Oxygenating plants (oxygenators) & submerged aquatics (biotype2)

There is a whole cross section (type2) of demure little plants whose diminutive stature belies their importance in the water garden. A healthy growth of submerged aquatics presents competition for light and nutrients which helps to inhibit the unrestrained growth of free-floating algae. At the same time as they photosynthesise carbon dioxide (CO2) is absorbed and oxygen released a process which can be observed on a hot summer day when they release a tiny stream of bubbles into the water.

As with all water plants they mostly have to be controlled and kept within bounds. Ponds choked with excess flora will crowd out less exuberant aquatics and cause oxygen depletion at night when the process of respiration is reversed-with fatal consequences to fish.

The listings, which follow, are arranged alphabetically and include some aquarium subject which could be use in the warmer parts of the country in the fishpond.

• Cabomba Fanwort closely related to the Nymphaea, the Fanwort is grown as an aquarium plant. In the pond it is semi-hardy with finely cut fan-like foliage. Aerial foliage is unexceptional and the flower tiny.

• Ceratophyllum demersum Hornwort this is the most commonly grown oxygenator and performs its function well. The plants float and form solid clouds of dark green foliage in mid-pond. In winter the foliage condenses and the whole mass sinks to the bottom. An excellent clarifier of ponds, but control its uninhibited growth. Only 20% of the pond volume should be occupied-the rest hook out with a rake or forked stick-to be used on the compost or as excellent mulch.

• Echinodorus cordata Amazon sword this plant has been used extensively in aquariums and lives well completely submerged. In shallow water the plant forms aerial leaves resembling an alisma and bears button sized white flowers which root and form new plants if allowed to rest on the water surface. Hardy to about 100C but unproved in colder water.

• Flosea Monkey tail spreading with alarming rapidity, this plant is in fact a proclaimed weed and hence prohibited for use in pond. It is in any event a dreary undesirable alien, which can choke a good water feature in one summer. A plant to be avoided.

• Egeria resembling a large leafed elodea but much more well mannered and of refined habits. It can romp away as well though, so control is required as before. The flowering stage is charming and occurs above water in the spring. There exist other species of great similarity.

• Myriophyllum Parrots feather another prohibited weed and rightly so because of it's vicious growth habits submerged and aerial leaves and fine cut and ferny.

• Najas pectinata Saw weed prickly curled leaved oxygen plant of little beauty.

• Sagittaria this is an extremely useful group of plants mostly grown as bog subjects. The foliage varies tremendously both above and below water while most flower in a rather unspectacular way if allowed to grow in shallow water. They root and prosper on the pond bottom consuming debris and rotting vegetation to the benefit of the whole ecology of the pond.

• Sagittaria natans the tiniest of the group with thin grassy underwater leaves and minute white flowers. An excellent oxygenator but not too easy to come by.

• Sagittaria platyphylla .. this is the largest of the genus. Wide submerged leaves are excellent oxygenators and the plants cleanse the surrounding mud as they grow. Don't plant it too shallow though because the leaves then thrust above the water and change their character. Aerial leaves are spear shaped and the flowers a dreary white.

• Sagittaria sagittifolia this is the gem of the genus with most decorative hastate or arrow shaped leaves. Low growing and non-invasive for sun, shade, deep or shallow water.

• Urticularia Bladderwort this is a fairly common weed in dams and lakes and can be easily identified by its fragile appearance and bladder like sac that keep the plant afloat. Useful if carefully controlled.

• Vallisneria spiralis Tape grass Vallis as this plant is affectionately called is the star of the water garden. An excellent oxygenator and most attractive as well. There are a few varieties i.e. a long giant one, a short corkscrew one and a dwarf one, all useful and indeed essential for the 'balance' of the pond.

Submerged aquatics can all be treated in similar way. Plant them in 5"pots weighted down by stones and sink to the bottom. Alternatively when planting or re-planting your lilies, scrape a few into the stone covering the lily container where they will flourish and disguise the pot. 'Blanket weed' or hair algae, so beloved of water gardeners is an efficient oxygenator in it's own right provided it is kept in check. It also serves as an indication that the water has been enriched in some manner, often wash from lawns, which have been recently fertilized. By harvesting the weed you are thus removing the excess from the nutrient cycle. It is very important, nonetheless to avoid any enrichment of the water from whatever source.