Oakleaf hydrangea has small blossoms

Asked July 3, 2019, 4:20 PM EDT

Hello. I have two oakleaf hydrangea that have been reliable bloomers with lush leaves and flowers every year. This year, one plant (in the left of the photos) looks great but the other one has sparse leaves and abnormally small flowers. Is there anything I can do?

Montgomery County Maryland

1 Response

Many plants that are showing extreme stress this year, or died, are showing the effects of abnormal rainfall last year which drowned many roots. If the site of this shrub had long periods of standing water, or fully saturated soil last year, that would account for the poor condition of the shrub. In that case, you would try to improve the drainage by diverting water.

However, in addition we see what looks like a solid plastic tarp being used for weed control (?) under the river rocks. This is going to block all moisture from reaching the root system for almost half the hydrangea. (The roots of the hydrangea reach out to the edge of its leaf canopy--and beyond.) This will effectively kill the hydrangea's roots by drying them up, and also simply prevent the shrub from getting enough water. (We are just coming off weeks of sparse rainfall.) So, this tarp may be killing the hydrangea.

If you need to put a weed barrier under the river rocks, use a breathable landscape fabric or plastic weed barrier that comes with holes. We would recommend you replace the tarp with this.

Water your shrub during dry spells. Fertilizer will not help and may hurt. Apply compost over the root area in the fall. An organic mulch is fine but always keep mulch off the trunk of shrubs and trees, and no more than 3-4" deep.