Submerged oxygenating plants are the essential maids-of-all-work vital to pool hygiene and balance That they are also known as ‘water weeds’ is a reflection of the fact that, if enough are planted initially to achieve balance in a short time, then in two or three seasons some types will have made so much growth as to become an embarrassment.
Which sorts will grow best in any particular pool cannot be forecast since it depends on subtle differences of soil and water chemistry that defy analysis. It is a fairly safe bet that elodea will do well, but rather than relying on any one kind it is best to plant a mixture and let them sort themselves out. The time forback or thinning the over-exuberant is August/September, when seasonal growth is dying back anyway and could create problems if left to decay in the pool. In the cold months when fish are inactive their oxygen demand is low and they do not rely on submerged oxygenating plants; dissolved oxygen from the surface is adequate.
All the following oxygenators are suitable for pools. Ranunculus aquatilis and the callitriches are the only ones that are likely to do well in running water.
Callitriche (Water Starwort). The narrow-leaved Callitriche autumnalis (hermaphroditica) is the best oxygenator of the family, growing entirely submerged. C. uerna (platycarpa) and C. stagnalis prefer water 6 in. deep or less and make bright green rosettes of foliage on the surface, particularly attractive in the autumn and winter months. They are good shelter and food plants for fish but give most of their oxygen to the air.
Ceratophyllum (Hornwort). CeratophyHum demer-sum and C. submersum make dense plumes of very narrow dark greenthat have a bristly touch. They thrive without conventional planting (’just dropped in’) and in most situations, including deep, cold or shaded pools.
Elodea (Anacharis) canadensis. Has a reputation for vigorous growth but is probably the shortest and tidiest of the good oxygenators once it has expended its first burst of energy and settled down. Elodea crispa (Lagarosiphon major). Makes lengthy, sturdyclothed in curled leaves and is highly regarded as a valuable producer of oxygen. Fontinalis antipyretica. Willow Moss makes soft patches of almost black-green foliage. Less easy than others to establish: best attached to a rough stone. Hottonia palustris (Water Violet or Featherfoil). The beautiful light green divided foliage is at its best in March/April and the autumn, which are good times for planting. If moved in summer the plant disintegrates but may reappear the following summer. Whorls of yellow-eyed stand 6 to 9 in. above the surface.
Myriophyllum (Water Milfoil). Myriophyllum spi-catum and M. verticillatum have soft, very finely cut feathery foliage.
Potamogeton crispus. Has branchingbearing leaves up to 4 in. long and \ in. wide, wavy-edged, green or reddish brown, translucent and delicately beautiful.
Ranunculus aquatilis (Water Crowfoot). This has very fine submerged foliage (in running water, long waving tresses) and lobed surface leaves. In May it sprinkles the surface with small white flowers.