Growing Rhododendron Maximum Is Easy—Here's How to Do It Like a Pro
Arx Fortis / Wikimedia Commons / CC BY-SA 3.0
Rhododendron maximum is a hardy evergreen rhododendron native to the northeastern U.S. and abundant in the Great Smoky Mountains.
It's a multi-stemmed shrub frequently found growing in dense thickets and featuring clusters of large, (1- to 3-inch) flowers, with orange or olive green spots. Oblong, 4- to 8-inch, leathery, dark green leaves are rusty orange underneath. Rhododendron maximum is toxic to people and animals.
Common Name | Great Laurel, Rosebay Rhododendron |
Botanical Name | Rhododendron maximum |
Family | Ericaceae |
Plant Type | Broadleaf evergreen |
Mature Size | 5'-15' tall, 5'-12' wide |
Sun Exposure | Part to full shade |
Soil Type | Organically rich, moist, well-drained |
Soil pH | 4.5 to 6.0 |
Bloom Time | June, July |
Flower Color | Reddish purple, pink, white |
Hardiness Zones | USDA 3 to 7 |
Native Areas | North America |
Toxicity | Toxic to people and animals |
Rhododendron Maxiumum Care
Finding the right outdoor conditions for this acid-loving plant is essential. It's rabbit resistant with an open form that suits natural areas including woodland margins and shrub borders.
Provide dappled shade with two hours or less of direct morning light.
Plant in well-draining acidic soil with a pH of 4.5 to 6.0
Fertilize in spring with a formula specified for rhododendrons or acid-loving plants.
Protect from late winter frosts for best flowering
Light
Choose a location in dappled shade. This rhododendron will grow in nearly complete shade but flowering may be sparse. Direct sunlight, especially hot afternoon sun will burn foliage. Shrubs should receive no more than two hours of direct morning sunlight daily.
Soil
The right type of soil is critical so it's a good idea to test pH before planting rhododendron maximum, and lowering it to appropriate levels, when necessary. Consider planting in raised beds instead of heavy clay soils. Add compost or other organic materials to enrich and aerate soil for adequate drainage.
Water
Water new plants twice weekly in the first year. Once established, water only during dry periods lasting two to three weeks. A shallow root system makes consistent moisture a factor, but soggy soil damages the fine, hairlike roots and can lead to root rot. Mulch with wood chips, pine needles, or straw to improve moisture retention.
Temperature and Humidity
Fully hardy in USDA zones 3 to 7, Rhododendron maximum tolerates temperatures to -30 degrees F. Mulch protects roots, however foliage may need additional winter protection. Heat is the greater concern with ideal temperatures between 45°F and 60°F. Humidity levels between 40 and 60 percent provide optimum health.
Fertilizer
Little to no fertilization is needed except in poor soil. Rhododendron maximum needs nitrogen for healthy foliage and phosphorous for flowering. Apply organic nitrogen fertilizer in spring according to product directions. In cold climates withhold fertilizer after June.
Types of Rhododendron Maximum
Rhododendron maximum is a single species in the genus. Here are some other cold-hardy, evergreen rhododendrons with exceptional foliage and flowers.
Rhododendron 'April Rose': Early-blooming evergreen with double, purplish-red flowers up to 2 inches. Deep green oblong leaves turn reddish-bronze in autumn. Hardy in USDA zones 4 to 8.
Rhododendron 'Nova Zembla': Highly prized for vibrant red blooms with maroon splotches from late spring to early summer. Tolerant of heat and sun. Hardy in USDA zones 4 to 9.
Rhododendron 'PJM Elite': Rounded clusters of reddish-purple, trumpet-shaped flowers from late spring to early summer. Oblong, glossy, dark green leaves turn reddish-purple in autumn. Hardy in USDA zones 4 to 8.
Pruning
Many types of rhododendrons look best left to grow naturally with little to no pruning. Remove dead and damaged branches and save heavy pruning for late winter when shrubs are dormant. Rejuvenate an overgrown shrub over several seasons by cutting back selected branches up to one-third. Light pruning is usually done after the bloom period, however for shrubs that bloom heavily, thinning buds prior to flowering can improve foliage. Remove branches that cross or touch the ground to improve air circulation and discourage pests.
Propagating Rhododendron Maximum
Propagation is done with stem cuttings or by layering. Stem cuttings are taken in autumn and root fairly quickly. Layering is started when the shrub is actively growing but can take as long as two years to develop into a viable plant.
New Plants From Stem Cuttings
Materials include sterile planting media, small hand pruner, sharp, sterile knife or razor blade, a flat or small pots, rooting hormone, and acid-based plant food. Follow these steps:
Use the hand pruner to remove a stem cutting 6- to 8-inches long.
Pinch out the terminal bud and any flowerbuds and remove all but the top set of leaves. Cut those in half.
Use the knife or razor blade to make two vertical slits about 1/4 to 1/2 inch long at the bottom of the stem. Cut just deep enough to remove a thin layer of green bark.
Take a fresh cut from the bottom of the stem and immediately dip in rooting hormone to cover to the top of the slits.
Fill a flat or small pot with a 50-50 percent mix of dampened sphagnum moss and perlite or vermiculite.
Use your index finger or a pencil to make a hole in the planting medium and insert the cutting carefully to retain the hormone powder.
Cover with a plastic dome or bag supported by a stake to keep plastic from contacting the cutting.
Place in a warm location that receives indirect light. Bottom heat of 70 to 75 degrees F. encourages root growth.
Rotate the container once or twice a week and spritz the planting medium to keep it evenly moist.
Evergreen rhododendrons root in about six weeks. Give the cutting a gentle tug to check for resistance.
Once roots have formed, pot cuttings up in a mix of 60 percent moss and 40 percent perlite or vermiculite. Fertilize monthly with an acid-based plant food. Pinch out terminal buds to encourage branching.
Tip
Take stem cuttings in the morning when they contain the most moisture. Softwood cuttings root more easily than hardwood stems, however they are more vulnerable to fungal infections. Treat with rooting hormone that includes a fungicide.
How to Grow Rhododendron Maximum From Seed
Rhododendron maximum is not a hybrid, so plants started with seed are identical to the original. Collect seeds in autumn after they turn brown, allow them to dry, and store them for up to a year in a glassine envelope.
The germination rate is excellent and sprouts appear in three to eight weeks. Gather together a sterile potting mix, pot with drainage holes, and plastic bag then follow these steps:
Sterilize a 50-50 mix of sphagnum moss and perlite or vermiculite in boiling water and allow it to cool.
Fill a pot with this planting material and place seeds on top. Do not cover seeds with potting mix.
Cover the pot with a plastic bag supported by a stake to keep plastic from contacting the soil.
Place in a location that receives indirect light and warm temperatures between 65°F and 75°F.
When the first set of true leaves appear, sprouts can be planted into individual pots with a 60-40 mix of moss and perlite or vermiculite.
Keep seedlings out of direct sunlight and continue to grow out in pots.
Begin fertilizing monthly during the growing season with a diluted acidic rhododendron or plant food.
Expose the seedlings to colder temperatures slowly as young shrubs are more vulnerable to frost damage.
It may take up to two years before new seedlings can be transplanted into the garden.
Overwintering
Protect Rhododendron maximum from strong winter winds, heavy snow and ice. A thick layer of mulch is adequate to safeguard the shallow root system.
If heavy winter weather is predominant in your area, choose a sheltered location or consider caging your shrub with chicken wire and insulating branches and foliage with straw.
Common Pests and Plant Diseases
Rhododendrons are vulnerable to a number of insect pests and diseases. Choosing the right environment and maintaining healthy shrubs helps limit some of the following problems.
Pests include aphids, borers, lacebugs, leafhoppers mealybugs, mites, nematodes, scale, thrips and whitefly. In cases of infestation, apply an appropriate pesticide following label directions.
This ornamental shrub is susceptible to canker, crown rot, root rot, leaf spot, rust and powdery mildew. Plant so roots lay just below the soil surface. Develop good pruning practices and deadhead spent flowers. Apply fungicide when needed for leaf spots, rust and powdery mildew.
How to Get Rhododendron Maximum to Bloom
Rhododendron blooms profusely with the right growing conditions. Add organically rich compost to the soil and support flowering with an acid-based fertilizer in spring.
Bloom Months
Buds appear in late spring opening to abundant flowers in June and July.
How Long Does Rhododendron Maximum Bloom?
Length of bloom period is dependent on weather conditions but flowers often last about three weeks.
What do Rhododendron Maximum Flowers Look and Smell Like?
Flowers are lightly fragrant attracting pollinators including bees, butterflies, and hummingbirds. Multiple 1- to 3-inch, bell-shaped flowers, appear in flat rounded groupings called umbels in reddish purple, pink, and white with orange or olive green spots.
How to Encourage More Blooms
Protect buds from late spring frosts which are the foremost cause of poor blooming. Apply fertilizer formulated for rhododendrons and acid-loving plants in early spring.
Caring for Rhododendron Maximum After It Blooms
Other than removing spent flowers, Rhododendron maximum does not require much attention after it blooms. Dead and damaged branches should be removed but avoid heavy pruning during the growing season. It promotes new growth that won't reach maturity before dormancy and will be vulnerable to winter damage.
Keep shrubs free from weeds but avoid mechanical cultivation which can damage shallow roots. Water during periods of drought lasting two weeks or longer.
Deadheading Rhododendron Maximum Flowers
Deadhead faded and spent flowers by clipping them at the stem base.
Common Problems With Rhododendron Maximum
Yellowing Foliage
Chlorosis occurs when soil lacks certain nutrients, most often nitrogen. Nitrogen also plays an important role in acidifying soil. If leaves appear washed out or yellowing, try spraying with organic fish emulsion or working bloodmeal into the soil around the base.
Root Rot
Soggy soil results in root rot and this shrub's shallow roots and preference for shade make well-draining soil a must. Prevention is the best cure. Transplant new shrubs with the crown slightly above soil level to improve drainage.
Leaf and Flower Galls
Galls cause distortions in leaves, flowers and stems. Protect plants in autumn and early spring with an application of dormant oil. Treat just before dormancy in autumn and just after bud break in spring.
Frequently Asked Questions
How big do Rhododendron maximum get?,
Rhododendron maximum grow from 5- to 15-feet tall and 5- to 12-feet wide. This ornamental shrub has a open form and is usually left to grow naturally without much pruning for shaping or size.
How much light does Rhododendron maximum need?
Rhododendron maximum grows best in dappled light and should receive no more than two hours of direct morning sunlight daily.
What is the difference between Rhododendron maximum and catawbiense?
Rhododendron catawbiense is found more often, but not exclusively, at higher elevations. Flowers are purple to lilac while R. maximum flowers have a reddish tinge. The undersides of R. maximum leaves are coppery orange and R. catawbiense leaves more yellow-green underneath. It also can grow larger than R. maximum, up to 20 feet tall.
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Read the original article on The Spruce.
