Slow Growing Rhododendron

(Question)

I have had my Rhododendron bush for 5 years and it has barely grown and has buds that haven’t opened. what can I do to help it? It used to be in a shady area but last year I moved it to a sunnier spot. It grew a little bit but seems to have stopped again. We live in southern Ontario. The new spot has good soil we bought at a garden center but our area is know for sandy soil.

(Answer)

Rhododenrons are prized for their spectacular flowers.  Hundreds of different species and hybrids exist. The most important thing to know about growing Rhododenrons is that a mildly acidic soil is essential for them to flourish and produce these flowers. They will fail to grow properly in an alkaline soil or in hot, dry situations.   It’s probably a good idea to determine the pH level of your soil.  There are commercial soil labs that will do the testing or you can purchase a home test from a good garden centre or nursery. If you need to make your soil more acidic mulch with fir and pine bark. Add pine and spruce needles if they are available. Rhododendrons greatly benefit from growing in humus rich, moisture retentive but well drained soil. Note that deep planting, poor drainage and alkalinity will cause the leaves to yellow.

According to the Reader’s Digest Illustrated Guide to Gardening in Canada, the most common reason for the buds not opening is too little light. Although these plants will grow in dappled shade they need to get 3 or 4 hours of high-intensity light every day or they will not set many flower buds.  As you’ve recently moved your plant to a sunnier location maybe it hasn’t adjusted to its new location.   Another possibility is that the buds have succumbed to some frost damage.  Alternating warm and cold spells in winter can affect bud formation. It’s a good idea to protect plants with burlap or something similar over winter. Carefully deadheading your rhododendron after blooming helps promote the formation of strong flower buds for the following year.  Allowing the development of seed heads will drain the plant of energy which otherwise could go into new flower production. You could also have a fungal problem. Something called “bud blast” spoils developing flower buds. It is thought to be associated with the activities of an insect called a leafhopper. Infected flower buds turn brown and die. Flower buds infected with bud blast should be picked off and disposed of to reduce the amount of fungal spores being released in the vicinity of the plants.

Rhododendrons are shallow rooted and therefore need to be watered frequently in prolonged dry weather. Most plants are hardy in zones 5 to 9 (Canadian). Keep them clear of other plants as they will compete for nutrients and water. There are fertilizers specifically for rhododendrons. You may wish to try these but follow the instructions carefully because you do not want to over fertilize.

Hopefully, there is something here that will work in your situation. The following websites are excellent sources of information:

https://www.rhodoniagara.org/     (Rhododendron Society of Canada – Niagara chapter)

https://www.rhododendron.org/soil.htm     (The American Rhododendron Society)