Foliar application can refer to either foliar feeding or foliar pesticide application. If a pesticide or fertilizer says that you need to do a foliar application, you are being advised to put it directly on the foliage. Leaves are able to absorb some nutrients and chemicals through the pores on their surface.
The absorption takes place through their stomata and also through their epidermis. Transport is usually faster through the stomata, but total absorption may be as great through the epidermis. Plants are also able to absorb nutrients through their bark.
Foliar feeding, a term referring to the application of essential plant nutrients to above-ground plant parts, has been documented as early as 1844 when an iron sulfate solution was sprayed as a possible remedy for “chlorosis sickness.” More recently, foliar feeding has been widely used and accepted as an essential part of crop production, especially on horticultural crops. Although not as widespread on agronomic crops, the benefits of foliar feeding have been well documented and increasing efforts have been made to achieve consistent responses. For instance, foliar feeding was earlier thought to damage tomatoes but has become standard practice.
The purpose of foliar feeding is not to replace soil fertilization. Supplying a plant’s major nutrient needs (nitrogen, phosphorus, potassium) is most effective and economical via soil application. However, foliar application has proven to be an excellent method of supplying plant requirements for secondary nutrients (calcium, magnesium, sulfur) and micronutrients (zinc, manganese, iron, copper, boron, and molybdenum), while supplementing N-P-K needs for short and/or critical growth 3 stage periods. However, foliar application has been shown to avoid the problem of leaching-out in soils and prompts a quick reaction in the plant.
For all landscape and garden plants, the major pathway for nutrient uptake is by way of the roots. Leaves have a waxy cuticle, which actually restricts the entry of water, nutrients, and other substances into the plant. To a limited extent, nutrients applied to leaves can be absorbed and used by the plant, but for the major nutrients (nitrogen, phosphorus, potassium) the quantity absorbed at any one time is small relative to plant needs. That means that foliar application of these three nutrients can only supply a very small fraction of the total needed by the plant, so foliar application should be considered only a supplement to regular soil application of these nutrients.
Foliar Pesticide Application
For instance, I treated my fruit trees by foliar application for an infestation of spider mites during the summer. I used horticultural oil, which is an organic acaricide. I tried doing it in the evening when it was not as hot, but some of the leaves still burned. It is always a good idea to aim to use chemicals as little as possible unless necessary. Look for biological and mechanical controls when available.
Foliar Application Tips
Foliar feeding is generally done in the early morning or late evening, preferably at temperatures below 24°C (75°F), since heat causes the pores on some species' leaves to close. You should also make sure that the leaves do not stay wet for long periods of time as this can sometimes encourage pests and diseases like fungi.
During the summer, you should be cautious when doing a foliar application. The label will tell you what temperatures to avoid lest your leaves become scorched. It can still happen, though, even if you aim for cooler temperatures.