This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
So far as is at present known, this pretty and interesting genus of aquatic plants comprises only two species - the one a native of North America, and unknown to cultivators in this country; the other inhabits ponds and sluggish streams in England and Ireland and other parts of central and northern Europe. H. palustris (Water Violet) is the European species. It is valuable for introducing into ponds and streams where aquatic vegetation is desirable, being interesting and ornamental for a long period during summer. The leaves and barren branches are all submerged; the former are deeply cut into fine thread-like segments, giving a feathery appearance to the submerged growth, and suggesting strikingly the other common name (Featherfoil) by which it is known in some parts of the country. The flower-stems are leafless, and rise erect above the water, bearing several whorls of rather large flowers, deeply divided into five broad lobes. The flowers are variously coloured in different individuals - pale purple is the most common colour, but blue and white and pink are also to be met with, and they appear in June, July, and August, and often also in September. It is easily propagated by division, and also by seeds.
If the latter method is adopted - and it is the simplest, if they have to be transported a distance - they should be sown immediately they are ripe in the quarters they are to occupy permanently, the only care requisite being the prevention of the washing of the bottom by floods if the pond or stream in which they are sown be liable to such disturbance, and the destruction that would be caused by waterfowls, till such time as the plants are strong enough to take care of themselves, which they will be the year after sowing.