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Severe pruning disturbs root-to-shoot ratio

: My cape honeysuckle is growing very well but only has about one flower on it. It seems as though the flower production has been declining for the last year or two, or maybe more. Would some Miracle-Gro or some other fertilizer help? We cut it back pretty severely last winter.

A: When a plant is cut back severely, we alter what is called the root-to-shoot ratio. This means that when a plant has been growing well over a period of several years, the amount of shoots produced is balanced by the amount of roots produced. If a large amount of shoots are removed from the plant, the root-to-shoot ratio favors the roots over the shoots.

The plant responds with vigorous top growth to bring back its optimum root-to-shoot ratio. When the plant has vigorous top growth, it frequently will produce no flowers because all of the energy is going back to establishing that root-to-shoot ratio. Once the ratio comes close to its optimum, flowering will be initiated again.

If you give it fertilizer and lots of it, you will encourage even more top growth, particularly if that fertilizer is high in nitrogen. When stimulating flower production, you want a fertilizer with a high percentage of phosphorus.

My suggestion to you is to wait until this plant reaches its optimum root-to-shoot ratio (signaled by flowering) and withhold applications of fertilizers unless that fertilizer is high in phosphorus.

Q: I hope that you can answer a question about roses. A couple of my bushes have leaves that are turning yellow. No black spots, just yellow. What is causing this and how can I take care of the problem?

A: Roses are notorious for developing yellow leaves in our soils. This is primarily due to the high alkalinity or pH of our soils. Because of this high alkalinity, plants that do not originate from these types of soils need to have decomposing organic material in the soil surrounding their roots.

This is why it is important to grow roses with organic amendments applied to the surface of the soil surrounding them. What I mean by this is to use organic wood mulch so that it will slowly decompose and add organic matter to the soil. With some types of roses, even this is not enough and you will still get yellowing leaves.

When the alkalinity is too high, many of these plants develop an iron shortage in their leaves that turns them yellow. This yellowing is called chlorosis. When the chlorosis is due to iron, it is called iron chlorosis.

If we can anticipate or know this yellowing is going to appear in the coming year, it is best to apply an iron chelate to the soil in January and water it into the roots. If this application is missed and leaves begin to emerge that are yellowing, then it is probably too late to apply iron to the soil.

In this case, it is best to apply an iron chelate spray to the leaves. The iron chelate spray probably will have to be applied several times to get a good green up. You will want to apply iron chelate spray during the very early morning hours when it is still cool or it may burn the foliage.

When using iron chelate sprays, it would be best to use distilled or reverse osmosis water or you might deactivate the iron chelate. It is also wise to add a few drops per gallon of a good quality liquid detergent such as Ivory liquid, which will help the iron spray penetrate to the inside of the leaves.

Q: I was wondering why some of my rose bushes have waxy leaves that shed insecticide the moment it hits them.

A: There is a tremendous variation among plants, even when they are in the same species such as the domestic rose. That waxy surface on the outside of the leaf is called cutin and it is there to help protect the leaf from abrasion and reduce water loss from the leaf surface.

Just like roses, humans are in the same species and we also have tremendous variation. This variation comes from the parents that were used to produce the offspring. Plants will produce leaves with more cutin in the presence of strong sunlight, like in our desert climate. We can get around this waxy surface when a applying liquid fertilizers or pesticides by using an additive to the spray solution.

There are additives called adjuvants or wetting agents that can be purchased at many nurseries that help liquid sprays penetrate to the inside of the leaves. They do this by preventing the liquid spray from beading up on the surface and allowing it to enter through pores or holes in the leaves called stomates.

Otherwise you can use a high-quality liquid detergent, which will do the same thing. But, you must not use too much detergent or it can cause the spray to roll off the leaf surface without penetrating.

Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.

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