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Supplying food to plants through their leaves is a practice that’s been found to be sound, practical and necessary or desirable under certain conditions.

Lest the dirt gardener misunderstand and so disapprove of foliar feeding, it is not a substitute but, in most cases, a supplement to the food taken by plants from the soil to aid in getting more vigorous growth and greater yields of fruits and flowers.

Where Did Foliar Feeding Start?

Where it all started I cannot say for sure but one of the first introductions to the general public came from Dr. Pirone at the New York Botanical Garden where he conducted tests on hundreds of plants to determine the effects of foliar feeding.

For example… one of the experiments was with 36 Golden Muscat grape plants all rooted from the same parent six months earlier. Plants were set in individual pots containing a good potting mixture of 2 parts soil, 1 part leafmold and 1 part sand. All were grown in a greenhouse in which the temperature and humidity were carefully controlled. Precautions were taken to prevent any of the nutrient from reaching the soil so that growth, if any, could be established conclusively to be due to the foliar spray. The plants were divided into four lots of nine each. Lot one was sprayed once a week; lot two, three times and lot three, five times. The check lot, number four, was left unsprayed except for an occasional application of tap water.

The difference in vigor and growth between the sprayed and unsprayed plants was clearly visible. Sprayed plants were also a much deeper green. Chemical analysis of the foliage were made and showed that approximately 50 per cent of the nitrogen and 15 per cent of the phosphorus applied to its surface entered the leaf within one hour of spraying.

After nine weeks, plants were removed from pots and further measurements were taken. The sprayed plants showed significant increase in weight of the entire plant as well as of roots, stems, leaves and length of stems and leaves. Unfortunately, the plants were too young to bear and so there is no data on fruit yields.

Dealing with Pavers and Sidewalks

One of the most interesting tests, carried out in cooperation with the Park Department at that time, was with approximately 300 shade trees growing along the streets of New York City. Because soil around trees growing in cities is covered with paving blocks and sidewalks, it was impossible to feed them by means of the customary soil applications. To overcome this difficulty, a complete nutrient spray was applied to the foliage of Pin Oaks, London Planes, Norway Maples, Little-leaf Lindens and American Elms varying in height from 15 to 35 feet.

A 300-gallon sprayer was used. Six applications were made starting May 25 and ending August 4. The results were favorable. Sprayed trees showed a decided increase in vigor and produced larger, greener leaves. Foliage remained on the trees longer in the fall.

Low Soil Fertility

In addition to the difficulty of applying fertilizer to the soil of most city trees, it is also true that most city soils are low in fertility. Smoke, dust, toxic gases, drought, sun scorch and countless other factors also interfere with the growth and development of shade trees in the city. Nevertheless, trees in a vigorous condition are better able to withstand these unfavorable conditions and foliar feeding helped to maintain their strength despite their environment.

More Experiments

Hundreds of plants growing in the greenhouse and outdoors were used in another experiment. In the greenhouse were azaleas, Baltic Ivy, boxwoods, chrysanthemums, coleus, euonymus, fuchsias, geraniums, hollies, Japanese Barberries, lantanas and yews. Outdoors were azaleas, boxwoods, mums, grapes, London Plane trees, rhododendrons, roses, yews and tomatoes.

Again the difference in size, abundance and greenness was obvious. Furthermore, boxwoods (212 plants about 12 inches high), usually rather difficult to grow in New York, produced bushy, compact growth and deep, green foliage when sprayed. And, contrary to what usually happened, young nursery stock that had been newly transplanted suffered no setback in growth when sprayed. An increase in size, abundance and greenness of foliage was observed in the sprayed plants.

In all the tests, spraying was thorough, covering both the upper and lower surface of the leaves. A complete chemical fertilizer with a nitrogen-phosphorus-potash (N-P-K) ratio of 23-21-17 plus other minor elements was used. The trade name of the complete fertilizer used in tests was called Ra-Pid-Gro. It is was non-corrosive, completely soluble and did not clog the nozzle of the sprayer. The spray was prepared by dissolving one pound of Ra-Pid-Gro in 30 gallons of water. To increase its wetting power, a tablespoon of synthetic detergent (Dreft) was added to the water. Most of the recommended insecticides and fungicides were also mixed with the nutrient to combine fertilizing and pest control in one application. That’s the way they did it back then!

None of the sprays showed any injury to plants even when applied in the noon day sun. Best results, however, were obtained when spray was applied in the morning, late afternoon or evening. If rained upon immediately after application, it did not matter too much because the unabsorbed nutrients were washed into the soil and became available to the plants anyway.

Application of sprays is simple and requires no expensive or complex equipment. The best time to start is early in spring when soil nutrients are not so readily available. Then, continue applications every two weeks. Always follow the directions on the fertilizer label.

Foliar feeding is no cure-all for plant problems, nor, as a substitute for good garden soil and proper care of plants. But, those with the small garden or a large commercial grower will find foliar feeding a handy tool by means of which you can be sure your plants are receiving all the necessary nutrients. It is a helpful ally, a valuable stimulant, ready to increase your garden pleasure and profits. Plus it has been used successfully for decades.