Foliar Feeding – Folly or Wisdom?

, written by gb flag

Spraying nutrients directly onto tomato leaves

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.

Foliar feed spray

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.

Seaweed extract can be used as a foliar spray

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

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Comments

 
"Curious about this statement "...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...." Almost everything I read commercially indicates 7-10 days, and organic home brews 3-7 days. Plants like peppers and tomatoes benefit from a good shot of fertilizer after they start to produce fruit --- and that is less than a month before harvest."
Norm on Friday 7 September 2012
" "...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...." That seems an absurdly cautious period. Anyone who doesn't wash their fruits and vegetables any better than that would imply is asking for troubles that reach well beyond any that might be presented with foliar feeding up to wthin a week of harvest. If we are talking about leafy greens any foliar feed needs to be biologically inactive anyway."
George on Friday 7 September 2012
"Hi, Norm, you're right, of course, that plants need feeding after they start fruiting. I wouldn't expect the fertiliser to be delivered by foliar feeding as a matter of course, though, especially as the major NPK nutrients are better delivered via the roots. Some feel differently, but I guess I should have made it clearer that, for me, foliar feeding is best used as an "emergency" treatment, when a plant seems to be ailing. Generally, I'd prefer to ensure that the soil is in good heart with applications of compost, comfrey etc, or liquid feed on occasion(again, though, not something that should be relied on). The seaweed spray I favour I use mainly because it seems to fend off blight and other disease and I leave off spraying it on a couple of weeks before I'm likely to be picking; it's pretty much done its job by then. I wouldn't expect to be foliar feeding very close to harvest, because applying feed at that point isn't going to achieve much. If the plant is ailing, you'll want to wait a bit longer than a couple of days for it to recover its strength. So, I don't really ever spray less than a fortnight before harvest. Having said all that, though, as far as leaving off a month before eating is concerned, yes this is rather a long time and I'd like to say I put that in to see who spotted the deliberate mistake, but I have to hold my hands up and say a plain old non-deliberate one crept in there. I personally would leave it no less than a week. What you do want to ensure, especially if you're using compost tea from compost with animal manure, is that the food is thoroughly washed before eating. There's a run-down on precautions against E coli bacteria at the end of this article http://www.ext.colostate.edu/pubs/foodnut/09369.html. "
Helen Gazeley on Tuesday 18 September 2012
"Thank you for the update clarification. Makes a lot more sense now!"
Norm on Tuesday 18 September 2012
"Am interested in the process of preparing seaweed as foliar feed.Are there other plants extracts that can be used as foliar feeds?"
Nicholas Muyale on Sunday 12 July 2015
"Hi Nicholas. You can use other home-grown liquid fertilisers as foliar feeds - e.g. nettles and comfrey. See here: http://www.growveg.com/growblogpost.aspx?id=253 "
Ben Vanheems on Monday 13 July 2015
"If your reason for foliar feeding is for plant uptake there is one important fact to take in mind. Plant leaves have mouthlike pores which are only open for brief peried during the coolest time of day usually early morning and are mostly located on the underside of leaves. These pores are called "stigmata" Look it up. I like to do this early at least before 8am. "
Jessie on Sunday 31 July 2016
"Thanks for the tip Jessie."
Ben Vanheems on Monday 1 August 2016
"Wood Vinegar Ubiquity - This season I have been using Pyrolitic acid, Bamboo Wood Vinegar, WV, as a foliar feed solution, 1 oz per Gallon, in my garden and in my home as a pesticide for ants and general spray cleaner. Thank you Michael Wittman at BlueSky Biochar. I have found - It kills ants in 30 seconds and disrupts their acid trails. Squash Bugs, aphids, lace wing are detoured, flea beetles killed Japanese beetle are disturbed and soon go back to munching. No veggie garden this year, but as a general organic Fungicide, Pesticide, Foliar Feed, Soil Wee-Beast booster ya just can't beat it for cost, utility and ease of use. At full strength even a good post-emergent Herbicide, - plants with waxy cuticle layers; American ivy, succulents, need repeated doses I had a small, 1.5 mm, black ant problem around the sink, finding food in dish scrubbers. Now I use WV-spray for everything. "
Erich on Tuesday 2 August 2016
"I have never,ever, had tomato end rot before, but one of volunteers had suggested spraying our plants with water to help keep them moist. This has caused tomato end rot it seems. Also some stems have gone black (few) so we have stopped spraying. Regulat deep watering every other day is what I've always done and never had any disease problems. Richard Good Gardeners International. www.ggi.org.uk"
Richard Higgins on Tuesday 27 September 2016
"Hi Richard. I have never heard of spraying plants to keep them moist to prevent blossom end rot. Your deep watering is a much better method, though unless you have very fast draining soil every other day seems too much. Spraying the leaves is generally NOT recommended; it invites many diseases. End rot is caused by many factors, not just calcium shortage as many people think. Too much water, too little water, cold nights during pollination, magnesium shortage, big swings in temperature, etc. I would say if what you have done in the past has been successful, then keep doing that! I am a Master Gardener in the U.S. so things may be a little different there, but tomatoes are tomatoes. :) "
Norm on Thursday 29 September 2016
"Perhaps spraying with water was intended to combat red spider mite, which can be a pest in greenhouses? It's certainly recommended for that purpose. "
Helen Gazeley on Thursday 29 September 2016
"The photo of the black marked stem which describes the injury as chemical/fertilizer burn is misleading. That is a clear case of late blight. Scorching with too strong foliar feeds will be light in colour and will mostly appear on the edges of the leave where the concentration will be the highest due to run-off. Spraying the tomato plant with water increases the risk of late blight and other fungal disease. As for blossom end rot - it is without a doubt due to a lack of Calcium. However, there are many reasons why Calcium doesn't get to where it should be. Cold weather, hot weather, irregular watering, low soil Calcium, and my pet hate, Nitrates. Too much Nitrogen causes the plant to take up more water, which is not balanced with the right amount of other nutrients. If you want great tasting tomatoes that will store longer, then cut on Nitrogen. Instead, start building the humus in the soil with green mulches, planting peas during the winter etc. Add some Humic acid to your soil. Cut out over the counter fungicides, as these also kill of the good guys in the soil. Copper being one of the bad ones. Now here is a little magic bullet for your blossom end rot, Boron (B). Most soils are deficient in B. Now you my ask , so what? Boron is the driver of the truck called Calcium. No driver, no decent fruit. Poor fruit set, hollow fruit, blossom end, reduced shelf life. It is important to know that there isn't a single remedy for curing blossom end rot, but it is preventable by adding Calsitic lime to your soil, bone meal will also do. Make sure that you have sufficient Phosphates in the soil, and then you do a foliar spray with Boron. Add a heaped teaspoon of Boric Acid (available from the drug store) to a liter of warm water. Dissolve properly. Use 10ml of the solution to 1 liter of water. Spray plants to the point of run-off, at intervals of ten days BEFORE blossom set. You can also dose the soil at each plant with 0.5L of the diluted mixture. Foliar sprays work best when applied late afternoon. If you are really serious about foliar spray, I certainly am, invest in a EC meter. On young seedlings you should have a ppm reading of 1200-1500. On productive plants you should not exceed 3000 ppm. As a previous reader commented, the underside of the leaves should be your target area. "
Wills on Friday 25 November 2016
"Hi Wills, thank you for alerting us to the error in the photograph above. We've replaced the image."
Ann Marie Hendry on Saturday 26 November 2016
"Hi. I'm new to this and read foliar spray is good. I sprayed it last night on my chilli plants with mixture of 1tsp Epsom salt and 1.5L water using mist pressure spray. This morning the young leaves on the top curled up and are looking very sad. any suggestions what can be done? Will the wilted curled leaves come back to life? Need help! Thanks."
Polly on Thursday 21 September 2017
"The strength of Epsom salts you've applied certainly shouldn't have caused any problems. Anywhere between one and two teaspoons of Epsom salts diluted in one litre of water would be absolutely fine, with regular spraying every two weeks. The only thing I can suggest is that the salts weren't properly diluted, so you may have concentrated the salts where they have curled. Have the plants now come back to life?"
Ben Vanheems on Tuesday 3 October 2017
"Thanks Ben. Maybe that is the case. I decided the wipe off the salt residue on the leaves and waited with fingers crossed. Luckily the plant did survived though the leaves dried out so I decided to cut them off. From a full grown beautiful plant now has gone baldy sadly. "
Polly on Tuesday 3 October 2017

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