Floating water plants are those that are not anchored by roots, although some do produce a few straggly roots that simply dangle in the water.

These plants tend to die down during late autumn, surviving the winter as dormant 'buds', resting in the mud at the bottom of the pond. In spring, when the weather warms, they burst into life again.

Some floating plants increase their size and number rapidly and can cause overcrowding in the pond. It is for this reason that there is a restriction in the sale of such plants in some warmer climates, including some of the southern states of the USA, where the winters offer less of a check to their growth. Let's consider some of the best floating plants:

- Common duckweed (Lemna minor)
The origin of the common name is, to say the least, hazy, although ducks on larger ponds and lakes will certainly nuzzle around looking for small creatures hiding in it. Some fish eat it, and they lurk under the shade it affords. The form that is generally found in garden ponds is Lemna minor and, if you introduce it to your pond - either accidentally or on purpose - the pond will soon be covered with a pale lime-green 'carpet'. Don't walk on it though, like a child did when I was visiting Wisley gardens in the 1970s. Fortunately the pond was very shallow, but the youngster had a nasty shock, and so did his parents.

FLOWERS
These appear in summer, and are tiny, grey-white and wholly insignificant.

LEAVES
Minute almost translucent, deciduous fronds in various shapes, but mostly oval. Thousands of them abut each other to form a dense carpet over the surface of the water. Each individual frond usually produces a single, dangling root.

CULTIVATION
One doesn't really 'cultivate' duckweed -it just happens! Planting comprises simply dropping several leaves with roots into the pond, generally in spring. It grows best in still, or nearly still water, and in either full sun or light shade. Duckweed is left to its own devices. All you need to do is use a net to thin it out occasionally to prevent it from covering and choking the whole pond.

HARDINESS
Very hardy, tolerating temperatures as low as -20•°C. Duckweed survives in winter as dormant buds on the bottom of the pond.

PROPAGATION
Again, one doesn't really 'propagate' duckweed. Just redistribute some of the fronds to a different pond.

ORIGINS
A one-species plant, originally from the Atlantic side of North America.

HEIGHT
None - to speak of.

SPREAD
A couple of individual fronds can increase and cover up to lm2 (9 sq ft) in a year, but thereafter rapid covering of ponds can take place.

MINIMUM WATER DEPTH
15cm (6in).

MINIMUM POND SIZE
Small - up to about 2m (18sqft).

ALTERNATIVE
Perhaps the best form to choose is the ivy-leaved duckweed (Lemna trisulca). This is less invasive than the common kind and produces a mass of star-shaped leaves, frequently floating just beneath the surface. Lemna gibba, often referred to as the 'thick' or 'gibbous' duckweed, has rounded, swollen leaves and is particularly invasive. Lemna polyrhiza (also known as Spirodela polyrhiza, and commonly referred to as 'great' duckweed) has larger, rounder leaves than the normal kinds, and each has a small 'tuft' of roots attached. Very invasive.

EXPERT TIP
Most gardening experts are asked how to get rid off duckweed, rather than how to grow it. Be warned: if you don't want it, it is important to check carefully each new water plant for the presence of duckweed before it is introduced to the pond.