hoot1(z5bNS)February 22, 2006

Should I fertilize my rhodos and azleas each spring?


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morz8(Washington Coast Z8b)

Dawn, these have low nutritional requirements compared to most shrubs...are they showing some symptoms of needing your intervention or are they growing and blooming as you would like?

My own (in acidic soil, amended clay) don't receive fertilizer - mulching with compost each Spring has been enough.

    Bookmark   February 22, 2006 at 2:57PM
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rhodyman(SE PA, USDA Z6)

Plants that have been given a soil mixture rich in organic matter probably will not need feeding for several years. Do not stimulate fast growth because it produces long weak stems and few flowers. But if a plant seems weak or sickly, use cottonseed meal or a special rhododendron-azalea-camellia-holly fertilizer such as Hollytone dusted on the soil in early spring. Supplemental feeding later is not normally needed, but phosphorus and potassium may be applied any time.

Rhododendron guru Harold Greer wrote: "Oregon State University did extensive testing on peak fertilizer ratios for rhododendrons. They came out with a 10-6-4 formula. I have modified this a bit, and made it 20-12-8-8, the last 8 being sulphur. I also added some slow release nitrogen in the "20" of the mix. It has been a very successful mix for years, and is now sold to many people who swear by it. I have even sent bags of it to Alaska, where the cost of transportation is more than the cost of the fertilizer! Other mixes will work just as well, there is no one perfect secret fertilizer." Phosphate and potash do not disappear from the soil, but build up little by little with successive fertilizing. Therefore, the high phosphate formulas do not provide extra help to the plant.

For most garden situations the old rule of "once before they bloom" and "once after they bloom" is still a sensible approach. Actually the fertilizer timing has nothing to do with the time the plant flowers, it simply means once in the early spring, probably April then again in June. Fertilizing after late June in a northern climate promotes tender growth in the fall which doesn't harden off before the first frosts of winter. This gets killed by the frost. This growth may have the buds for next year's flowers on it which would also get killed by the frost. Recent research indicates that plants reasonably well supplied with nutrients, including nitrogen, are more resistant to low temperatures than those that are starved.

Magnesium is an essential element and lack of it will cause yellowish areas between the leaf veins on older leaves. If the leaves are a solid green the addition of Epsom salts (magnesium sulfate) would not be useful.

Lack of iron causes much the same symptoms as lack of magnesium, but with the younger leaves showing yellowing between the veins. Iron deficiency is frequently caused by too high a soil pH, often the result of mortar or mortar building debris in the soil near the roots. A soil test should be performed to see whether high pH is a problem and if it is the soil should be acidified. For a quick but temporary solution, ferrous sulfate can be added to the soil or chelated iron can be sprayed on the foliage, but the pH should be corrected for long-term good growth.

Calcium is also essential to good rhododendron growth. Calcium can be obtained either from gypsum or from agricultural lime. Gypsum will not raise soil pH, while lime will, therefore, lime is not generally recommended in areas with naturally alkaline soil or water.

Miracid can be more of a problem than a solution. It is a 30-10-10 fertilizer. The 30 is nitrogen which promotes foliage growth and inhibits flower bud production. For acidity, flowers of (powdered) sulfur or iron sulfate are best. Do not use aluminum sulfate since aluminum builds up in the soil and is toxic to most plants eventually. For flower bud production and hardiness, super phosphate is best. Around the base of each plant I use a tablespoon of dry sulfur and a tablespoon of dry super phosphate when a plant shows signs of problems. For a general fertilizer for rhododendrons and azaleas, Hollytone is preferred by many growers.

Do not mistake the normal wilting action caused by extreme heat or cold as an indication of a problem. It is normal and will go away when milder temperatures return. Desiccation of the roots can be serious in cold or hot conditions. Watering may be needed in winter or summer.

The chart below shows the relative availabilities of most plant nutrients at various pH levels. The width of each horizontal bar indicates maximum availability as the widest point and diminishing availability as the bar narrows. Applying nutrients in the proper balance is essential for proper plant nutrition, but maintaining the pH of the soil mix so that these nutrients are available for uptake is just as crucial. The optimum pH for rhododendrons and azalea is around 5.5-6. Too low a pH is just as bad as too high a pH.

    Bookmark   February 22, 2006 at 5:26PM
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I have azalea bushes that are old, still bloom but seem to need something. Can I feed them now, they are just starting to bloom. What is the life span of them.

    Bookmark   March 13, 2006 at 11:37PM
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morz8(Washington Coast Z8b)

Virgiia, azaleas are long lived plants when their requirements are met; the Azalea Society has documented azaleas in Japan which are hundreds of years old.

What are your azaleas showing you that makes you think they need fertilizer, it's best to address symptoms.

A lack of fertilizer may show up in a number of symptoms including stunted growth, smaller than normal leaves, light green to yellowish leaf color and early leaf drop. Before fertilizing, have a specific reason for doing so.

These same symptoms can be caused by other problems such as heavily compacted soil; stresses from insects, disease organisms and weeds, and excessively wet or dry soil.

Fertilization will not correct those problems, so be certain that you know the cause of the symptoms and treat them appropriately -- a description of your plants might help us to better answer your question.

    Bookmark   March 14, 2006 at 3:12AM
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dottyinduncan(z8b coastal BC)

Our resident Rhodie expert recommends slow release Osmocote, 20-20-20 in early Spring and in early July. It's expensive but sure is easy to use and you can't hurt the plants with it.

    Bookmark   March 15, 2006 at 10:36AM
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I have been using Scotts for Azalea's and Rhodies. The first time I used it you could really tell a difference. This year I may try the Hollytone. I am behind on some things. Weather too warm too early and now there is frost in the near future. My early bloomers are sure to be hurt and they are too large to cover. Last year the deer pruned them for me.

    Bookmark   March 15, 2006 at 9:43PM
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