What is a foliar spray and why might it be helpful to plants? Astrid Muschalla, Master Gardener, enlightened us on her approach to the use of these sprays in the home garden or orchard.

What is Foliar Feeding?

All plants require nutrients for growth, which are normally obtained from the healthy soil in which they grow and which are most often transported from the roots to the rest of the plant via the vascular system.  Foliar spraying is the application of nutrients to the foliage of a plant. The premise is based on scientific research from the 1950’s, which demonstrated that leaves can take up nutrients very quickly via stomata, small openings located on the underside of the leaf. 

Image credit: plant-magic.co.uk

A soil test can indicate when soil may be deficient in the nutrients needed by a given type of plant. Ideally nutrient deficiencies are remedied by addressing the soil condition (e.g. addition of organic material may be helpful), but in the short term a foliar spray may be considered. 

When derived from compost, foliar sprays also contain beneficial microorganisms, although this will vary with the source of compost used. These organisms may consume or out-compete pathogens present on the plant. 

Compost Tea Recipe

Astrid makes a compost tea to use as a spray. Made from well aged (> one year) compost, or better yet, vermicompost, it is a well aerated solution teeming with beneficial microorganisms. Aeration for 24-48 hours ensures aerobic bacterial growth, avoiding the unpleasant odours associated with anaerobic microbes.  Her recipe uses 500 mL compost in 20 L of water. Adding 25mL of organic molasses will feed the microorganisms in the compost and they will multiply, which is why she then dilutes 10x with water when spraying.

Foliar Feeding. Image credit: advanced nutrients.com

She targets both the upper and the underside of the leaves, trying to get at least 70% coverage, according to research by Dr. Elaine Ingham, a leading soil microbiologist. Spray every 2-4 weeks starting when the buds are almost ready to open, and stop when flowers appears. In Astrid’s garden, spraying is typically done in the evening. Mid day timing may result in leaf scorch.

The addition of a kelp product (such as Old Age Kelp 0.30-0.25-0.15 or Down to Earth brand Kelp + Potash 0-0-8) for flowering plants such as berries, peppers and tomatoes, sprayed once a week until fruit set can also be used.

Commercial Foliar Sprays

There is ongoing research into the use of commercial foliar sprays, particularly in the agricultural sector. Current research includes the use of these sprays to potentially swing plants such as orchard trees from growth to fruiting. Sprays may also be beneficial in alleviating transplant shock and to create the healthy environment a plant needs to ward off diseases.

Caution Regarding Neem Oil

On an unrelated note…. On several occasions during our Ask a Master Gardener calls, participants have enquired about the use of neem oil as a pesticide in the home garden.  The Master Gardeners of Ontario (MGOI) indicates that there are currently no registered domestic class pesticidal products in Canada that contain neem oil. The Pest Management Regulatory Agency (PMRA) of Health Canada has not certified it as safe for use. 

The full MGOI statement can be found at:
https://www.mgoi.ca/resources/Gardening-FAQs/Neem-Oil–Why-MGs-Do-Not-Recommend-It.pdf