Notes on Keeping Wolffia arrhiza (Rootless Duckweed) in Captivity

Source: Hollings, Michael, and Olwen Hollings. “Notes on Keeping Wolffia arrhiza (Rootless Duckweed) in Captivity.” Sussex Botanical Recording Society Newsletter, no. 61 (January 2006).


Way back in July 1998, following a kindly tip-off from Frances Abraham, we pottered along the footpath beside the east bank of the River Arun.  From just below Amberley Castle the path runs to the river bank opposite the former Bury Ferry crossing, then south with the river.  We scanned with binoculars all the ditches feeding to the river, looking for duckweeds.  At the third ditch which we came to, at TQ018124, we found a ditch emptying into the river, but held back by a hinged sluice.  Here duckweeds in abundance had piled up behind the sluice; the surface was choked with them for at least 5m back from the sluice bar.

An empty cocoa tin on a length of twine brought us a sample from the middle of the patch, and we tipped it into a large white plastic tray.  There were abundant Lemna minor, numerous L. trisulca, fewer Spirodela polyrhiza, and a few Wolffia arrhiza – the latter clearly discernible with an ordinary magnifying glass.   This was The Place, we exulted. A search of other nearby stretches of ditch, mostly heavily overgrown and with few duckweeds to be seen, confirmed this.  We carefully removed the other species of duckweed to a separate container and made a rough count of the Wolffia: well over 250. If our single sample were typical of the duckweed mass, there must have been at least a million Wolffia individuals at this spot.

In 1998 Frances had found Wolffia at TQ025138, which is on the opposite side of the R. Arun; and in 1997 at TQ026132, which is on the footpath just below Amberley Castle.  As the crow flies, this last site is just about 1km from our spot, and about 1.4km along the network of drainage ditches. The plants could possibly have been floated along in the slow moving waters.

Meanwhile, back at the ranch, the various cultures were flourishing, multiplying rapidly. Initially we tried flat, white plastic ‘butter packs’ 17x17cm, 5cm deep, which seemed ideal for the job.  We also tested rather flat, wide-mouthed clear glass jars used for fruit preserving.  Into each container we put c.500ml of filtered water from our garden pond (pH 6.5).  Every two weeks we added 10ml of a solution (5g/litre) of Phostrogen, a very bland inorganic fertiliser containing all major and most minor nutrients.  On 12 July we put 50 Wolffia ‘units’ into each of five containers; the easiest way to transfer them individually was with fine forceps, the tips wedged 1.5-2.0mm apart.  Surface tension seemed to lock each ‘unit’ into a small drop of water, and each then dropped out when the forceps were dipped into the culture jars.

Three containers (two plastic, one glass) were put outside in full sunlight (Weston Meter readings 150-1600 foot-candles in shade and full sun respectively) under a wire netting fruit cage (with nylon netting top) to exclude birds, who not unreasonably might assume that the jars were for their use.  The other two containers (one glass, one plastic) were kept indoors on a south-facing windowsill (Weston meter readings 60-150 foot-candles).

On 30 July (18 days culture) we counted the lot.  The two plastic containers outside held 166 and 183 Wolffia units, many in mid-budding.  The other plastic container (windowsill) had 144 units.  The glass jar outside yielded 364 Wolffias, the one indoors held 333.  We now had nearly 1200 Wolffias. Those kept outdoors had increased 4.75 times, those indoors 4.77 times: no apparent difference.  Those kept in plastic containers, however, increased by 3.3 times, whereas those in glass multiplied 6.97 times.  It looked as if glass was far better than plastic, although all cultures (especially those outdoors) had developed unwelcome growth of green algae (Cladophora etc.).  But in a subsequent repeat trial, 264 units multiplied to 954 by 12 August (x3.6 in 13 days), against 200 units put into glass jars which had reached 596 Wolffias (x2.89 in 13 days).  Both treatments used sterilised tap water (pH 6.8), and were kept on the windowsill in the hope of avoiding algal contamination.  All cultures were given 10ml of Phostrogen solution weekly.

Although the initial growth in plastic trays seemed much better than in glass, as the days passed the green algae developed in both cultures, and crowded out the Wolffias; budding slowed down and, instead of normal oval to spherical bright green cells, increasing numbers of much larger (c.1.5mm) thick-walled structures appeared, many apparently lacking chlorophyll, and showing no signs of budding.  After five weeks the cultures were in very poor shape, and green algae had almost taken over.

Preventing algae from overwhelming Wolffia became our main concern.  Although we tried putting Wolffia into containers with L. minor, L. trisulca and Spirodela polyrhiza, the results were inconclusive.  Another series of experiments had suggested that Lemna trisulca combated algal growth, so it was Lemna trisulca that we decided to back. Even with L. trisulca present in the ‘butter packs’, 100 Wolffia units introduced on 30 July had become 291 by 13 August, and 318 by 3 September, giving multiplication values of x2.91 in 14 days and x3.18 in 35 days.  But by 4 October (66 days), there were only 11 ‘normal’ Wolffia individuals, with 67 large spherical bodies, of which 56 were colourless – apparently totally lacking chlorophyll.  In another similar plastic box, 260 Wolffia introduced on 30 July had become 954 by 12 August (x3.67 in 13 days), but by 4 October there were only 16 ‘normal’ forms plus very many large spherical bodies.  By 24 January the following year we could see no Wolffia in these two containers, but masses of green algae.  So we tipped the whole lot into our garden pond – and never saw any of them again!

We put 240 Wolffia units into a large flat glass jar with glass lid and with plenty of Lemna trisulca on 30 July; by 13 August these had become 2042 – and counting was a real chore.  This was about the highest multiplication rate (x8.5 in 13 days) we observed.  This culture survived until late September and algal growth seemed noticeably less than in uncovered containers.  But the Wolffia multiplied more slowly as the summer turned into autumn, and by the end of the month only the large spheres were visible.

There are clearly several different factors that control growth and multiplication, and survival, of Wolffia.  Over the longer term, glass containers were better than plastic, sterilised tap water better than pond water, because algal growth seemed to develop more slowly. From mid-July to mid-August multiplication under favourable conditions was most rapid, but slowed up inexorably by late August, with increasing numbers of the large spherical bodies appearing.  If these are over-wintering resting forms, they have not so far turned up again the following spring – presumably we have not yet discovered the right over-wintering conditions for them.

To our surprise, we found less difference than we expected in multiplication between open sunlight outdoors and a south-facing windowsill.  Growing Wolffia with Lemna trisulca was consistently helpful, and went some way to preventing algae from taking over.  With shortening days as summer turned to autumn, growth and multiplication of Wolffia stopped; this seasonal decline was not due to ‘staling’ of the growth medium, for when we sub-cultured ‘normal’ Wolffia individuals to fresh medium in September, they rapidly developed into the large spherical forms.  Extra lighting, for example with high-pressure mercury vapour lamps as used in horticulture, might have some effect, as it does with many cultivated plants.  But we simply do not know how best to sustain Wolffia over the winter, or even whether the thick-walled spherical bodies are indeed over-wintering forms.

Under field conditions, life in a drainage ditch is somewhat fraught; a thirsty Dobbin or herd of cattle could swallow the lot and not notice.  The 1:25000 OS map clearly shows the drainage ditches connected up, and leading to the R. Arun and thence to the sea.  Many ditches have a hinged sluice flap where the final ditch empties into the river, and at one spot certainly this was where a high concentration of duckweeds, including Wolffia, occurred. But a sudden downpour could sweep all of them over the rim of the sluice and into the river; they must surely have safer refuges for over-wintering.  From a conservation standpoint, it is easy in July/August to turn a few hundred Wolffia units into many thousands, which could be restored to sites that have lost them.  If only we knew how to tide them over winter!