What I Have Observed About Some Rhododendron Troubles
By J. G. Bacher
Nature is well adapted in the Pacific Northwest chiefly west of the Cascade range for rhododendrons to be at their best as garden shrubs. Not only do we find several species growing in the wild here, but also the majority of the known species from elsewhere seem to do well here in a variety of locations. Yet the genus rhododendron is so immensely diversified that many of them can be grown in other parts of the country equally well.
But wherever they may be found some few pests and insects seem to go with them, to follow them so to speak, and make a gardener wonder.
However, compared to most other garden shrubs and trees, rhododendrons fare better than average in the number of troubles interfering with their success much to the relief of the gardener everywhere. It is a frequent observation however that some plants appear to remain stationary, show a lack of vim and vigor yet without trace of insects or other pests. In this instance when the foliage assumes a yellowish cast, remains abnormally small or incurved go about your garden, stop, look, and get a shovel to lift them out of the ground. Whether the season be winter, spring or fall, do it, as it won't hurt the plants provided you lift the entire ball for you will invariably find but very few roots beyond the original ball which remains very firm and intact. You will note that the earth is most likely rather heavy and too tight for the root system of this shrub. What to do now is simple. Enlarge the hole to double the size required for the ball, downwards as well as sideways, then fill this cavity with two thirds peat moss and one third sandy soil, also a good handful of cottonseed meal mixed into the combination. Now use a small pick or pointed trowel to rip up the surface of balled roots so that a new root system can be encouraged to grow. Then plant this ball back into the hole with the surface barely a fraction of an inch below the average earth level. Remove as much of the heavy soil as can be eliminated and if space remains to be filled use peat moss or old saw dust for a refill.
During dry weather spells and windy days bear in mind to water the bushes once or twice a week not just with a little sprinkling over the surface but a thorough soaking. However, bear in mind that rhododendrons are benefited by spraying over the foliage particularly at night, since this creates the right atmospheric condition for their well being. Should the season be very dry there is nothing more congenial for rhododendrons than a mulch of peat moss. As the root system is always near the very top of the soil do not cultivate around them as you might do for roses, simply leave them alone and if weeds bother pull them up by hand and let them become mulch since they produce the best natural plant food. Here and there a rhododendron may become the prey of scale insects especially in dry locations and if such plants are not treated promptly they become sickly looking on short notice. If scale of one or several sorts settle onto your foliage you will always notice them in those spots first where water seldom strikes and that is your first cue for the remedy. This consists in spraying the plants forcefully late in the evening with a strong force of water trying to dislodge the scales and discourage them by wet conditions over night. Inside of a week or two the plants will acquire a new look of vim and virility and an examination will reveal the departure of the scales that sapped the life out of the leaves and bark. However if conditions have prevailed bad for a long time it may be well to begin with, to spray your bushes freely under the foliage with a solution of Vapotone as per directions on the container. This eliminates the insects very quickly and is harmless to the plants if directions are followed. Another method where insects are firmly established is to spray the bushes during the winter, that is while dormant with regular lime and sulphur solution as used for orchards. This is a marvelous cleaner for all insects as well as their eggs and fungus troubles. The draw back to such a treatment is that the leaves retain a dirty appearance from the discoloration of the solution deposited on the foliage but which will disappear after a number of weeks.
One of our chief sources of rhododendron troubles is the so-called strawberry weevil or black vine beetle known to science as Brachyrhinus sulcatus . This beetle is of nocturnal habit and comes up from the ground at night and feeds on the young foliage by eating out semi-circular holes on the edge of young leaves. Being usually only about 5/8 of an inch long with body of brown or blackish color ¼ inch diameter, its presence is often overlooked for it drops to the ground in the morning. It may be observed early and during cool cloudy days when it delays its departure to the ground. While this foliage damage is comparatively unimportant since plants do not suffer much from it, yet in a few weeks this beetle lays its eggs at the base of these feeding plants and they generally begin to hatch out during the fall season. The small white grubs get to work devouring the bark around the stem just under the ground and as they feed in our open climate all winter long very frequently these white grubs manage to peel the bark completely around the stem and then circulation stops with the result that on the first dry windy day the foliage begins to have a wilted look much to the distress of the gardener who frequently wonders why bushes should wilt in soaking wet ground. That there is no remedy for that kind of damage is plainly to be seen for the bush is already dead before you notice it. The aggravating fact then comes to light for when you discover this state of affairs the mischief makers cannot be found as by this time they have transformed into their chrysalids or have emerged as full grown beetles and flown the coop. To my opinion the most effective remedy is to spray all the bushes that they are likely to feed on with arsenate of lead solution 1 tablespoonful to a quart of water. This poisons the adult so that no eggs will be laid for next season. However bear in mind this beetle can fly and may come into your garden from elsewhere again in the future so remain alert to its presence. It must also be said that this insect has a predilection for various perennials often grown in gardens and a favorite crop is the popular garden Primulas . It is my inclination to believe that they even prefer Primula roots to rhododendrons for one finds them there much oftener than on the azaleas or rhododendrons. If the primulas are nearby it may be good policy to lure the beetles to the primulas and take up these plants late in November and transplant them to another location. You will find the little maggots of the weevil quite easily on account of their creamy white color and, shaken loose from the primrose roots they can be destroyed before having done much harm to either plant. This may be troublesome for some gardeners but common sense is nevertheless on your side in this battle of wits with the world of insects for you will win and that's what counts.
Where rhododendrons are really difficult to bring on successfully is in those localities where water is alkaline. This insidious poison works slowly, and in lime soils it is far better to plant rhododendrons into specially prepared soils free of all lime with applications of sulfate of aluminum to help check alkalinity. It is unwise to give any exact formula, for soils are so varied that one had better make a series of trials to find out what works best, for your ideal proportion might be injurious even in the next door garden so keep your mind open on the subject of aluminum applications. Also it has been my observation many times that overdoses of nitrogenous fertilizers such as cotton seed meal may bring about a yellowing of the foliage, that reminds me to some extent of jaundice speaking from personal experience.
Lack of drainage or too tight a soil that remains wet is inclined to stunt the growth of rhododendrons but basically that is due to lack of oxygen accessibility to the root system of plants. There are however species of rhododendrons able to function in heavier soils than average, and to my observations the triflorum group with R. augustinii at its head and the heliolepis series seem to thrive in soils too heavy for the average rhododendron.
However for general success it is wise to have porous soils to plant them in, and decaying wood seems highly desirable for their root systems.