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By spring, you’d expect your rhododendrons to put out showy flowers. It is the reason to plant them.
These are among the first flowering shrubs of the season. The vast majority are spring bloomers. Some earlier than others and a few won’t bloom until early summer.
By late spring, flower buds should be opening. When they aren’t, there will be a reason for it. It could be as simple as pruning too late in the season last year, or it could be more complex like growing in alkaline soil.
In all cases, when there is a problem with getting rhododendrons to flower, the leaves can tell you a lot.
Healthy rhododendrons maintain their lush, large, green leathery leaves all year. If those are discolored, burned, curling, or falling off, there’s a problem with the soil.
If it’s just a case of the flower buds not opening, it’s more likely to be the growing conditions that need to be corrected.
6 Reasons for Rhododendron Flower Buds to Drop or Fail to Open
1 – Pruning Was Done Too Late in the Season
The best time to prune rhodies is immediately after bloom. Leave it until later, and you could be pruning away next year’s flower buds.
This isn’t a plant that blooms on the current year’s growth. Flower buds develop in mid to late summer. Those are next year’s blooms. Prune it too late in the season, you’ll be removing next year’s flowers.
To avoid that, prune rhododendrons very soon after flowering.
Late summer or fall pruning is only advisable on very heavy bloomers when you deliberately want to reduce the number of flowers so that each bud can use the energy to produce bigger flowers.
Asides from removing next year’s buds by pruning late in the season, there is a bigger risk that new growth will not be hardened off for the winter weather arriving.
That can risk serious damage to rhododendrons and azaleas. When these are pruned, it spurs on growth. You don’t want new growth late in the season as the leaves will likely die when the temperatures drop.
2 – Early Season Bloomers Need Frost Protection
Most rhododendrons bloom in late spring, around the end of April in most zones. There are exceptions. PJM cultivars (named after Peter John Mezitt) flower as early as March.
Other types of early flowering rhododendrons include April Snow, Blue Baron, and Molly Fordham. These will do better with frost protection.
A late spring frost can kill the flower bud. Warmer weather brings the shrub to life, but when a sudden cold snap occurs, bud freeze will stop it from flowering.
For early flowering rhododendrons, keep an eye on the weather forecast. When a late spring frost is due, use a horticultural fleece blanket (if you have one), or anything warm to cover your shrubs.
This is one of the many tricks of the craft on how to garden in the winter. The same needs to be done to protect tender shrubs from late spring frosts.
Adding mulch around the base will only protect the roots. It’ll keep the shrub alive, but it will not protect the flower buds. Only insulating the entire shrub will do that.
For light frosts when temperatures are expected to drop to around 26oF to 32oF, an antitranspirant can be sprayed on the plant to protect it from frost damage.
The sudden temperature drop is not the main concern. It is the freezing morning dew that coats the flower buds that can stop a rhododendron from flowering. It is surprising that a plant can survive harshly cold winters, only to get damaged by light and late spring frost.
The reason that plants survive cold weather is by developing antifreeze proteins (AFPs), protecting them from cold damage. As the temperatures drop later in the summer, those aren’t present.
In the spring, temperatures warm-up, then the late spring frost takes the plant by surprise. The energy will be put into flowering and growth instead of survival.
Plant antifreeze is what to use to prevent cold damage.
Some brands of plant antifreeze include Bonide’s Wilt Stop, CloudCover®, and Wilt-Pruf. Do note that these only work on cold-tolerant plants. They’re essentially a manual intervention to top-up the ice-binding proteins (ICPs) that plants use for overwintering.
If they can’t develop their own ICPs, spraying it on won’t be of any use. They can protect rhododendrons and azaleas from light frosts from 26oF (approx. -3oC).
3 – It’s Under-Hydrated
Rhododendrons get stressed when they’re thirsty. A stressed plant will struggle to flower.
The flower buds can be there and undamaged, but the stress caused by drought will cause the plant to focus on survival instead of the flower buds opening.
Prolonged dry weather is not the only cause of rhododendrons lacking water.
A dense canopy over the shrub can cause rainwater to be deflected away from the soil. When that’s the case, manual watering will be needed.
Another easy to overlook cause is competing plants. If you have your rhododendron growing near a tree, the tree will soak up a lot more nutrients from the soil. Shrubs cannot compete with trees for soil nutrients. The tree will starve a rhodie.
Direct sunlight causes water to evaporate at a faster rate. Dappled sunlight is ideal for rhododendrons as the leaves can store the water for longer.
Fast draining soil is another concern, but it is easy to fix that problem. Mix in several inches of organic mulch to improve moisture retention. Leaf mold, shredded bark, grass clippings, perlite, and straw are all organic materials that can help the soil hold water for longer.
An early sign of drought is leaf curl on rhododendrons. When you see that happening, water the soil. Mature rhododendrons need to be watered every two to three weeks during dry weather. Tender new rhodies may need to be watered twice weekly.
4 – It’s Over-Fertilized
Over-fertilizing will stop rhodies from flowering. Nitrogen is the usual culprit.
Nitrogen overdosing is more common when there is organic mulch used.
Some sources cite nitrogen being robbed from the soil when wood-based mulch is applied. This isn’t the case. Nitrogen gets tied up in the decomposition phase. It releases nitrogen back into the soil, but until it does, the plants can develop chlorosis.
Then, if nitrogen is added to fix that, leaf burn happens once the mulch releases the nitrogen from those that the organic materials tied up. Then you’ll get leaf burn.
As with drought stress, fertilizer burn will prevent flowering. It puts the energy into leaf growth instead of flowering. The bigger issue will be the leaf discoloration.
If you suspect there is too much nitrogen in the soil, the simple fix for fertilizer burn is to flush the soil with purified water. This is easier done in container-grown rhodies. In garden soil, it’s a tricky balance to fix the problem without drowning the plants’ roots causing root rot.
The same fix applies to reviving an azalea from fertilizer burn as it is in the rhododendron genus.
When there are fertilizer issues, pay attention to the signs the plant shows.
On the topic of fertilizing rhododendrons…
Phosphorous is important, but it can also be dangerous!
Phosphorous is necessary for rhodies to flower. But it isn’t just a case of adding it. It doesn’t move through soil easily. It needs to be added when you plant rhododendrons.
After planting, phosphorus-rich fertilizer should only be added once a soil test confirms there is a phosphorous deficiency.
For that reason, do not apply bloom buster fertilizer. Those are higher in phosphorous and potassium than they are in nitrogen. It’s supposed to help with flowering but since phosphorous can’t move easily in the soil, the shrub won’t absorb it.
Timing is crucial: Rhododendrons should not be fertilized in the summer. There is a deadline. July 1st to be precise. As a cautionary measure, add fertilizer in May. It causes new growth and that needs time to harden off before winter. Late season new growth will see most of it damaged by early spring next season.
As for the type of fertilizer to use, cottonseed meal does the job.
5 – The Soil pH Needs Amending or the Shrub Needs to Be Moved
The soil pH is imperative to get right. Rhododendrons are easy to grow but they are not forgiving. They are acid-loving plants favoring a soil pH of 5.5 or less. Any higher than that will not only prevent it from flowering but also risks killing it.
The cause is not always planting in alkaline soil. Planting a rhododendron too close to a sidewalk, driveway, or building can cause lime to leach into the soil raising the pH.
If it reaches a pH of 7.5, it will be too high to fix. The only way to save a dying rhododendron when the soil becomes too alkaline is to move it 5-feet away from structures, or pot it up in a container.
Generally, rhodies need to be planted around 5-feet away from buildings.
6 – Lacking Sufficient Sunlight
Most rhododendrons are sun-loving plants requiring six hours of full sun. Any less, flowering will be affected.
The only workaround when you have to grow in the shade is to grow a winter bloomer.
Three possible varieties for growing in shade are the Christmas Cheer, Rosamundi, and the white flowers of the Snow Lady.
For full shade rhodies, true rhododendrons will struggle because of their larger flower size.
When you need to grow in the shade, drop the flower size by growing an azalea instead. Azaleas are in the same genus, just with a smaller flower petal. Those will bloom in partial and full shade. These do not need as much sunlight for photosynthesis as rhododendrons do.
Rhododendrons need at least six hours of full sun for flowering. If the location you’re growing yours in doesn’t get that, plant an azalea instead.