Garden Plants Encyclopedia Marginal Aquatics M – O

Marginal aquatics comprise those plants that grow happily around the edge of ponds in mud or several centimetres of water. They also inhabit stream-sides where the water depth may vary. In modern pre-formed or liner ponds they are usually grown on the marginal shelves in individual containers. This is preferable, as many of these plants have strong and vigorous root systems. These can overwhelm neighbouring plants if they are allowed to become established in close proximity. Container cultivation also permits the water level under which each plant is growing to be easily adjusted when necessary without affecting any other plants.

The measurements following the descriptions of each plant are the approximate ultimate height of that plant. Most will grow in conditions of mud and up to 15cm of water. Where there is specific intolerance this is noted. Frost-hardiness is indicated. Tropical; indicates that for the plant to continue growing successfully all the year around, it must have tropical conditions. Such plants are not frost-tolerant.

Note for North American gardeners. The Garden Plants Encyclopedia is an international project and is written in standard English with measurements using the metric system.

Marsilea angustifolia – Thin-leafed Nardoo
This is one of the less-common native Marsilea species, which sometimes grows totally submerged, but is most useful and attractive in a pond when growing in a shallow margin, where its narrow four-segmented, clover-like leaves, up to 1cm long, can float freely. A pleasing little perennial plant it is also ideal for cultivation in a sink or trough garden. Like all Marsilea species it creeps around and can easily be lifted and divided at any time from late spring through the summer. Australia. The leaves usually float, although occasionally it behaves as an emergent plant up to 10cm high.

Marsilea drummondii – Common Nardoo
This is a scrambling perennial clover-like plant with broad silvery-green leaflets 5-30mm long and almost as wide. These are mostly emergent, but in deeper water often float. Nardoo grows from a long creeping rhizome which is covered with orange-brown hairs. It grows well in sun or shade, but dies back during winter in cool areas. Marsilea species are botanically members of the fern kingdom, but do not remotely resemble a garden fern. Australian Aboriginal peoples grind the roots of this species into dough, which they then cook. The spores can also either be roasted or ground down to make flour. Increase during spring and early summer by dividing the creeping root system. Australia. Up to 20cm.

Marsilea hirsuta
A short-growing perennial marginal aquatic that will also colonise muddy areas outside the main pond when growing in natural conditions. Of clover-like appearance, it is plain green and densely hairy, with leaflets that have either entire or indented margins. These are up to 20mm long and 13mm wide. It grows freely from a creeping rhizome that has pale brown hairs at the point where the foliage emerges. Easily increased by division of the scrambling root system. Australia. 5-20cm.

Marsilea mutica – Rainbow Nardoo, Australian Water Clover
Beautifully patterned shiny green rounded clover-like leaves with ochre and red markings are produced both above and on the surface of the water. These are 10-50mm long and 20-40mm wide. This handsome perennial grows from a long, creeping, much-branched rhizome and is more adaptable than any other species to deeper water. It grows in sun or shade and dies back during winter in cool areas. Easily increased by division of the scrambling rhizome. New Caledonia, Australia. Up to 30cm.

Note: Prohibited in some regions of New Zealand.

Marsilea quadrifolia
An attractive perennial plant of four-leafed clover-like appearance, which produces foliage that is either submerged, emergent, or both. When submerged the leaves often take on the appearance of silver foil. This is caused by tiny air bubbles being trapped by the fine hairs of both the upper and lower surfaces of the foliage. The leaflets are up to 20mm long and 15mm wide. It dies back in the winter in cool areas. Easily increased by division of the scrambling rhizome. Europe, Azores. Up to 30cm.

Maundia triglochinoides
This is considered a botanical curio rather than a valuable decorative water garden plant. An interesting, but not showy perennial, which grows from a creeping rhizome no more than 0.5cm thick. The strange leaves are spongy, inflated, triangular in cross-section, and up to 80cm long. The curious flowers are produced in a spike, up to 10cm long and 2.5cm across. Of an off-creamy colour, they are followed by heavy clusters of squat fruits up to 1cm long. Increase by early spring division of the rhizomes. Australia. 60-80cm.

Mentha aquatica – Water Mint
A strongly aromatic scrambling perennial marginal aquatic with rounded, hairy green leaves 2-6cm across, which are borne on slender reddish stems in crossed pairs. The soft lilac-pink blossoms are produced in tight groups, and look like miniature powder puffs. There is a main terminal group of flowers on each stem and then many subsidiary whorls below. These densely clothe the plants during mid and late summer. Like most members of the Mint family this species has a vigorous root system and is best grown confined to a planting basket. When growing naturally in deep water of around 2m, it has been found to adapt and becomes a sterile non-flowering form. When established normally in pond margins or alongside creeks, it will often produce fertile seed. Stem cuttings root readily at any time during the growing season. Frost-hardy. Northern Europe. 30-45cm.

Mentha cervina syn. Preslia cervina
Spreading clumps of slender, erect stems clothed with small lance-shaped leaves, no more than 20mm long and 5mm wide, are crowned during late summer with small stiff whorled spikes of blue or lilac flowers. A perennial the plant is strongly aromatic and happiest when growing in shallow water at the pond edge. There is a white-flowered variety, Mentha cervina var.alba. Stem cuttings root readily at any time during the growing season. Frost-hardy. North America. 30cm.

Mentha diemenica- Slender Water Mint

An Australian native with short erect stems that are clothed with evenly arranged pairs of slender aromatic, mid to dark green, oval or lance-shaped leaves, 4-12cm long. Light lavender-mauve to lilac blossoms are produced during late spring and summer. Up to four flowers are produced in the axil of each leaf, which means that there may, on occasions, be eight blossoms at a leaf joint. This species is as often discovered in damp woodland glades as at the waterside in the bush. Unlike Mentha aquatica it is unwise to cover the roots of this species with more than 1-2cm of water. Stem cuttings root readily at any time during the growing season. Frost-hardy. Australia. 30cm.

Menyanthes trifoliata – Bog Bean
This is a very distinctive perennial plant for shallow water with showy white five-petalled flowers, which are delicately fringed with soft pink, and have purple stamens. These are produced during spring and early summer above dark green foliage, which is rather like that of broad bean. The leaf consists of three oval leaflets, each up to 10cm long and 5cm wide, which are arranged clover-like on a single leaf stalk. This clasps the creeping rhizome with a long sheath. The flower stalk is 25-40cm tall and ends in a dense spike, comprising upwards of ten flowers, which is up to 12cm across. Cuttings of sections of the creeping stem-like rhizome, each with a bud, if removed during spring or early summer, produce fine plants quickly. Bog Bean can also be increased by division and separation of the creeping rhizome. The vigorous young terminal growths, if removed with a tuft of roots and a few centimetres of rhizome, are ready for immediate replanting. Frost-hardy. Europe, Morocco, temperate Asia, India, North America. 20-40cm.

Note: Prohibited New Zealand.

Mimulus ringens – Blue Aquatic Musk
This is the only true and reliably perennial aquatic Mimulus. A North American native, it prospers best in cool districts, although it will grow quite well in sub-tropical areas. It produces tall, erect stems from either fibrous roots or rhizomes. These are hollow, square, and slender, and carry opposite pairs of narrow lance-shaped leaves up to 10cm long and 4cm across. The lavender-blue to purplish flowers which have a yellowish centre, are up to 2.5cm long, and have pronounced spreading lips. These blossoms are produced singly from the leaf axils. Tiny fruit capsules follow and are filled with very fine viable seeds. Flowering takes place for much of the summer. There are two varieties recognised by botanists, M.ringens var. ringens, where the leaves clasp around the stem, and M.ringens var. minthodes, in which the leaves have a tapering base and do not clasp the stem. It is the former which appears to be grown in Australia. Propagation is by early spring division, short summer stem cuttings or seed. Frost-hardy. United States. 30-60cm.

Monochoria cyanea
Although regarded by botanists as being a perennial, it generally behaves as an annual under cultivation. A very beautiful plant with great potential for the water garden. However, insufficient is yet known about its cultivation to make it a popular garden plant. Currently work is taking place in Queensland with a view to introducing it widely to Australian water gardeners. It is a handsome plant with glossy, more or less oval dark green leaves that are produced on scrambling stems. These often root at the leaf joints. The leaf blades are quite variable in size, from 4-15cm long and 1-6cm wide. The spikes of beautiful deep-blue or violet blue blossoms are subtended by a leaf. The star-shaped flowers have five or six petals, each 1-2cm long, surrounding a creamy to yellow centre. Seeds are freely produced. Increase by spring sown seeds or summer separation of pieces of rooted stem. Seed is currently the preferred method of propagation. Tropical. Australia. 60-80cm.

Montia australasiaca syn. Claytonia australasiaca
A fast-spreading, creeping perennial plant for the pond margin. This is a fleshy-leafed carpeting aquatic with dark green to grey leaves up to 1.5cm long, which are arranged alternately on scrambling stems that root readily at the leaf joints. Small white or pale pink star-like five-petalled flowers, no more than 2cm across, are produced freely. These usually appear in clusters, in the axils of the leaves. In water that is deeper then 5cm it rarely flowers. It must be grown in the full sun. This species is usually evergreen, even in cooler districts. It can readily be increased by seed sown during the spring, or division of the plants at the same time, just as they are about to break into active growth. New Zealand, Australia. Scrambling.

Myosotis scorpioides syn. M.palustris – Water Forget-me-not
Rather like the border Forget-me-not, but differing in so far as it is a reliable perennial, and has smooth lance-shaped evergreen leaves rather than being a biennial with coarse hairy foliage. The leaves are up to 8cm long, and the plants are smothered during much of the summer by heads of bright blue Forget-me-not flowers with distinctive yellow centres. This pretty little aquatic grows from a creeping rhizome, the plants scrambling around and forming a neat carpet of oval to elliptical, alternately arranged foliage. Myosotis scorpioides is ideal for clothing the edge of a water feature where the pond meets the garden. Easily increased from a spring sowing of seed, or division of established plants during spring or early summer. There is also a white-flowered form, M. scorpioides var. alba, and ‘Mermaid’, a free-flowering consistent bold blue cultivar. These should both only be increased by division. Frost-hardy. Europe, western Asia. Naturalised in the United States. 20-40cm.

Myriophyllum trachycarpum
A perennial that unlike most of the other members of the genus is frequently emergent as well as submerged. The stems grow up to 1m long and root variously into the pond floor. The emerging foliage is erect, with upright stems that are crowded with narrow, almost needle-like dark green leaves in a random arrangement. The leaves are up to 2cm long. The tiny pinkish flowers are produced during late summer and autumn in the axils of the leaves. The submerged foliage is finer and more feathery than the emergent. Unlike other Myriophyllum species this is not a particularly fast grower. It can be increased by early spring division, separating portions of rooted stem which are then replanted. Australia. It rarely grows more than 30cm above the water surface level.

Neptunia oleracea syn. N.natans – Water Mimosa, Touch-me-not, Water Sensitive Plant,
This is one of the most remarkable and fascinating tropical perennial aquatic plants. A shrubby character with an overall look of the Sensitive Plant, Mimosa pudica, and like the Mimosa, having the same ability to respond to external stimuli. The finely-divided leaves of this aquatic fold and collapse if touched; a source of great amusement, especially for children. Not content with this, Neptunia also produces some quite remarkable stems, these developing copious white spongy aerenchyma. This is plant tissue that is characterised by very large intercellular spaces, which give the stems the ability to float. During this part of the plant’s life cycle the branches periodically detach from their rooting anchor and floating free.

Apart from these intriguing asides, Neptunia is a most attractive waterside plant, prospering in about 15cm of water, but sending floating branches out towards the middle of the pond. These can be irritating, but they can be easily cut back and controlled. The foliage is very similar to that of the true Sensitive Plant, consisting of dense ranks of narrow lance-shaped mid-green leaflets, which produce a delicate fern-like appearance. These are sprinkled for much of the summer with attractive fluffy-looking yellow balls of fifty or more tiny crowded blossoms held several centimetres above the foliage. Tropical Asia, Africa, Central and South America. Naturalised in other parts of the tropics. 60cm-1.5m.

Oenanthe javanicum ‘Flamingo’ – Variegated Water Parsley
This handsome perennial aquatic is a selection from the weedy Java Dropwort, Oenanthe javanicum, (syn. O.stolonifera), an unimpressive dull green, creeping plant with parsley to celery-like foliage and uninspiring umbels of tiny white or greenish-yellow flowers. The variegated selection is a different proposition, with handsome cream, green and pink foliage. Although, like the wild species, it can be rampant, spreading quickly when unrestricted by a planting container, it is a fine marginal plant for any but the most restricted of ponds. The much-divided leaves are up to 30cm long and arranged on erect stems which give rise to umbels of very small white flowers. Most gardeners remove the emerging flower heads immediately they are seen thrusting upwards in order to retain good quality foliage. Flowering causes elongation of the stems, and this opens up the foliage, which in turn becomes of poorer quality. Increase from early spring division. 60cm-1.2m (when flowering).

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