Great Laurel (Rhododendron maximum)

Great Laurel, Great Rhododendron, Rosebay Rhododendron, American Rhododendron, Big Rhododendron

Rhododendron maximum, commonly known as great laurel is a late-blooming evergreen shrub endemic to North America. It produces clusters of gentle, white or pink flowers and glossy oval leaves, typical for Rhododendron genus. Hill slopes of the Appalachian region are the natural habitat of this vigorous, hardy plant.

I. Appearance and Characteristics 

Rhododendron maximum is a species of Rhododendron native to the Appalachians of eastern North America, from Alabama north to coastal Nova Scotia. Its common names include great laurel, great rhododendron, rosebay rhododendron, American rhododendron and big rhododendron.

Rhododendron maximum is an evergreen shrub growing to 4 m (13 ft), rarely 10 m (33 ft), tall. The leaves are 9–19 cm (3.5–7.5 in) long and 2–4 cm (0.79–1.57 in) broad. The flowers are 2.5–3 cm (0.98–1.18 in) diameter, white, pink or pale purple, often with small greenish-yellow spots. The fruit is a dry capsule 15–20 mm (0.59–0.79 in) long, containing numerous small seeds. The leaves can be poisonous. Leaves are sclerophyllous, simple, alternate, and oblong (10 to 30 cm long, 5 to 8 cm wide). It retains its waxy, deep-green leaves for up to 8 years, but once shed is slow to decompose. It produces large, showy, white to purple flowers each June and July.

Seeds from rosebay rhododendron are minute and it is estimated that approximately 11 million are contained in 1 kg. Commercial seed production is generally from cultivated hybrids. Seeds from wild sources are not commonly sold commercially. Rosebay rhododendron is a slow-growing shrub and has a very high sprout potential. If mechanical removal is attempted in the case of forest management, extremely high densities are attained by this species in a matter of a few years. Prescribed fire has also been used to control this species but with limited success.

II. How to Grow and Care

Sunlight

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.

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.

Watering

Great laurel grows in well-drained, moist soils. It does best in areas that retain moisture as the roots do not like to be dry. Simultaneously, the great laurel does not like to be excessively wet. Gardeners should water regularly to maintain moist, but not sodden, soil.

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.

Fertilizing

It is important that great laurel is mulched due to its shallow roots. This will help maintain its moisture and soil temperature. The great laurel can be fertilizer post establishment with a 12-4-8 or 15-15-15 complete fertilizer.

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.

Propagation

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, 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 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 cuts up in a mix of 60 percent moss and 40 percent perlite or vermiculite. Fertilize monthly with acid-based plant food. Pinch out terminal buds to encourage branching.

How to Grow 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.

Pests and Diseases

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.

Common Problems 

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.

III. Uses and Benefits 

Rosebay rhododendron is a striking and aesthetically pleasing feature of mesic southern Appalachian forests. It is one of the largest and hardiest rhododendrons grown commercially. Several cultivars with white to purple flowers have been selected for the horticultural trade. Where it occurs naturally, it produces a showy, white, pink, or light purple flower primarily in June, but occurs from March into August. Rosebay rhododendron maintains deep-green foliage year round. This species affords protection to steep watersheds and shelter for wildlife. The wood is very hard and is occasionally used for specialty wood products.

Great Laurel (Rhododendron maximum) Details

Common name Great Laurel, Great Rhododendron, Rosebay Rhododendron, American Rhododendron, Big Rhododendron
Botanical name Rhododendron maximum
Plant type Native Plant
Hardiness zone 3a, 3b, 4a, 4b, 5a, 5b, 6a, 6b, 7a, 7b
Harvest time Fall
Height 5 ft. 0 in. - 15 ft. 0 in.
Width 5 ft. 0 in. - 15 ft. 0 in.
Sunlight Deep shade (Less than 2 hours to no direct sunlight)
Soil condition High Organic Matter
Flower color Green
Leaf color Green