How to Plant Rhododendron

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Whether planted as a single specimen or in masses, rhododendron draws the eye.
Whether planted as a single specimen or in masses, rhododendron draws the eye. (Image: Axel Ellerhorst/iStock/Getty Images)

The evergreen or deciduous rhododendron (Rhododendron spp.) seems to welcome spring each year with its red, purple, white, orange or pink clusters of trumpet-shaped flowers that stand out against a backdrop of dark green leaves. Hardy in U.S. Department of Agriculture plant hardiness zones 4 through 8, rhododendron grows best in partial shade and fast-draining, light, acidic soil that contains plenty of organic matter. With the use of proper soil preparation and planting procedures, you can set up your new rhododendron for success.

Things You'll Need

  • Soil test kit (optional)
  • Trowel
  • Agricultural sulfur
  • Rototiller (optional)
  • Shovel
  • Compost, pine bark mulch or peat moss
  • Rake
  • Tape measure
  • Pruning shears or knife
  • Watch, clock or timer
  • Large, plastic tub or bucket
  • Garden hose
  • Wagon or wheelbarrow (optional)
  • Pine bark, hardwood or leaf mold mulch
  • 70 percent isopropyl rubbing alcohol
  • Bowl

Site Selection and Preparation

Select a planting site that contains moist, loamy soil, receives partial shade with filtered sunlight and is protected from wind. Choose a site that has 3 to 6 feet of unobstructed vertical space and 3 to 7 feet of open horizontal place, depending on the mature height and width of the rhododendron variety you wish to plant.

Remove all weeds, rocks and other debris from the planting site, and discard the removed materials. Test the pH of the site's soil by using a soil test kit, or collect soil samples and deliver them to your county's Cooperative Extension Service office for pH testing.

Check the soil test results to determine whether the soil's pH level falls within the 5.0 to 5.5 range. Spread agricultural sulfur over the planting site in early fall or early spring, at least 6 months prior to planting, if the pH level is above 5.5. Apply the sulfur at a rate of 5 1/2 pounds per 100 square feet if the soil's pH is 8.0, 5 pounds per 100 square feet if the soil's pH is 7.5, 3 1/2 pounds per 100 square feet if the soil's pH is 7.0, or 1 1/2 pounds per 100 square feet if the soil's pH is 6.5 to decrease it to 5.5. Mix the sulfur into the top 6 to 12 inches of soil with a rototiller or shovel.

Placement in the Site

Prepare the planting site in early spring after the last average frost date or in early fall after summer temperatures begin to cool. Plant rhododendron no sooner than six months following the addition of agricultural sulfur to the planting site.

Spread a 6- to-12-inch-deep layer of compost, pine bark mulch or peat moss over the planting site. Mix the compost into the top 6 to 12 inches of the soil using a rototiller or shovel. Smooth the soil's surface with a rake.

Dig a hole in the planting site that is two to three times wider than the rhododendron's root ball and 1 inch less than the root ball's height. Position the hole 3 to 7 feet from stationary objects, such as buildings and plants, using the estimated mature spread of your rhododendron species as a guide.

Cut through all strings or rope that secures any waterproof wrappings to the plant's root ball, using sterilized pruning shears or knife. Pull the wrappings slowly and carefully away from the root ball. Leave the root ball of a potted rhododendron in the plant's container.

Place the exposed or containerized root ball gently into the center of a large, plastic tub or bucket. Direct water from a garden hose into the tub or bucket, stopping when the water level is 1 to 2 inches above the root ball's top. Let the root ball soak for 5 to 15 minutes, until it stops floating or releasing air bubbles, signs of complete hydration.

Remove the rhododendron from the water. Transport the plant to the planting site via a wagon or wheelbarrow if it proves too heavy to carry.

Slip the root ball of a potted rhododendron out of the plant's container. Place the rhododendron gently on the ground next to the planting hole.

Cut through all roots growing in a circle around the root ball, using sterilized pruning shears or knife. Cut off all broken, cracked, mushy or dead roots.

Place the root ball in the bottom-center of the planting hole. Ensure that the plant is vertical and upright. Add or remove soil from the hole's bottom as needed to position the root ball's top even with or 1 inch higher than the surrounding soil surface.

Fill the bottom one-fourth to one-third of the hole with the displaced soil. Tamp the soil firmly around the rhododendron's root ball. Add another layer of soil to the hole, filling it half full. Tamp the loose soil.

Fill the hole with water from the garden hose. Wait for the water to soak into the soil completely. Add additional soil to the hole, tamping it down. Continue to add soil and tamp until the hole completely filled and level with the surrounding soil. Do not overfill the hole or cover the top of the root ball with soil.

Spread a 2- to-4-inch-deep layer of pine bark, hardwood or leaf mold mulch around the plant, covering the planting site and the top of the root ball. Keep the mulch 2 to 3 inches from the rhododendron's trunk, which helps prevent the development of rot. Slope the inside perimeter of the mulch toward the ground to direct water to the plant's root ball. Water the mulched area thoroughly to completely moisten the mulch and the soil in the top half of the planting hole.

Tips & Warnings

  • Apply at least 1 inch of water per week, keeping the mulch and root ball evenly moist but not soggy.
  • Sterilize your pruning tools by pouring one part 70 percent isopropyl alcohol and one part water into a bowl and soaking the blades for five minutes. Remove the tools from the solution and let the blades air-dry. Always sterilize your tools before and after using them to help prevent the spread of plant diseases.
  • Never use aluminum sulfate to lower soil's pH level because the aluminum may harm rhododendron.
  • Never plant a rhododendron deeper than its previous planting depth.

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