Garden Detective

Garden Detective: Sick rhododendron may be drowning on soggy soil

I hope you can tell me what to do to help my very sick rhododendron. A local nursery told me to hold the water, since I was watering it every other day (I’ve cut back to every four days). I cut off a branch that seemed to be infected with something, but now the rest of the bush is starting to get the same thing. It always has droopy leaves as if needing water, then the leaves turn brown and die. Do you have any suggestions?

JoAnn Drone, Foresthill

From the symptoms you describe (and follow-up photos), it appears likely that your rhododendron is suffering from over-watering (as you previously diagnosed) possibly leading to a disease that flourishes in wet conditions, according to UC master gardener Rachel Tooker.

Rhodendrons have fibrous, shallow roots that need good drainage to perform well. In the natural environment, the plants grow in soil conditions with lots of organic matter. In the clay soils of our region, drainage frequently is a problem.

By watering your rhododendron every other day, the soil must have become over-saturated, thus driving out the oxygen (found between particles in the soil structure) needed for roots to thrive. You may want to carefully monitor the area around your plant at the end of the watering cycle to see how much moisture remains in the soil. Dig down about 8 to 12 inches (near the shrub, but don’t disturb the roots) and check to see whether there may still be sufficient moisture under the surface. If there is, you may be able to wait longer intervals before you water again.

You can keep those shallow roots cool, moist and happy and extend your watering intervals by applying a light application of mulch over the soil. (Make sure the mulch is 4 to 6 inches from the base of the plant.)

Understanding how much to water your rhodie can be tricky, because just as they don’t thrive with over-watering, they also dry out easily. Because their roots tend to stay in shallow soil depths, they need regular watering (particularly during hot weather). Watering twice a week for new plants is important to establish roots. Then, as the plant matures, with good mulching practices and awareness of weather, it is possible to extend the interval to weekly or even biweekly.

If the issue with your plant is simply the amount of water, adjusting your watering practices and making sure that you are watering at the drip line and not just at the trunk may help to get your plant back on a healthy track.

Rhododendrons are affected by Phytophthora root and crown rot pathogens that thrive in wet conditions. Spores that develop into Phytophthora typically enter from the soil into the root and crown areas. The leaves of these plants may appear to be drought-stressed.

Wilting symptoms may develop first on one branch or stem, and then spread to the rest of the plant. Slow decline occurs when the roots are attacked. Rapid decline occurs when the crown is attacked. Sap or gum oozing from the diseased trunk area often is a strong indicator of the disease.

Unfortunately, Phyphthora stays present in the soil. Certain management techniques such as developing good drainage by planting in raised beds or mounded soil, monitoring the amount of water (and avoiding standing water), avoiding movement of infested soil from one area of the garden to another and selecting disease-resistant rootstocks or plant varieties can help control the disease.

For more information, send for the University of California Cooperative Extension Pest Note 74143, “Phytophthora root and crown rot.” Send a self-addressed, stamped business-size envelope to: PN 74143, UC Cooperative Extension, 4145 Branch Center Road, Sacramento, CA 95827.