Water gardening Top five plants for vegetable pond filter

What would you say are the top five plants that work well in a vegetable filter? ... Kerry Show, Kent

Answer: The top of the pops, the plant that outperforms and outsells all others is the Norfolk reed, Phragmites australis. Not only does this hoover up pollution it oxygenates as well.

Basically after that, you are after anything that grows fast and that can easily be cut down to the rootstock and disposed of, things like Cyperus longus (Sweet Galingale), Juncus effusus (Soft Rush) and Scirpus lacustris (the true bulrush). For a flora impact try the Butomus umbellatus (the flowering rush). Any of these will relish any excess nitrates and many of the other compounds in polluted water.

Here's the answer to a previous question about veggie filters

Vegetable filter (biofilter) garden aquatic plants ... We are thinking of constructing a vegetable filter this spring. Our pond is 10ft x 6ft x 5ft deep so what is the ideal size for a vegetable filter? And what are the best plants to choose and how many should we buy?

Norfolk Reed (Phragmites australis) is undoubtedly the best performer. It has the amazing ability to absorb the oxygen from the air and release it at the roots. This feeds the essential micro-organisms with the necessary oxygen for the break up of organic matter and cleaning up polluted water. It is a pretty rampant plant and needs has to be solidly contained in a robust container. Any water or marginal plants are effective to a certain extent, but fast growing ones are the best. Cyperus longus (sweet galingale),Watercress, Spearworts (Ranunculus lingua), or you could even try Water Hyacinth planted in the gravel.. There are one or two readymade systems on the market including the Oase Filtofall. The maximum pond size with fish with which it can cope is 2m and the manufacturers say you need will need to pump 5000 l/hour (1000gallons per hour) up through it. Although it seems a bit small it would lend a useful back up to a biological filter, particularly in the summer months. The water supply to the filter seems a bit excessive to me.

A larger system could be created out of concrete blocks. It could be a series of beds flowing into each other or just one large bed. The beds need to be roughly half a metre deep allowing the water to flow down past the roots of the plants. The plants would be planted at 200mmm to 250mm centres in clean gravel. I suspect for a fully stocked Koi pool of the size you have, you would need a filter bed of 20sqft, just over 2sqm (3ft x 7ft - 1m x 2m) but I think a water flow of 1000 gallons per hour (5000 l/hour/) should be quite adequate once the system is mature. Rules of thumb: (in old English)1. No more than 2inches of fish per square foot of surface area. 2.The filter system should turn over roughly half the system every hour. 3. The filter system should have roughly one third the surface area of the pool.

I have a small pond and recently all the plant watercress in it died. Please can you tell me what tests I can do to see why it might have died. Would it have died due to high nitrate levels? Andrea Fletcher, Leeds

Peter May says: Most people, in fact whole continents have the problem of watercress taking over, filling ponds and blocking waterways. It is usually quite difficult to kill once it is established and excess nitrates will make it flourish even more. However it is delicious stuff and it is not just us human beings that find it nice to eat. If you have any snails in the pool or a slug and snail problem on dry land, I would look to these as the culprits. It is very sensitive to cool air which slows down its growth, and if it is being grown like a marginal plant, it will tend to duck its foliage below the water level where it would be particularly at the mercy of water snails and even fish.

Thinking of 'duck', ducks love it too. There haven't been any itinerant quackers visiting, have there?

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