Easy Does It
By Cecil Smith, Aurora, Oregon
Many English gardeners suggest very elaborate soil preparations for their shrubbery border. They spade or trench the whole area to be planted to a depth of two feet or better. In the process, most any type of organic material available is worked into the full depth of spading. These materials include leaves and leaf mold, sod and grass clippings and peat.
That is fine. In fact, the only objection that I have is that it is too much hard work, especially when I have to do it myself.
With this deep trenching of the whole bed, a large layer of earth in which the shrubs are planted is of the same consistency, with no break in the capillary attraction, and a fairly constant supply of moisture should flow to the root ball.
A second method advised by some writers and practiced by many amateur gardeners like myself is to dig a hole somewhat deeper and wider than the root ball, perhaps two feet deep and three feet wide, and then add considerable organic matter. That is the way I formerly planted rhododendrons.
It was observed that with this large hole type of preparation, it was very difficult to keep the root balls of rhododendrons moist during our normal period of several months with little or no rainfall and with our very low daytime humidity here in Oregon west of the Cascade mountains. It would take several years for the plants to get established to the point where they would endure a few weeks of drought without too much distress. It appears that the six inches or so of prepared soil all around and below the root ball was quickly freed of its moisture in a dry period, and that there was not sufficient contact with the surrounding soil to allow the uninterrupted capillary flow of moisture to the roots. One might almost say that the plant had been insulated against the moisture of the surrounding earth. This is more likely to be noticed with rhododendrons than with most other genera because rhododendron roots are relatively slow growing, and it would take them longer to reach the solid ground at the edge of the hole.
We have soil in our garden, the top ten or twelve inches of which is a clay loam, with a higher than average humus content, as it is "new" soil. The clay subsoil is sufficiently porous to let heavy rains trickle down through it with no run off, and holes do not fill up with rain water. Rhododendron roots seem to travel through this soil as it is at the rate of several inches a year. From this and other observations, I believe that rhododendron roots will penetrate stiffer soil than is many times believed. This is not saying, however, that they will thrive in stiff clay without large amounts of organic material added.
A third method of preparing ground for a rhododendron planting, if it needs humus, is to mix the organic material into the top eight or ten inches of soil of the whole area to be planted. This can be done with a shovel, or, if available, a power tiller. If the top soil is of medium porosity and is "new" soil, or has about the same amount of humus as "new" soil, I believe plants will do well if a hole is dug barely large enough to contain the root ball. If the root ball is small for the size of the top, it might best be lined out in a specially prepared bed having a very high humus content. and left there until a desirable root ball has grown.
When vigorous plants are put into the holes made in the unprepared ground just large enough to receive the ball, the roots have contact with solid ground on the bottom and all around the sides as well. There is no break in the capillary flow. The moisture can constantly flow from the sides and from the clay below the plant. In the case of the shallowly prepared bed, the bottom of the root ball may rest on undisturbed ground, thus allowing that part to receive moisture without a break. If in these two methods the planting is done in the early fall, the roots will have penetrated firm ground by the following June or July, and should be in an excellent condition to withstand dry weather.
Over a period of years, the roots in the shallowly prepared ground will spread rapidly horizontally, thus giving the plant an opportunity to get air from a wide surface and moisture from a wide base. With a mulch spread over this wide area of roots, constantly rotting but maintained, the rhododendrons should thrive.
The question might be asked as to how these plants would move after a number of years, by which time the root ball would have resembled a huge pancake. Several years ago I acquired a plant about seven feet tall, which had been undisturbed for many years. It was September, and the ground was quite dry. A trench was dug around the plant leaving a ball about three feet in diameter. The roots had not penetrated over ten inches deep. It was obvious that a ball of that depth and width would not hold together, and above all, it was too heavy for two of us to lift. About four or five inches of the half dry soil was removed all around, to the depth of the root ball, leaving a large number of roots up to the size of a lead pencil bristling out all around. It was not a neat looking job, but much easier to handle. Some hours later, by the time the plant was in my garden, the exposed roots, from which, of course, all the fine roots had been lost with the loose soil, were looking quite dry and unhappy. The plant did well the following summer with plenty of water. That fall, a year after planting, I dug around some of the roots which had been exposed, and found each surrounded by a mass of fiber roots about two inches long, shooting out in all directions. In a year these bare roots had become the source of a great mass of fiber roots extending clear around the plant. These would not have been there, had the exposed roots been cut off when the plant was dug.
In a climate where deep trenching and additions of large amounts of organic material might make provisions for supplemental irrigation unnecessary, I can see this expensive and laborious method might pay in the long run. If irrigation has to be provided for, I believe that in established plantings, one extra watering of a shallowly prepared bed will bring the plants through in as good condition as those in a deeply prepared bed, and almost all of the back breaking work will be eliminated. That's for me!