Neglected to fertilize this year

kimcocoJune 29, 2009

I haven't yet fertilized my rhody this year, I'm assuming I should have done this early spring...I have Rhody Cunninghams White. Lots of buds last year (1st year, no blooms), no buds this year that I've seen, but nice new growth, some winterburn.

The only thing I did this year was add small amount of gypsum to the soil...I was told this would increase acidity and promote looser soil in future years.

Should I fertilize now? No fertilizing after June for my zone?

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luis_pr(7b/8a Hurst, TX)

Except for mulch and some amendments to acidify the soil, I do not fertilize much, perhaps adding some cottonseed meal in Spring of some years. Rhodies and azaleas are not large feeders like roses and other plants so they get most of what they need from the decomposing mulch. Of course, if your soil were to be deficient of some minerals, you would need to add some "weak" fertilizers like liquid seaweed, liquid fish or coffee grounds. But the last fertilization of the year should be done right about this time (late June or sometime in July) in order to make sure that tender growth and/or buds are not killed by early freezing Fall temperatures.

    Bookmark   June 29, 2009 at 5:18PM
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rhodyman(SE PA, USDA Z6)

It is a little early to see next years flower buds. They may be there, but are not large enough to see yet. Poor bud set can be related to low phosphorus levels. Phosphorus and potassium may be applied any time.

There have been two recent research projects on fertilizing rhododendrons. One detailed project showed no difference in results no matter whether the plant was fertilized or not fertilized. That will be published in the JARS soon hopefully.

However, in the other study, Dr. Dirr of Arnold Arboretum and others found that high nitrogen levels created by chemical fertilizers killed mycorrhizae which are the microbes that help the root use nutrients in the soil. They didn't find the same problem with organic fertilizers. So once a plant is fertilized with chemical fertilizer, it may need fertilizing in subsequent years since the micorrhizae have been killed. However, if it is fertilized with a good organic fertilizer, it may recover to a point where it no longer needs fertilizing.

Holytone is an organic fertilizer, while the miracle grow products are chemical.

In general, most people don't fertilize rhododendrons and azaleas. Fortunately, the plants show when they need fertilizing. Then, if and when fertilizing, it is best to use an organic fertilizer at half the rate on the package, and, as you mention, only fertilize around bloom time, not later, unless you use a fertilizer with no nitrogen. Here are some symptoms of plants that are malnourished:

1) Yellowing of a leaf between dark green veins is called chlorosis and is usually caused by an iron deficiency. Many conditions can be responsible for an iron deficiency. Poor drainage, planting too deeply, heavy soil with poor aeration, insect or fungus damage in the root zone and lack of moisture all induce chlorosis. After these conditions are eliminated as possible causes, soil testing is in order. Chlorosis can be caused by malnutrition caused by alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency, magnesium deficiency or too much phosphorus in the soil. Iron is most readily available in acidic soils between pH 4.5-6.0. When the soil pH is above 6.5, iron may be present in adequate amounts, but is in an unusable form, due to an excessive amount of calcium carbonate. This can occur when plants are placed too close to cement foundations or walkways. Soil amendments that acidify the soil, such as iron sulfate or sulfur, are the best long term solution. For a quick but only temporary improvement in the appearance of the foliage, ferrous sulfate can be dissolved in water (1 ounce in 2 gallons of water) and sprinkled on the foliage. Some garden centers sell chelated iron, which provides the same results. Follow the label recommendations for mixing and applying chelated iron. A combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem. Chlorosis caused by magnesium deficiency is initially the same as iron, but progresses to form reddish purple blotches and marginal leaf necrosis (browning of leaf edges). Epsom salts are a good source of supplemental magnesium. Chlorosis can also be caused by nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots, root weevils or root death caused by extreme amounts of fertilizer.

2) Uniformly yellowish-green leaves is often just the need for more nitrogen. This will be more noticeable in the full sun. Some less sun tolerant varieties will always be light green in full sun.

3) Wind and cold damage is indicated when the edges of the leaves become distorted and turn brown on the side of the plant more exposed to wind and cold. Note, boron poisoning or fertilizer burn will create the same symptoms except uniformly over the plant rather than just on areas exposed to the sun and wind.

4) Rhododendrons require phosphorus, as well as nitrogen, and adequate sunlight to produce flower buds. Not always understood is the length of time required for phosphorus to reach the root system and be taken up by the plant. As long as six months may be necessary for this process. Rhododendrons which have formed few, if any, flower buds by fall should receive an application of granular phosphate some time during the winter to assure flower bud development during the following summer months. High levels of phosphorus in the soil can lead to chlorosis.

The more avid gardener should test his soil for deficiency or surplus of basic elements. Sometimes the soil of a particular location is known for excessive boron, for example, or for low phosphorus content. A further problem could be the water supply which might contain too much salt or some of the heavy metals. The zealous grower should concern himself with this potential problem.

    Bookmark   June 30, 2009 at 9:15AM
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