How to Grow Pacific Rhododendrons for Perfectly Pink Blooms Every Time
Pacific rhododendron, Rhododendron macrophyllum, belongs to the group of large-flowered, acid-loving plants including azaleas and more than one thousand species of rhododendron. What sets the Pacific rhododendron apart is where it grows. It is specific to the North American Pacific Northwest.
Considered the showiest flowering shrub in its natural environment, this evergreen rhododendron features masses of 2-inch, pink to purplish-pink blossoms in clusters above leathery, oblong, deep green leaves.
Increasingly popular in home gardens as foundation plantings, in woodland borders and prairies, this shrub is toxic to people and animals.
Fun Fact
Pacific rhododendron is Washington's state flower and puts on a spectacular native display on the Olympic Peninsula and in the Cascade mountain range from April to July.
Common Name | Pacific Rhododendron, Coast Rhododendron, California Rosebay |
Botanical Name | Rhododendron macrophyllum |
Family | Ericaceae |
Plant Type | Evergreen shrub |
Mature Size | 4 to 15 feet tall, 4 to 12 feet wide |
Sun Exposure | Part sun to part shade |
Soil Type | Moist to dry, well-drained |
Soil pH | 5.0 to 6.0 |
Bloom Time | Mid spring to mid summer |
Flower Color | Pink, rose, purple, white |
Hardiness Zones | USDA 6 to 9 |
Native Area | N. California, NW Pacific Coast, British Columbia |
Toxicity | Toxic to people and animals |
Pacific Rhododendron Care
Success with Pacific rhododendron depends on location and growing conditions. While this West Coast native grows beautifully on its own in its natural environment, you'll need to meet specific needs regarding light, soil, and temperature in the home garden.
Light
Direct sun exposure between two and four hours daily generates exceptional flowering. Naturally an understory shrub, Pacific rhododendron fares well in dappled sunlight. In the home garden, aim for a planting spot that gets morning sun and afternoon shade. Inadequate light reduces flowering.
The amount of direct sunlight affects growth habit along with bloom potential. Shrubs receiving more sun tend to stay compact. Those grown in more shade tend to grow rangier and taller.
Soil
Acidic soil is essential to grow Pacific rhododendrons successfully. These shrubs excel in low-nitrogen soils so save time and effort by testing before planting and make any necessary amendments to lower pH to between 5.0 and 6.0. Mulch with organic materials like pine bark or needles.
Choose a planting location with dry to moist, well-draining soil. This shrub does not tolerate heavy clay and will succumb to root rot.
Water
Water needs are seasonal with a greater amount needed during spring and summer and less in autumn and winter. Water new transplants consistently, about once a week to help establish a strong root system. Keep soil moist to slightly dry.
Temperature and Humidity
Pacific rhododendron flourishes in cooler temperatures between 45°F and 65°F. Protect shrubs from excessive sunlight, especially hot afternoon exposure. They withstand temperatures to -35°F, though foliage may wilt or curl.
Interestingly, these conditions reverse once temperatures return to the preferred range. Choose a location with protection from strong winter winds, mulch, and insulate plants against extended frosts and freezing. Humidity requirements are average.
Fertilizer
Feed this rhododendron in early spring with a fertilizer formulated for rhododendron, azaleas and camellias or sprinkle sulfur around the plant's base.
Pruning
Pacific rhododendrons look and bloom best when allowed to grow naturally without excessive pruning. Remove dead, damaged, and crossing branches immediately after flowering is complete. Remember shrubs grown in more sunlight stay more compact but adhere to recommended light requirements to keep plants healthy.
Older rhododendrons can sometimes be rejuvenated by cutting back to within 6 inches of the ground over several seasons. Choose one or two main trunks to cut back in early spring.
Propagating Pacific Rhododendron
Pacific rhododendron spreads naturally through seed dispersal. Propagation with stem cuttings taken during active growth is easy with a good success rate. New plants are also developed by air layering.
Propagate With Stem Cuttings
Stem cuttings are usually harvested in autumn as new green growth begins to harden off. For good results, take cuttings in the morning when stems are full of water. Gather sphagnum moss, perlite or vermiculite, small pots, plastic covering, a sterile hand pruner, a sharp knife or razor blade, and rooting hormone. Follow these steps:
Fill a flat or small pot/s with a mix of 50 percent sphagnum moss and 50 percent perlite or vermiculite. Moisten.
Use the hand pruner to remove a 6-inch cutting. Make the cut below a leaf node or in the internode space.
Remove any terminal buds and lower leaves keeping the top set. Top leaves may be cut in half to direct energy into root growth.
Use the razor blade to cut two slits in the bottom stem about 1/2- to 1-inch long. Make the cut shallow, just deep enough to open the green outer bark.
Dip the stem bottom in rooting hormone to cover two or three leaf nodes and insert the cutting in the pot center.
Cover the pot with plastic, using support to keep it from contacting the cutting.
Place in indirect light in a warm location of 70°F to 75°F and do not allow cuttings to dry out.
Rotate the pot weekly to adjust for variations in light and temperature.
Cuttings develop roots in three to four months. If resistance occurs with gentle tugging, roots have formed.
Slowly remove the plastic and transplant it into pots with a 60 to 40 mix of sphagnum moss and perlite.
Fertilize monthly and remove terminal buds to encourage branching.
Propagate By Air Layering
Start this propagation method in spring or early summer.
Select a low branch and use a razor blade or sharp knife to cut two, shallow, 1/2- to 1-inch slits on either side of the branch where it lies closest to soil.
Dip the slit portion in rooting hormone and anchor it into soil (not mulch) with a rock or landscape pin.
Once roots hold the branch in place, it can be cut from the main trunk and planted in a new location or grown out in a pot until transplanted into the garden. Rooting can take up to two years.
How to Grow Pacific Rhododendron From Seed
Seed pods develop on plants once they reach five years of age. They do not go dormant and remain viable for two years. Success growing Pacific rhododendron from seed, including saved seed, is reliable as long as soil is kept consistently moist during germination and early growth. They are light determinate, sown indoors in February, and begin flowering in about three years. Gather together sphagnum moss, perlite, small pots, and plastic coverings. Follow these steps:
Mix together 50 percent sphagnum moss and 50 percent perlite. Sterilize the mix by boiling it and allowing it to cool.
Fill a small pot or pots with this mix and sprinkle seeds on top. Do not cover with growing medium.
Do cover the pot with a plastic bag or dome for increased humidity.
Place pots in a warm room with temperatures between 65 and 75 degrees F. in a location that receives indirect light. Keep evenly moist.
Seeds germinate in three to eight weeks. Slowly remove plastic coverings exposing the seedlings to room temperature for an hour or two longer each day. Young sprouts are extremely sensitive to temperature changes.
Once seedlings are sturdy enough to be handled with one or two sets of true leaves, transplant them into individual pots.
Overwintering
In higher elevations and areas with extended freezing, protect plants with a 3- to 4-inch layer of mulch. Erect a wire cage to surround the shrub and fill it in with straw, leaves, or other insulating material.
Common Pests and Plant Diseases
Root weevils and lacebugs are two pests that damage leaves and roots. Lacebugs cause stippling on leaf surfaces and tarry excrement on the undersides. Root weevils feed on leaves at night, damaging the tips, and return to the soil during the day to feed on roots. Apply pesticide that specifically targets these pests.
Other common insects—including fungus gnats, aphids, scale, and whitefly—seldom cause extensive damage. Use neem oil or another horticultural spray or try knocking them down with a strong spray from your garden hose.
Pacific rhododendrons are vulnerable to fungal leaf spots, powdery mildew, galls, root rot, and chlorosis. Providing adequate conditions is key to prevention. Overwatering or heavy clay soils lead to root rot which is difficult to correct. Apply appropriate fungicide for leaf spot issues and acidify soil when necessary to maintain low pH.
How to Get Pacific Rhododendron to Bloom
Bloom Months
In coastal areas Pacific rhododendron blooms spring to early summer from April to June. Shrubs in higher elevations flower somewhat later beginning in May and lasting into early July.
How Long Does Pacific Rhododendron Bloom?
Flowering continues for about three weeks.
What Do Pacific Rhododendron Flowers Look and Smell Like?
Flowers are pale to deep pink and sometimes white or purple. The 2-inch, tubular flowers consist of five lobes with ruffled edges and are borne in clusters called 'trusses'.
How to Encourage More Blooms
Removing spent flowers encourages branching which results in more blooms next year. Fertilize in early spring with an acidic formula specific for rhododendrons.
Caring For Pacific Rhododendron After It Blooms
Immediately following bloom is the ideal time to prune Pacific rhododendron. Although this shrub looks best with an open form, removing dead flower clusters makes way for healthy new growth. Seed pods should be removed along with spent blossoms.
Common Problems With Pacific Rhododendron
Bud Blast
When buds turn brown and drop before opening, it's likely due to a fungal infection caused by poorly drained soil or overwatering.
Failure to Bloom
Winter injury including late freezes and heavy frosts can damage flower buds leading to a loss of flowers for the year. If extreme late-season cold is forecast, insulate plants with frost coverings.
Rust
Rust is caused by a fungal infection that leaves orangish-yellow spots on foliage. Infected leaves eventually turn brown and drop which slows growth and adversely affects the overall health of the shrub. Provide adequate air circulation and avoid wetting leaves with overhead watering.
Frequently Asked Questions
How big do Pacific rhododendrons get?
Shrubs grown in more shade get taller while those that get more sun stay compact. Mature size ranges from 4 to 15 feet tall and 4 to 12 feet wide.
How far away from the house should you plant Pacific rhododenron?
Allow 2 1/2 to 3 feet between Pacific rhododendron and buildings with concrete foundations. Concrete can affect soil pH.
How long does it take Pacific rhododendrons to grow to full size?
Pacific rhododendron grows slowly in a garden setting and may take up to ten years to reach full size. This depends on pruning methods and sun exposure.
How to Grow and Care for Rhododendron
Read the original article on The Spruce.
