Myriophyllum spicatum

Myriophyllum spicatum, or spiked watermilfoil, is an aquatic perennial from temperate regions. It has a stoloniferous base that produces very long floating stems, up to 3m (10ft) long, reaching the water surface to form a dense yellow-green carpet of finely divided, feathery leaves. The inconspicuous flowers rise above the surface in spikes of small white flowers, pink when in bud. It is an excellent plant for oxygenating and purifying slow to stagnant, even polluted water. Its very vigorous growth should be limited as it can quickly become invasive.

Myriophyllum spicatum belongs to the Haloragidaceae family. It is native to Europe, Asia, and North Africa. It now proliferates worldwide in stagnant or slow-flowing freshwater. It is one of the most common aquatic plants in ponds and waterways. It forms loose floating stems that can reach 3m (10ft) (sometimes 6m (20ft)) which are often reddish and branch out and grow horizontally on the surface of the wate. It produces spikes of tiny white flowers 1 to 2mm (0in), pink in bud, 15cm (6in) above the water. The finely dissected foliage appears yellow-green below the surface and bluish-green above water. The leaves are arranged in whorls of 12 to 24 leaflets around the stems and are semi-evergreen. The roots anchor into the soil to 50cm (20in) deep.

Plant spiked watermilfoil in a pond, in baskets filled with loamy, humus-rich soil, in full sun under 10 to 150cm (4 to 59in) of water. Weigh down the watermilfoil when planting as it has few roots. This plant is hardy to about -15°C (5°F) in full sun or partial shade. It shows great tolerance to pH and water hardness. The stems may suffer from frost but will reappear the following spring. Where it thrives, its exceptional vigour can make it invasive. Therefore, it needs to be controlled either by installing it in a basket (when possible) or by regularly removing some of the stems.

Spiked watermilfoil is a first-rate oxygenating and purifying plant. It can concentrate nitrogen and phosphorus. Through its filtering action, it helps stabilise the delicate balance of a garden pond. Its presence will contain the growth of green algae. If the pond is big enough it is better to plant two to three species of submerged plants to vary the foliage. This plant can also be used in tropical fish aquariums where you must provide sufficient light.