Blossoming Roddi

steve276June 10, 2013

We have multiple roddis in our yard and the biggest produces a large amount of flowers. The problem is the flowers do not open up. All the others do. I feed it miracid. And it gets the same amount of rain the other plants do.

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rhodyman(SE PA, USDA Z6)

Miracid is more of a problem than a solution. It is a water soluble fertilizer high in nitrogen. It is exactly the opposite of what a rhododendron needs. First, they don't need much fertilizer. Second, nitrogen tends to hinder flowering but produces an abundance of leaves. If the leaves are green and not yellowish with green veins, then the acidity is OK.

If you must fertilize only use a fertilizer with organic nitrogen. Organic nitrogen is a slow release form, but more importantly, it doesn't kill the mycorrhiza in the soil which are necessary for good health of rhododendrons and azaleas.

Probably the best rhododendron fertilizer is HolyTone. Only use once per year around bloom time and use at half the rate on the package.

Failure of flower buds to open could be due to a number of reasons. On a mature plant if they ever bloomed they will have a few of the seed pods still here and there. If you can't find any old seed pods, then they may have never bloomed. In any case, here are a few suggestions that may help:

Bud blast. Disease may attack the buds before they open. If it is a fungal infection, the dead bud will be covered with short hair-like structures.

Bud set. The buds could be foliage buds rather than flower buds. In this case check the link below for causes of that.

Fall Blooming. Some causes of fall blooming include a dry summer and moist fall and/or warmer climates including a warm fall. For some reason, fall blooming is more common in Georgia according to Dr. Sandra McDonald. Plants which are not sufficiently hardened off or are exposed to unseasonable warm spells can start bloom prematurely. These blooms are seldom satisfactory and many times get frozen before opening fully. In any case, the seasonal bloom is lost. Avoid application of nitrogen after late spring, but phosphorus and potassium may be applied during the growing season. Application of nitrogen too late in the season can hinder hardening off in the fall.

Low temperatures. The buds could be flower buds that froze. Early frosts are especially destructive. Plants harden off progressively in the fall. If a frost occurs before hardening is complete, the frost can kill flower buds. Late frosts in the spring after dormancy is broken is also a problem. This is especially true of early bloomers such as PJM that break dormancy very early. Cold climates may be too cold for many rhododendrons. Most rhododendrons have a low temperature at which the flower buds are damaged and will not produce flowers. It varies greatly from variety to variety and somewhat from season to season. Flower buds can also be damaged by cold, dry winds, particularly when warm winter weather is followed by a period of bitter cold.

Nutrients. Improper nutrients my be a problem that affects cold hardiness and flower bud set. A few things you can do are to fertilize with phosphorus (super-phosphate) per directions to increase hardiness and flower bud set. This can be done any time. Do not use nitrogen rich fertilizers as they may inhibit flower bud production and also reduce cold hardiness. Lawn fertilizers are notoriously high in nitrogen and should be kept away from flowering plants.

Acidity. Measure the pH and acidify if necessary. Flowers of sulfur (powdered sulfur) or iron sulfate are the best chemicals to use to increase the acidity [lower the pH]. Do not use aluminum sulfate since aluminum salts build up in the soil and eventually becomes toxic to many plants including rhododendrons and azaleas. If soil is too acid, the symptoms can be the same. Very acidic soil can prevent the roots from taking up nutrients. As many of my rhododendrons are planted in very acidic forest soil, an application of Dolomite and a light top-dressing of mushroom manure in late spring is all they need. Sprinkle the lime on in late winter, very early spring. Don't overdo it - just a light sprinkle. If it is mid-spring, get the lime on right away so the rhododendron roots will be able to take up the soil nutrients in time for new growth. If you don't have rain, water it in well.

Protection. If the plants are wrapped in burlap during the winter, they may gain a few more degrees in hardiness.

Drought. When soil moisture is too low, the buds will not open as shown in photo on the right. Watering will usually resolve this condition if detected soon enough. Also, summer drought with a moist fall can contribute to fall blooming. [Photo courtesy of Harold Greer]

Deer Damage. Deer and rabbits may eat many of the flower buds as they browse in the winter, particularly if the weather is harsh and other food is scarce.

Here is a link that might be useful: Flower buds fail to open:

    Bookmark   June 12, 2013 at 9:20AM
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If the problem is buds not opening... This was a particularly on and off spring in northern New England. I found a far higher incidence of buds which looked great in March failing to open at the usual time, for exactly the reason rhodyman cited above: the buds begin to swell then experience cold low enough to cause them to abort. What is unusual is all buds not flowering.Usually, and spring was no exception, there will be some buds that survive and flower, even if they do not have the full complement of individual flowers.

    Bookmark   June 13, 2013 at 12:13PM
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Thanks for your responses. We have had this problem for over 10 years with this plant. It is the largest of all the surrounding rhoddies. It always has a full complement of flower clusters. However the individual petals do not open up. All the other plants next to it flower perfectly.

    Bookmark   June 13, 2013 at 12:38PM
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rhodyman(SE PA, USDA Z6)

It may be a seedling that never had normal flowers. What you describe is one of the characteristics of "defective" hybrids. In that case, it is genetically programmed to form these defective flowers. Stamens can be expressed as petals, maybe petals can be expressed as stamens, or at least something other than a petal.

Here is a species in which such behavior is normal:

R. spinuliferum - 4', +5F. It is an upright plant with attractive leaves with prominent veining. However, it is best known for its strange upright tubular flowers. They range in color from pink to crimson with protruding stamens and style. Spinuliferum is native to Yunnan and S Sichuan where it occurs in dry pine forests and thickets from 5,500 to 8,500 ft.

    Bookmark   June 13, 2013 at 6:27PM
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In NH you shouldn't need anything to acidify your soil. We have a great pH for Ericaceous plants like rhodies and heather.

If you can add photos of the buds in the fall or spring and what they look like while your other rhodies are blooming you may get more specific answers to your question.

Another possibility that might cause your problem is that you have a plant that doesn't have hardy enough buds, so they are getting killed by low temperatures each year. The plant as a whole can be hardy even though the flower buds aren't.

This post was edited by nhbabs on Wed, Jul 3, 13 at 18:41

    Bookmark   July 3, 2013 at 6:39PM
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