red spires of Lobelia splendens (L. fulgens). In the foreground is Limonium perezii, a feathery, spreading artemisia, pink-flowered Sedum spectabile ‘Autumn Joy’, Penstemon ‘Hidcote Pink’, and some late-flowering, low astilbes.

The earlier flowering side bays are mostly of plants with pink, blue, and violet flowers, but both have a pure white centerpiece. In the far bay, there is a single plant of Crambe cordifolia, a cousin of the common cabbage which gives rise to a mass of tiny white flowers five feet high and as much across. This is surrounded by numerous plants of Salvia x superba, veronicas, campanulas, and pink phloxes. The nearer bay has three plants of Gypsophila paniculata ‘Pacifica’ to match the crambe. Noteworthy among the pink plants in this section is a California native, a plant of Achillea borealis collected by Wayne Roderick on Santa Cruz Island, that makes a near-perfect dome of rose-colored umbels and is a vast improvement over A. millefolium ‘Cerise Queen’. There are displays in the three bays from June to November.

Plant Form

So far we have been following the modern tradition of “color in the garden” for which perennials with their long flowering period are a principal source. There is also, however, a sizeable number of perennial plants that are more notable for their habits of growth and their formal properties than for the color of their flowers. For example, many umbelliferous plants — giant heracleums, ferulas, and Angelica archangelica — have dingy flowers but are well worth growing for their imposing stature, remarkable foliage, and stalwart flower panicles. These plants can be used to create bold effects.

A large number of flowering plants called “thistles” by laymen are frequently effective forms to be used by the creative gardener. The best known of these are the stately white hairy plants of Onopordum nervosum (O. arabicum), which is really a biennial, and the cardoons and artichokes, but there are many more plants of similar stature and interest among the genera Cnicus, Cirsium, Carlina, Carduus, Echinops, and Sonchus. Their leaf forms, flowers, and particularly the flower buds of several centaureas can make beautiful additions, as can the marvelously spiky forms of eryngiums, from the very tall and gracefully slender spires of some South Americans, such as Eryngium pandanifolium, to the silvery, spiky knobs of E. giganteum and the beautifully colored, sharply pointed ornaments of E. ‘Violetta’.

Probably many euphorbias belong here and Anchusa azurea, cephalarias, scabiosas, Catananche caerulea, and many bulbs such as Allium giganteum and Urginea maritima. Such a border or bay would be a good home for an occasional dahlia, otherwise a difficult plant to place in the garden, and for such stately plants of the lily family as grass trees and furcraeas. It is apparent that if we add the combinations that are worth trying on the basis of form alone to what we learn about flower color over the years, the possibilities for novel and satisfying plantings are indeed endless.

Tender Perennials

In general, perennials are known as hardy plants; if the roots live from year to year in a garden they are by definition hardy. However, the word “hardy” has come to be somewhat ambiguous as it is often used to denote those plants that are hardy on the average in a given area, very often the British Isles, the source of so much of our garden literature. Plants that are hardy only in the milder parts of such an area are referred to as half-hardy, while those that cannot be grown at all, except perhaps as annuals, become tender perennials.

For those gardens in the West with few frosts, any number of so-called tender perennials are hardy and can be used to enlarge the repertoire of effective herbaceous flowering plants. Most of these come from the Canary Islands, South Africa, and parts of Central and South America. A special study would be necessary to do justice to these plants, and a survey of any class of plants is not our purpose here. Nevertheless, we should point to the existence of this wide range of plants for our garden planning, many, if not most, of which have been cultivated but not used — in the sense of creating effective plant combinations — in our gardens.

Californians are familiar with Limonium perezii from Madeira, a plant similar to L. Iatifolium except that it is larger in all its parts, has an extended flowering season, and is evergreen. This is an excellent sample of this class of plants; there are also other beautiful limoniums from the Canaries, culminating in that giant of them all, L. arborescens. Sonchus is a genus that flowers with large yellow daisies. The species that occur in western Europe are weedy, but in the Canary Islands they have developed into beautiful, large-leaved plants with distinctive brown-toned golden flower heads in green clusters.

The Mexican salvias, of which an increasing number are now being grown in California, are most often huge perennials with outsized flowers (those of Salvia gesneriiflora are nearly three inches long, and the spikes of some species are nearly a foot high). The great blue-flowered salvia, Salvia guaranitica, is easy to use among other perennials; it is like a larger, deeper colored S. azurea subsp. pitcheri. Most of the others, however, have such lush foliage and are so overwhelming that I have yet to find a place for them that is in scale and shows off their qualities to advantage.

A newly discovered perennial from South Africa has proved to be a veritable gem. It is Diascia rigescens, which makes a more or less evergreen mat three feet across from which arise an abundance of nearly foot-high spikes of pink penstemon-like flowers throughout the summer and well into fall. Still more remarkable, perhaps, is the fact that the plant hides its spent flowers and seed heads and always looks its best with no attention at all. There are other, smaller diascias that are useful as path edgings and groundcovers. Among these are Diascia cordata and the larger flowered cultivar, ‘Ruby Field’, both very popular in England, and the relatively untried D. hanaganii and D. feltonii. All are lesser plants than D. rigescens, and apparently less tolerant of hot situations.

Also from South Africa is the orange flowered, sun-loving Leonotis leonurus and its coarser relative, L. nepetaefolia, as well as three remark­able shade lovers, Plectranthus behrii, P. saccatus and P. fruticosus which combine beautiful foliage with very large heads of salvia-like flowers in shades of pink or violet blue.

All of these tender perennials differ from most of their hardy cousins in being evergreen, and some of them flower in winter. They are best used among plants with similar qualities such as quasi-tender evergreen shrubs and in warm, frost-protected places in the garden.

Drought Resistance

Verbena bonariensis and Gaura lindheimeri are two tall, handsome perennials that flower from midsummer to late autumn, and will lighten up any usually dull, unwatered or seldom watered garden in the late, dry season. Among

the best very tough plants for earlier flowering are Achillea borealis in the rose colored Santa Cruz Island form, Corethrogyne californica (also a California coastal native) and many such Mediterraneans as Centranthus ruber, Catananche caerulea, Stachys olympica, lavenders, origanums and Jerusalem sages.

Planting Perennials

Reginald Farrer, in his introduction to The English Rock Garden, speaks of the “natural interwoven carpet” of a garden and goes on to ask a rhetorical question about alpine gardens that could, mutatis mutandi, be asked about plantings of larger plants. “What can be uglier,” he says, “and less harmonious than the large unbroken stretches in which you sometimes see alpines laid out, each species in a broad irregular space to itself, with each plant inserted at a neat distance from the next, quite regularly like bedded out stocks, and with the bare ground between picked clean of weeds, and raked as tidy as a tablecloth?” A bed of perennials should, I think, be even more informal than a rock garden, and the plants should be even less regularly spaced and less obviously cultivated. There should be as little bare ground as possible, and this without resorting to what are usually called groundcovers.

How this is done, it is usual to say, is a matter of individual taste and judgment; nevertheless, experience has suggested a few generalizations that may be of some interest. In his book on perennials, Graham Stuart Thomas has quite accurately given their ultimate height and spread. It is a case of deciding how many plants we want to use. Part of the answer is given by the plants themselves: some plants, especially those with an imposing formal structure, are best used singly as specimens; others, Phlox paniculata, for example, seem to demand to be used in fairly large numbers for good effect; still others seem to appear at their best in fair-sized clumps, and for these three or four fairly closely spaced plants will do.

For the rest, we must visualize several things at once: the specimen plants that will dominate the plot; the plants we shall use many of, and that will set the tone of the plot or dominate the mise en scene; the plants that may mean little by themselves but add interesting complexity; and, finally, the plants that tie this plot into its neighbors or to the garden as a whole. When we have made these decisions, the number of plants to use will more or less fall into place. In an illustration earlier in this series (Pacific Horticulture, Spring, 1984) one part of a planting is a stand of about two hundred martagon lilies. If we look at this and imagine perhaps only five, we should see an entirely different picture and one far from the intentions of the masterful gardener who created this scene.

Cultivation of Perennials

Although our topic is basically where and not how to plant, this occasion perhaps should be used to say something about the general cultivation of perennials and other large herbaceous plants. Unlike rock garden plants, most of the larger perennials come not only from the lowlands but from areas with a comparatively rich, loamy soil. They are good plants for what are often called “ordinary garden soils,” a term that covers a wide latitude of soil types and drainage conditions. What it means is that perennials do not need the exquisite drainage most alpines and other rock garden plants find necessary for their existence. We should not be misled by this, however. Most plants, other than those that make water or bogs their home, thrive on good drainage and the average perennial is no exception.

Where possible, therefore, it is a good idea to cultivate perennial plots deeply before planting. Sawdust or other organic matter should be worked into the soil — any soil — and particularly one that is either silty-sandy or clay. If possible, it is desirable to lighten and aerate the soil to a depth of two feet or so, although the earthworms and moles will often do much of this for you.

Like good vegetables, healthy perennials are usually sizeable. Whether the ideal is one of large flower trusses or noble form and foliage, it can best be realized when the plant is growing with its utmost vigor. The tight, starved, exquisite buns of the rock garden belong to a different world. Good fertilization programs are in order, and if a plant that is supposed to put on a magnificent display fails for some reason to do so, it is a good idea to check one’s procedures and perhaps seek advice.

Recommended Perennial Plants

The Northwest

From a list as short as this many fine plants which command attention during their moment of glory are omitted in favor of those with a longer season — usually extended by their foliage. But within a framework of the dozen listed, others not on the list, which might be described as prima donnas, may by accommodated.

Hostas and astilbes have fine foliage which contrasts in striking fashion; both enjoy conditions in which rhododendrons thrive and make good backgrounds for them.

Primulas are also excellent; there are many of them, but a start can be made with Primula denticulata and P. polyantha. Siberian irises will be happy in the moist conditions enjoyed by primulas, and their leaf shapes contrast well. Tall bearded irises like drier soil and a more formal setting, but are also easy to grow. Start with these two kinds, and expand into others.

Most hybrid peonies flower with the irises in June, but don’t limit your garden to these; discover the handsome spring-flowering species and the Saunders hybrids.

The spiky blue stems of delphiniums are the traditional complement to roses, and they do best in the Northwest. Asters, like delphiniums, have a short season and their leaves are not remarkable, but they are the mainstay of late summer. Start with a few Michaelmas daisies (Aster novi-belgii), of which there are many cultivars, and graduate to Aster ericoides and A. cordifolius — rarer, but worth the search. Geraniums are workhorses of the spring and summer perennial border, and they are rich in pink, rose, lavender, blue, and white.

Pinks, old-fashioned or modern, need plenty of sun and good drainage if they are not to succumb to Puget Sound’s damp winters, but their leaves and scent make all efforts worthwhile.

Euphorbias I find indispensable in a good perennial scheme, because they are great flatterers of their neighbors. Rodgersias, however, are obviously superior; among them are the dark handsome strangers with foliage worth dying for that many gardeners have yet to meet. They are plants for moist soil.

Among anemones are some wonderfully permanent and dependable plants. Begin with Anemone nemorosa and A. japonica for late summer and fall.

Marvin Black

Northern and Central California

Garden conditions in Northern and Central California vary enormously depending mostly upon distance from the coast. Where there is abundant ocean influence and where wind protection can be provided, perennials flower for very long periods. Midway between the Northwest and southern California, our region has the advantages of both, but also the disadvantages; it is in a sense a more extreme and seasonal climate, experiencing the hot, dry summers of the south and the rainy and windy winters of the north.

More perennials can be grown even in hot, inland areas than is usually supposed. Phlox paniculata, for example, puts on a brilliant but comparatively short-lived display in warm areas. However, if the plants are cut back after bloom, they will flower again. A long season of bloom can be assured by planting the beautiful and disease-free white hybrid of Phlox maculata and P. carolina, ‘Miss Lingard’, in front of the white Phlox paniculata ‘Mt. Fuji’. Miss Lingard is cut back after flowering, whereupon the larger phlox blooms. It then is cut back, to be replaced by the reblooming ‘Miss Lingard’, which again (with luck) is followed by ‘Mt. Fuji’. In this way, months of bloom are assured for the white garden.

In general, plants from the central and southeastern United States have proved more reliable for hot areas than those from the Himalaya. Some familiar examples are stokesia, echinacea, rudbeckia, lythrum, liatris, physostegia, and Gaura lindheimeri. Plants that fall apart in hot spells should be ruthlessly discarded. This includes such otherwise useful plants as the larger geraniums, Centaurea montana, Sedum maximum and S. telephium, and even Shasta daisies.

When the year comes to its end and the garden is subject to heavy winter rains and wind another group of problems appears. One can be illustrated by the gorgeous Mexican sages. Whereas the early blooming Salvia uliginosa, S. guaranitica, S. involucrata, S. cacaliifolia, and others are superb garden plants, the huge winter-blooming bushes of Salvia gesneriiflora, S. madrensis, S. holowayii, S. mexicana and their like are so often broken down by rain and wind as to be of dubious garden value.

Another winter problem in the wetter areas of central California comes from the combination of mild weather and saturated soils. Since most herbaceous perennials come from regions of snow and cold winters, they often are subject to fungal and bacterial attacks in warm wet winters. In a sense many perennials are less winter-hardy here than they are in the continental areas of the country.

Lester Hawkins

Southern California

Gardeners all over the world talk of having something to enjoy in their gardens twelve months of the year, and in southern California, even more than the rest of the generally mild Pacific Coast, we expect every month to be equally interesting. Even when gardeners take the sensible approach of designing a number of plantings, each to take the stage for a limited period and then fade into the background, year-round visibility puts a high priority on good-looking foliage and long season of bloom as factors in choosing perennials. Our arid climate and lack of winter chilling limit the use of many of the traditional perennials, but there are others. Here, in no particular order, are some I have found useful and accommodating in coastal and valley gardens of southern California.

These are for sun and any reasonably welldrained soil:

Limonium perezii — perennial statice with purple flowers and good leaves, will naturalize along the coast.

Penstemon gloxinioides hybrids — tall with pink, red, or purple flowers all year if old stems are cut to the ground.

Tulbaghia violacea — society garlic, an eighteen­-inch onion with lilac flowers all the warm months.

Achillea taygetea — pale yellow small yarrow with fern-like gray foliage (I like this better than the brighter cultivar ‘Moonshine’).

Hemerocallis and bearded irises — both have many re-blooming cultivars, which may not be as spectacular individually as the latest introduction, but give better value in the garden.

Salvia ‘May Night’ — twelve- to eighteen-inch spires of deep purple flowers that will often re-bloom if cut back to basal leaves after first flowering.

Geranium sanguineum, G. sanguineum var. lancastriense, G. ibericum — excellent plants for a sunny place in good soil; G. macrorrhizum will make a splendid groundcover in light shade.

Euphorbia characias subsp. wulfenii, E. rigida, E. epithymoides (E. polychroma) — all green flowered, gray leaved, and bloom in winter or early spring.

Sedum ‘Autumn Joy’ — for its gray leaves as much as its pink to russet flower clusters.

Centranthus ruber — valerian, a fixture on wild hillsides in southern California, but good in gardens too; cut spent stems to ground for all-year bloom.

These will take some shade:

Heuchera maxima — white coral bells on a grand scale; Rancho Santa Ana hybrids come in red and pink in the same size.

Justicia brandegeana ‘Chartreuse’ — a shrimp plant with yellow-green bracts; blooms all year in shade.

Some good plants wouldn’t bloom or look good for a long enough season to list but for their willingness to time-share space in the garden, either by one plant growing through the other and both blooming together, such as Verbena rigida and Convolvulus mauritanicus, or by one plant replacing the other completely. AIstroemeria ligtu hybrids come up through the early foliage of Physostegia virginiana, flower in early summer and disappear completely in time for the physostegia to put on a display in August. Ceratostigma plumbaginoides has intense blue flowers in fall, after which the twelve-inch stems turn brilliant red and die back completely, leaving space for sprawling winter bloomers such as Euphorbia rigida.

Chris Rosmini