The average evergreen or deciduous rhododendron expands to a diameter of nearly 8 feet at maturity, so correct spacing in the garden when planting several rhododendrons is essential to their survival. The rhododendron looks best as a border or in an ornamental group. Here are tips to gauge the proper spacing for rhododendrons.
Step 1: Consider the Mature Size of the Rhododendrons
Some fast-growing rhododendrons can reach their mature size of almost 8 feet in width in just 2 to 3 years, crowding out other plants in that part of the garden. In addition, the plant height at maturity is about 25 inches. Make certain that you plan the intended space for the rhododendrons with these measurements in mind.
Step 2: Choose a Location Away from Buildings and Tree Trunks
As rhododendrons do grow to encompass a broad base, plant well away from buildings so the plant can expand equally in all directions. Avoid planting near deciduous softwood trees like maples and birches, as the shallow tree roots will draw off moisture needed by the rhododendrons.
Step 3: Count on Several Years of Sustained Growth
In the right soil, light and moisture conditions, rhododendrons can grow and bloom for nearly a decade, so planning on ten years of growth could be a challenge. Provide adequate space in the garden to divide and transplant the rhododendron at least twice during that interval. Rhododendrons transplant easily and effectively due to their shallow root systems.
Step 4: Choose Companion Plants that Can Tolerate Acid Soils
In the same garden area as your rhododendrons, you can plant tall flowers like irises, that thrive in the same acid soil conditions, with a pH rating from 5.0 to 5.5. Some ferns also make fine companion plants for rhododendrons. Choose ones whose mature fronds are tall and broad. To create an aesthetically pleasing group with the rhododendrons, choose perennials that you can move regularly and divide. You can include daffodils and larkspur in your grouping, near the outer edge of the rhododendron plantings where the soil will be somewhat less acidic.
Step 5: Plant Young Rhododendrons Well Apart
Space the young rhododendron plants you buy at least 3 feet apart in the garden area. With rapid growth in the first 2 years, you will only need to split and transplant the plants with their full root balls once after that time. As the plants reach toward each other, the massed blooms will look splendid in this part of your garden through the growing season.
Step 6: Divide and Move Rhododendrons
As your rhododendron plants start to crowd each other, divide them carefully at the roots and replant as you did the young plants with plenty of organic compost in the soil. Use forest leaf mold and peat moss to maintain soil acidity in the planting holes. Dividing should not be needed before their third full season of growth. Be sure to keep them in partial shade and water well, especiallly during hot dry weather.