Questions and AnswersThe following questions are excerpts from correspondence addressed to the Secretary of the Society.
Q. This summer the leaves of my evergreen azaleas were covered with a thick whitish mold and most of them were blistered as though some insect had damaged them. Can you tell me what this disease is and what can be done for it?
ANS. What you have described is known as the "Azalea blister-leaf." It is a fungus growth and can often be controlled by picking off and destroying all the infected leaves as soon as they are noted. If this sanitary measure is not sufficient, then advise spraying with a Bordeaux mixture of 6 lbs. copper sulphate, 2 lbs. lime and 100 gallons of water. Spray just before or just after blooming time and again in September.
Q. When should rhododendron seed be planted?
ANS. Early Spring sowing is preferred, late January or February. Light conditions are more favorable at this time and the plants will develop better. Seed will germinate in the fall, but the small plants are more subject to damping off and make poor growth during the winter months.
Q. Can you tell me what causes the following condition in rhododendron plants? A plant is growing well and is apparently healthy. All at once without apparent cause the leaves on a twig or even a full branch will wither as though the plant Deeded water, which it doesn't. The rest of the plant stays green and normal, but gradually the leaves and limb or branch dies. This has happened to several of my plants and I have lost whole sections of several of them. Tell me why?
ANS. This is known as "die-back" and is caused by a fungus, Phytophthora. It should be remembered that due to the severe frost of two years ago many limbs of plants are still being lost due to the damage of low temperatures. If the fungus Phytophthora is present little can be done to prevent eventual loss, but many times by moving the plant to a new location the fungus seems to be arrested.
Q. I have been told that one should not grow rhododendrons with primroses. Is this true?
ANS. Absolutely not, the two do very well together and compliment each other admirably. Both are attacked by the Strawberry weevil and the same remedy takes care of both plants.
Q. Is November too late for taking cuttings of my rhododendrons?
ANS. It is generally too late for the hybrids but there are many rhododendrons which can be rooted late in the fall and early winter. Of these we class the finer leaved species. The series Triflorum root very well from August to October with slight bottom heat or even in a cold frame. Others, as calostrotum and myrtilloides root well in early November and the little Lapponicums can be taken in December.
Q If a plant is growing vigorously what upsets its growing?
ANS. There are several things which can upset the growing condition of a plant. Possibly the three most common are: it can be undermined by moles which allow air into the roots through their tunnels; or the plant can be partially girdled by weevils or an over dose of fertilizer can check the growth and upset the entire development of some rhododendrons.
Q. Is it a detriment to a rhododendron if it is forced?
ANS. If a rhododendron is forced, as the commercial growers do, so that it develops a root system comparable to the top growth, then it is not a detriment. However, if the plant is forced to develop a larger top growth without the proper root system, then such feeding is harmful to the plant.
Q. Why does not the 'Earl of Athlone' make a good union with the root stock?
ANS. It does, if ponticum is used for the understock. Many rhododendrons will not graft well on certain types of understock such as fortunei or decorum. The 'Earl of Athlone' is one of these and works best on ponticum. It should be grafted as close to the ground as possible.
Q. How many rhododendrons are native to the West Coast?
ANS. There are three: R. occidentale, Western Azalea; R. californicum with its variety, macrophyllum and R. albiflorum, a rare azalea type found only in high mountains.
Q. Is it harmful to transplant rhododendrons into sawdust?
ANS. It is not advisable to set the roots of rhododendrons directly in sawdust. The sawdust should be applied on top of the ground as a mulch. Ammonium sulfate should then be used to supply the needed nitrogen which is taken from the soil by the decomposing sawdust. Please refer to our Quarterly Bulletin Vol. 5 No. 2; p. 58 for further information about Sawdust for Rhododendrons.
Q. Is it possible to have the soil too acid for rhododendrons?
ANS. Yes, most rhododendrons do best in soils having a pH reading from 4.0 to 6.0.
Q. How long should a layer be left attached to a plant?
ANS. It should be cut away from the parent plant as soon as it is well rooted.