Pool plants that delight to grow with their roots in shallow water, and raise their stems and flowers above it, are referred to collectively as marginals. They include some aggressive colonisers capable of swamping the more moderate – and more ornamental – types if given free rein. This is what happens if a mixture is planted in a trough: after a couple of seasons you have a solid hedge of the one survivor, inevitably the coarsest and least attractive of the lot.

Container planting, with only one variety to each container, is the ideal method of eliminating unfair competition and restricting growth to tidy, controllable clumps. Containers 6 in. deep on a shelf about 9 in. deep provide an effective water depth of 3 in. that suits the great majority of marginals. Effective water depth (e.w.d.) is mentioned in the following list only when it varies from this general rule. Where less is desirable the container level can easily be raised by the thickness of a tile or two.

Marginals perform no water-clarifying or sun-shading function, so they are not essential to the pool community in practical terms. They earn their place entirely on the strength of their flowers or, in some cases, on the ornamental value of their foliage. Many commonly listed marginals do not qualify on either score, and they are not included in the following list. The Figure given for height is the approximate average mature foliage height. E.w.d. 0 in. means that soil and water level are coincident. Acorus calamus variegatus. A form of the Sweet Flag with leaves striped green and creamy white. Tidy growth from a slowly creeping rhizome. 2^ ft. E.w.d. 0 to 2 in.

Butomus umbellatus. The Flowering Rush has narrow, twisted leaves and 3-ft. Stems bearing clusters of red-stamened pink flowers. Calla palustris. Bog Arum raises shiny rounded leaves only about 6 in. above horizontally spreading stems. Arum-type white spathes enclose a spike of tiny flowers that mature, if pollinated by crawling pond snails, into red autumn fruits. E.w.d. 0 to 2 in. Caltha. The Marsh Marigolds or Kingcups are a splendid family flowering early and freely in shallow water or marshy soil. Caltha palustris grows 12 to 15 in. high and has single sunny yellow flowers; C. palustris plena, a more compact 9 in., makes an almost solid mound of full, double blooms through April and May; C.polypetala has larger leaves, larger single flowers and grows up to 3 ft.; C. palustris alba is about 9 in. high and has white flowers. E.w.d. For all calthas 0 to 1 in.

Cyperus longus and C. vegetus. Sedges that make very attractive foliage clumps when container planted, but seed and spread too readily to be controllable in natural pool margins. 4 ft. and 2 ft. respectively.

Eriophorum. The name Cotton Grass sums it up. Narrow grassy foliage and silvery white plumes of silky down. L£ ft. E.w.d. 0 to 1 in. Glyceria aquatica variegata (G. spectabilis). A very popular foliage plant whose leaves are striped boldly with green, yellow and white, and pink flushed in spring and autumn. 2 ft.

Iris laevigata and its varieties. These have the finest flowers of any of the marginal plants, making June colourful when they are seen in company with the early water-lily blooms. I. Laevigata has flowers of clear violet blue. In the variety variegata these fine blooms are combined with handsomely striped green and cream foliage. Even when it is out of flower the fans of variegated leaves make a striking picture, as fresh in the autumn as in April. Undoubtedly one of the finest pool plants. Of many other /. laevigata varieties I particularly like col-chesteri and monstrosa, whose white flowers heavily overlaid and mottled with rich dark blue are so alike I find it difficult to distinguish them; albo-purpurea, with flowers of china-blue and white; lilacina, a white tinted with satiny lilac; and the fine large pure white Snowdrift. The very beautiful pink variety Rose Queen seems to me to be a laevigata/kaempferi hybrid; several significant differences between this and the other laevigata types include the practical one that it definitely does better with its roots just covered in water than in the 3 or 4 in. the others enjoy. All grow to about 2 ft. Iris pseudacorus. The vigorous 3-ft. Yellow Flag.

The most decorative of its forms is p. variegatus, a more restrained grower with yellow flowers and green-gold leaf striping. Unfortunately the leaf marking fades off by June to a uniform pale green. Iris kaempferi and Iris sibirica. Usually employed as moist-soil plants above water level, but self-sown seedlings are often found right on the waterline and I find that both do well in marginal containers when the soil is only just covered by water. Lobelia hybrida Queen Victoria. Normally grown as a border plant, with the injunction that it be protected in winter, if not taken indoors. Rather surprisingly it seems to flourish as a marginal and winters well with no more protection than 3 or 4 in. of water. The rich scarlet of its flowers – a really superb colour – in July and August associates well with the blue of pontederia. 3 ft. Menyanthes trifoliata (Bog Bean). Happy as a marsh plant or a shallow marginal. Broad-bean-type leaves lift about 9 in. above creeping rhizomes, and the clusters of flowers, delicate pink outside, pure white within, and finely fringed, are worthy of close examination. E.w.d. 0 to 2 in.

Mimulus guttatus. More familiar as M. luteus; its ease of cultivation and cheerful prodigality of yellow flowers all through the summer make it very popular. Its seedlings crop up in damp nooks and crannies all round the pool. 1 ft. E.w.d. 0 to 1 in. M. ringens is a more sober relative with soft violet flowers in late summer. L£ ft.

Myriophyllum proserpinacoides. Worth having if only to impress visitors with your ability to remember and pronounce its name. A plant for draping edges, it can share a container with upright-stemmed plants like butomus, iris and typha, without harm. It trails sprays of dainty emerald green foliage and gives the impression of being too delicate to survive the winter; in severe winters this may prove to be true.

Pontederia cordata. Has many good points. It is a good doer without being rampant; its tilted leaves, shaped almost like Norman shields, have a healthy gloss; it flowers late when many other marginals are well over; and its spikes of blue flowers, though not big enough or bright enough to be really showy, are an uncommon colour in the water garden. All in all, one of the best. 2 ft.

Ranunculus lingua grandiflora. A 3-ft. High water buttercup with typically shining yellow flowers from June onwards. When container grown its roving white roots can easily be spotted and dealt with. Given free range it can be a menace. Sagittaria japonica plena. The showiest of the arrowheads, with very full double white flowers, though I must admit to a personal weakness for the simple three-petalled gold-centred smaller flowers of S. japonica. The arrowheads are late starters, never showing growth (or being purchasable) before June. In one miserable summer I gave mine up for lost in July; they finally appeared in early August. The bold, deeply arrowed leaves grow to about 2 ft. while the roots tolerate an e.w.d. Of 6 in. or more. Scirpus albescens (Bulrush). A true bulrush, that is, not one of those plants with club-like brown bosses that are mistakenly called bulrushes but are really cat-tails or reed maces (for which see Typha below). The attraction of S. albescens lies in the creamy yellow rushy stems vertically lined with dark green. 4 ft.

Scirpus zebrinus. If a dubious botanical name, it is the most familiar one for the Porcupine Quill or Zebra Rush. It is a really striking foliage plant with broad alternate bands of green and white. When established the plant begins to produce stems of a uniform dull green, but if the root is lifted, divided and replanted the bold variegation magically reappears. It needs this treatment at least every three years, and is well worth the trouble. 3 ft. Typha. The Reed Mace (or, in America, Cat-tail), so often called Bulrush. The velvety brown boss consists of millions of tightly packed silk-tailed seeds: it breaks up in the winter into silky fluff and floats away on the breeze. Typha latifolia, the Great Reed Mace, grows to 8 ft. Much more suitable for garden pools are the narrow-leaved, graceful 4- to 5-ft. T. angustifolia and the regrettably scarce slender 3- to 4-ft. T. laxmannii. For small pools there is a miniature, T. minima, with fat round bosses, that grows to about l£ ft. Container planting is very necessary for the reed maces. Apart from curbing their invasive tendencies, root confinement seems to encourage the early production of flowering stems. Zantedeschia aethiopica. The Arum Lily, more familiar as a greenhouse plant than an aquatic. It is, nonetheless, perfectly happy in a container on a marginal shelf and considering what the lush, glossy leaves and fragrant white flowers can contribute to the character of the water garden it is a pity that this adaptability is not more widely exploited. The plant can either be taken inside in the autumn and dried off in traditional style, or left to overwinter in the pond. If the latter, the container should be moved to the bottom of the pool to give the plant the protection of 10 or 12 in. of water over it. If you forget to raise it again next summer it will remind you by pushing leaves above the surface even from that depth.

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