Q&A – Sparsely Blooming Rhodos

Q:  Why does my rhodo only put out one or two florets per bud instead of filling out the whole flower truss?

R. ‘Horizon Monarch’ only puts out a couple of florets per bud

 

Dr. Glen Jamieson says…

This year with its extreme cold snap on Vancouver Island in late January, following a very mild winter to that date, has been very rough on some rhododendrons that previously had been performing well, in some cases for decades. The significant drought we had last summer and fall did not help either, especially if the plants did not receive sufficient water. Some rhodos may have actually died, at least to almost ground level, while others may have buds that do not open, or only open partially, as occurred with the ‘Horizon Monarch’ mentioned above. I have both a ‘Rosa Mundi’ and a ‘Rubicon’ that this year for the first time in decades failed to have virtually all their flower buds open, although about 1% of the buds, all close to the ground, put out one or two florets per bud instead of filling out the whole flower truss. Both plants are in a more exposed location in our garden, being down by the road where they were exposed to more wind and possible greater desiccation.

Peter Cox stated that several sources mention frost, drought, and “bud-blast” (unlikely in the Pacific Northwest) as potential causes of bud failure:

Damaged flower buds and poor bud set: It is always most disappointing when fat, healthy looking flower buds either fail to open at all or only open a percentage of their buds, the rest being black and dead. Some rhododendrons regularly abort some or even all of their buds for no apparent reason. This may be due in some cases to a deficiency, perhaps magnesium, or to drought.  Reports from various places give mixed results from applying magnesium (usually as Epsom salts). By far the most usual cause of bud damage is frost. Flower buds are invariably less hardy than the rest of the plant so a really hard winter is sure to cause losses to flower buds. Early autumn frosts can damage buds that are not fully hardened off. This is a very annoying type of damage that may be overlooked and may not be noticed until the buds attempt to open in spring. Rhododendrons vary greatly in their ability to harden up enough to withstand early frost. In areas very prone to spring frosts, it is better to avoid growing plants that always burst into growth at the first sign of spring. Plants that frequently lose their first growth flush (and sometimes even their second) are liable to become stunted and rarely flower.” Reference: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 244. (from https://depts.washington.edu/hortlib/pal/rhododendron-wont-bloom)

Finally, please don’t give up on any of the cultivars mentioned above, as they are all great plants for our area.  Make sure they have everything they need to bloom next year – adequate watering and morning sun with protection, if needed, from the hot afternoon sun.  Mulch and remove damaged and dead buds, etc. if needed.

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