Robin Gale-Baker, from Sustainable Macleod, discusses wicking beds. This is one of a series of articles she has written about growing techniques (see right hand sidebar). She has also written a number of articles about growing vegetables and fruit trees.
The price of wicking beds has come down considerably in the last couple of years with the availability of corrugated beds with water tight bases. Corrugated beds do not need a pond liner, which is the expensive component of wicking beds made of macrocarpa, a salvaged wood. And they are ready made, saving on building costs. There are now so many advantages of wicking beds that they are well worth considering.
- 50% reduction in water usage compared to an in-ground or raised equivalent size bed.
- Watering (place hose in inlet) only once or twice a week in summer, less in winter.
- Waist height for easy gardening especially for people with mobility issues.
- 4-5 crops per year compared to one summer and one winter crop.
- Plants can be grown closer together and roots will grow deeper.
- Consistent and constant moisture prevents damage to roots through drying out, especially important for tomatoes.
- Leaves resist scorching even in extreme heat. A fully hydrated plant will not scorch, though the fruit may get sunburnt.
- Grows spectacular root vegetable including carrots, beetroot, radish, parsnip, turnip and kohlrabi.
- Don’t try to build a cheap wicking bed – they invariably fail. Apple crates are cheap construction of unsuitable wood and deteriorate quickly. The nails become loose as well and can puncture the lining.
- Don’t use builders plastic or normal plastic if you need a liner. They are not strong enough.
- Don’t put in a bottom outlet in the belief that you should drain your bed. Years of experience have shown that wicking beds do not normally become anaerobic. If you do feel compelled to flush the reservoir, then simply fill the reservoir so that it overflows and flush out by hose. The downside, which I have seen a number of times, is that a bottom outlet will leak, causing your wicking bed to empty. Repair of this often means removing the sand and Flo-cells or scoria as well as the soil which is a time consuming, tedious and disheartening job.
- Don’t cut the geotextile lining which separates the soil from the reservoir to fit. It must come up the sides (like a bag) to contain the soil and not let it filter down the edges into the reservoir.
- Do not top-water, except for the first week if you have planted seeds or seedlings. These need, like conventional plantings, time to establish roots to take up water. Top watering can cause fungal disease by wetting leave sand splashing spores from the soil onto lower leaves.
- Use a proven design. Water reservoir 300 ml depth, soil 250-300ml depth with inlet at the opposite end to the outlet. This is what works! Recognise that there has been much improvement in design by others since Colin Austin first invented wicking beds.
- Test your corrugated bed for water fastness by filling it with a couple of cm with water before proceeding to construction. If there is a leak, use a waterproofing compound to repair it and test again.
- Choose between using scoria, or flo-tanks (or similar) wrapped in geotextile surrounded by sand, in the reservoir. Scoria is cheaper but does not wick as well because of the size of the ‘rock’ and is a nightmare to dig out should something go wrong. Sand surrounding the flo-tanks wicks very well because of the small, dense particles it is made from but is more expensive when the cost of the tanks and cells upon which they sit are factored in (but much easier to remove).
- Use proper geotextile (not weed mat), to separate your soil and reservoir. Geotextile allows water to wick up but prevents soil dropping down into the reservoir.
- Use a good quality vegie mix for your soil – definitely not backyard soil! Add compost between crops but avoid manures.
- Remember to fill your beds once a week but check more frequently in a heat wave. The more growth you have the more water your bed will need as the bigger the plants the more water they need.