Feeding Rhododendrons—Plants Should Eat when they Are Hungry

A common quandary for gardeners—when is the best time to fertilize their rhododendrons? Of course, the best period is when the plant is best able to utilize those nutrients which you are providing, but the plant growth being so complex, this period is very difficult to determine.

A clue to the best period to fertilize is to observe the way the plant grows. This can denote what is happening within the plant and even what is occurring below the soil sunface.

In general, there are two types of growth patterns observed in plants. Some plants such as Juniperus and Thuja (cedars) have indeterminate growth. Once they come out of dormancy in the spring, the shoots grow fairly steadily, with no definite growth spunts followed by rest periods. Plants of this type have root systems which grow steadily throughout the growing season, drawing essential nutrients from the soil for the above ground parts.

With rhododendrons, the growth characteristics of the upper pant of the plant are different. Like Taxus (yews) and many other plants, rhododendrons have growth flushes. In the spring, the vegetative shoots flush, usually after the flowering period. This initial flush is at first succulent, but growth slows and a hardening of the initial shoot is seen as the leaves fully expand and the tissues become mature. For quite a long period, depending on the season and species studied, the plant seems to be resting, as there is a lag phase in the growth cycle.

Then there is often another flush of growth from new vegetative buds which were forming during the lag phase. Similarly, the new shoots harden up after expanding, and vegetative and floral buds are formed for the next season’s growth and bloom.

What are not seen, of course, are the events taking place within the plant and below the soil surface within the root system.

First of all, let us start in the springtime as the frost is leaving the ground. Once soil temperatures reach 2-3°C root systems begin new growth. This root activity is taking place long before there is any vegetative growth. In fact, root growth slows, and virtually ceases as the upper plant commences growth. What is happening is that the roots begin rapid elongation when the soil temperatures are sufficiently high for proper cell division and differentiation. They are growing rapidly in order to build up critical levels of essential nutrients. These levels are sometimes termed “threshold levels”. Once the specific amounts of nutrients are reached for the plant’s needs, vegetative growth can commence. As the buds break dormancy and begin to grow, the root growth slows drastically. The growing shoots are utilizing stored nutrients which are assimilated and combined through photosynthesis into building blocks for tissue generation. As the leaves expand fully and the tissues harden the upper part of the plant enters the lag phase of the cycle.

The root system now resumes growth in order to explore new sources of nutrients within the soil matrix. Again, these nutrients are translocated from the roots and stored until the “threshold levels” are reached, for resumption of the next vegetative flush. If a single element is deficient, the threshold level for that element will not be reached, and the time lag will be longer between growth flushes, relative to a plant which has sufficient nutrients available to its root system.

In our area, on young plants, two flushes of growth are seen in a normal growing season with elepidote rhododendrons. The second flush is usually the one which forms the terminal flower bud if the plant is mature enough and has been provided with the proper nutrient levels.

Fertilizers are best utilized by the plant while the roots are actively growing. Therefore there is little benefit in feeding rhododendrons while the shoots are developing, since the roots are relatively inactive.

Application of a well-balanced fentilizer should be made shortly after the ground frost has disappeared, prior to any activity of the upper part of the plant. As mentioned, this fertilizer will be absorbed by the growing root system and stored for subsequent shoot elongation. Once the first shoot flush appears to have hardened off and the new leaves become a mature green, fertilization should be resumed to supply the necessary nutrients for the next flush of growth. Of great impontance at this feeding period is the application of a high phosphorus/low nitrogen type of fentilizer, to encourage flower bud development rather than more vegetative buds.

Another neglected period of feeding is the time of root growth which occurs in the fall, prior to true dormancy when the soil temperatures drop below 3°C. True, fertilizing in early fall can cause rhododendrons to enter the winter in a non-hardened state and can result in winter damage, but fertilizing in late fall will not predispose the plant to tissue damage from winter injury. A good indicator of timing is when deciduous plants begin to colour heavily and leaves begin to drop. The root systems are still active and will absorb the nutrients given, converting them to organic forms for use the following spring. Many plants in fact, have been shown to winter better when fed in the fall as they are then entering the winter without being under stress due to a lack of essential nutrients. When feeding in fall, the fentilizer should be diluted to half strength, since absorption is somewhat slower in the cool soil media and leaching of the full application rate wastes fertilizer.

By studying the growth behaviour of plants which exhibit this flushing type of growth, fertilization timing can best be correlated with the plant’s needs. This method is more suited to the actual behaviour of the plant in a particular season than a rigidly-dated method of application.