Foliar Feeding – Folly or Wisdom?

For such a seemingly gentle world, gardening is full of controversy. After all, you only have to put two gardeners together for an animated discussion of the best way to make compost. Foliar feeding is one of the controversies.

What is Foliar Feeding?

With foliar feeding, instead of watering your fertiliser into the soil, you spray it onto your vegetables' leaves, generally from a small spray-bottle available from any garden store.

The idea grew in the 1950s, when research at Michigan State University found that "a leaf is a very efficient organ of absorption. The amounts may at first seem relatively small, but to offset this handicap, the efficiency is high." Since then, chemical companies in particular have promoted its general use, and their products, in order to boost crop health and yield.

Of course, not everyone agrees. It's suggested that, especially with home-made and organic feeds of unknown nutritional content, which don't include a surfactant in the solution to help it cling to the leaves, the feed drips off leaves far too quickly to do much good and that one of the reasons it appears to work is that it drips onto the soil and waters the plant in the normal way.

Meanwhile, in The Myth of Foliar Feeding, Linda Chalker-Scott, of Washington State University (and who also produces the Informed gardener podcasts), writes that, while not much good for general usage, it can offer a specialised, temporary remedy to nutritional deficiencies in fruit and is "best suited to intensive crop production under specific soil limitations." If you think about it, that description is pretty close to what goes on in the back garden.

What can Foliar Feeding do?

First it should be remembered that foliar feeding is never an alternative to building up a good, healthy soil. Your vegetables could never get enough of the major nutrients of nitrogen, phosphorous and potassium if you purely fed them through their leaves.

On the other hand, it is a fast way to supply micro-nutrients, such as iron and zinc, which your vegetables don't need in such quantities but which may not be so available in the soil and go a long way to keeping them healthy. If your crops are under stress from drought, pest-attack or disease, it's definitely worth giving them a squirt of encouragement. An ailing plant can perk up quite visibly after spraying with, for example, compost tea.

Is it worth spraying healthy plants, though? Well, anecdotal evidence suggests that it certainly has an effect. The Foliar Feeding thread on Garden Web follows an experiment that resulted in readers being able to pick out tomato plants that had been fed through their leaves, and those that hadn't.

What to use as Foliar Feed

Just as we can make our own fertiliser to spread around plants or water in, we can make our own foliar feeds. Any of the liquid fertilisers (or teas) mentioned in Using Home-grown Fertiliser can be used as a spray.

You'll need to strain them carefully, however, to avoid the spray nozzle becoming blocked. I've found a triple layer of old net curtain effective and any filtered-out sludge can be added to the compost heap.

One of the most popular organic foliar feeds, however, isn't usually home-made at all (unless you have access to a sea-shore). Seaweed extract is high in trace elements (which may not have made it into your home-made fertiliser) and, being a commercial product, doesn't clog your nozzle.

There's a reason for using it, other than its nutritional value, too, and this is the reason I return to it each year. Seaweed contains natural growth stimulants and research has shown that it makes plants less susceptible to pests and diseases, including the dreaded potato and tomato blight. The reasons for this aren't completely understood, but it seems likely that it, not only makes the plant stronger, but the micro-organisms in the solution compete with the spores and bacteria that cause disease.

How to Foliar Feed

Don't make your foliar feed too strong as there's a risk of scorching the leaves when salts in the solution are left on the leaf surface. The same dilution that you use for your liquid fertiliser (i.e. diluting it to the colour of weak tea) should be safe but, the first time you use it, try spraying a few test leaves and check the result a couple of days later. If they show signs of scorch, then dilute it further and try again.

Research has indicated that water droplet size is not important as far as absorption of the nutrients is concerned. On the whole, though, heavier drops slide off leaves more easily, so, if you have a choice, a finer spray is better.

Any vegetable with leaves can benefit from a foliar spray. Those vegetables with particularly robust leaves (indicating a thick and waxy cuticle or outer layer of leaf), are unlikely to absorb as much of the feed as other vegetables with softer leaves, but there will still be some benefit. A good example of a vegetable with robust leaves is a cabbage.

The leaves of vegetables in the greenhouse are softer, therefore they should absorb the feed well.

• Spray the whole plant, and make sure you cover both the upper and lower surface of leaves.
• Don't spray in direct sunshine, as the drying of the salts in the solution may result in leaf scorch.
• Avoid watering normally until the spray has dried, and try not to spray just before rain, as extra water will dilute the solution and wash some of it away.
• When it comes to using the vegetables, then it's a good precaution to stop spraying around a month before harvest, and wash your produce well before using. If you're using a proprietary product, then follow the advice on the label.

By Helen Gazeley