Rhododendron decline

Asked April 4, 2019, 11:24 AM EDT

Are these rhododendrons being crowded out by hostas? These 25 year old rhododendrons have been in decline the past two years. I’m trying to figure out the cause and the best path forward. They are located alongside a west facing wall of my house with a north side wall wall 20’ to the south and a 40ft pine tree to the east, keeping these plants in heavy shade most of the day. I know we’ve had some harsh winters in the recent past, but I wasn’t expecting to see such dieback or yellowing leaves after what I thought was a mild winter. Now I’m trying to diagnose the cause of the plant stress. The hostas weren’t always planted so close. They used to be about 5ft away and are now 1-2ft. Could they be crowding the rhododendrons out? I know I need to remove all the dead wood from the plants, but are there any other obvious actions I should take or other information I need to gather to assist these plants? Is the smaller one two far gone to be revived or should it just be replaced?

Washtenaw County Michigan landscape design rhododendron

1 Response

Thank you for the pictures. It looks to me like there are several things here that could be affecting these shrubs.
First - yellowing of the older leaves between the veins of the old leaves usually indicates a magnesium deficiency in the soil. If you have not done so already, I would recommend that you test the soil in this area to confirm this and also get more detailed information on the soil make up. You can purchase a soil test kit from MSU for a cost of $25. Below is the link to the soil test web site where you can click on the link to purchase through the MSU bookstore.
https://homesoiltest.msu.edu/
You can increase the magnesium in the soil by adding Epsom salts which will not affect the pH of the soil. Or you could also add lime but that will raise the pH at the same time (which may affect the health of the shrubs which are sensitive to pH levels).
Second - I am wondering if these shrubs are being affected by the heat pump or air conditioner I see in the background. Rhodos do best in shade/part shade with some wind protection so possibly the air from the equipment may be blowing on them and causing dryness in the plant and soil. I might suggest that you place a fence or barrier between the equipment and plants to better protect them from air being blown.
And last, it looks like you may have planted them too deep or added mulch too close to the shrub so that the crown is buried. The crown is the point where the above ground stem and below ground root meet. Pull back the soil/mulch from around the base of the plant and see if you can free the crown from below the soil. Never mulch right up to a plant (perennial, shrub or tree ) since this promotes diseases, encourages insects and can bury the crown.
I would not give up on the these shrubs. Prune the dead limbs off now and get your soil tested to determine what is lacking in the soil. Then begin to amend the soil as needed to ensure a healthy foundation for the shrubs. Make sure the rhodos are protected from wind (especially in winter) and give them consistent water during dry periods in the summer. I wouldn't be concerned about the hostas since their root structure is not significant enough to impact the
rhodos.

Hope that helps.