September is the best time of the year to transplant azaleas, rhododendrons, mountain laurel, andromeda, blueberries and other plants that thrive in acid soils. This is because these species have stopped growing new stems and leaves and are starting to generate and elongate roots.
So take advantage of fall garden center sales. If your existing plantings are too dense or wrongly placed, now is a good time to dig and transplant.
Here’s how to assure success in transplanting plants that prefer acid soils.
First, make certain that the soil you will be transplanting into is adequate. Acid soils are generally deficient in calcium and magnesium, but only a soil test of the area will correctly identify soil conditions. Each soil test should be made from a composite of five or more core samples. I rely on A&L Eastern Agricultural Laboratories in Richmond for all of my soil testing.
Plants like these also need well-drained, high-organic soils. Even if the soil test indicates an ample amount of calcium, I make it a regular practice to mix one-half cup gypsum (calcium sulfate) to the planting soil. To assure an abundance of organic matter, I also blend one-third by volume of compost or pine fines with the existing soil while blending the gypsum with the backfill. Compost adds not only organic matter but also slow-release nutrients.
Never amend the soil with peat moss, especially when transplanting rhododendrons. Peat moss holds too much water, making conditions favorable for water-borne fungi that attack the roots of rhododendrons.
All species that grow in acid soils are shallow-rooted. So never dig the planting hole deeper than the depth of the root ball. There is no need to place compost or back-fill under the root ball because of the shallow-rooting nature of these species.
If you are digging plants that need more space to grow, the outside edge of the root ball you are digging should begin mid-distance between the drip line of the branches and the stem of the plant. If the soil is dry, irrigate the plant well at least two days before you dig.
After digging, lift the plant by the root ball and not by the stem. If you are transplanting container-grown plants, after removing the plant from the container, use a sharp knife and slash the outside edge of the root ball an inch deep from top to bottom making the slashes two to three inches apart. Since most container plants are grown in soilless rooting media, slashing the root balls and pulling out some of the roots will hasten new root development.
The top of the root ball should be visible at the surface of the finished grade. Before mulching, water the plants thoroughly to settle the backfill around the roots and eliminate air pockets. A good heavy watering helps to firm the soil in place.
Apply no more than one inch of compost or pine bark mulch. Never use hardwood bark mulch because it is basic in nature and contains high levels of manganese.
Azaleas, rhododendrons, mountain laurel, andromeda, blueberries and other acid-lovers will tolerate light to medium shade, but they will produce more flowers and be more cold-tolerant in full sun. In commercial nurseries, all of these species are grown in open fields and sometimes covered with light shade in late fall simply to give the plants a better appearance for sale in the spring.
Ask Dr. Gouin your questions at DR.FRGouin@gmail.com. Please include your name and address.