Garden Q&A: What can I do for better bigleaf hydrangea blooms?

Q: My bigleaf hydrangeas didn’t bloom well this year, but they have in the past. The foliage looks OK … some typical leaf spot, but otherwise nothing amiss. What can I do?

A: No intervention this time of year will have much, if any, impact on bigleaf hydrangea (Hydrangea macrophylla) blooms for this season. Plants will soon begin developing their flower buds for next year, which will lie dormant on the stems all winter. Several factors can cause a lack of blooms, but they largely come down to weather, accidental removal of the buds, or growing conditions.

Bigleaf hydrangea blooms on old wood, which refers to growth produced the prior year. Some varieties can also bloom on new wood, which is growth produced in the current year. As an example, early summer blooms on the shrub originated from old wood, but any flowers opening in late summer and into early autumn will be from new wood. Old wood flower buds have to overwinter before opening and must rely on innate cold-hardiness or a sheltered spot in the garden to avoid damage. They are also vulnerable to deer browsing, plus susceptible to the eager gardeners wanting to cut back what appear to be dead stems.

Resist the urge to cut any hydrangea stems until leaves fully emerge by late spring, so you can tell what wood is actually dead or weakened enough to not bother keeping.

Sometimes basal growth (emerging from nearest the roots) is faster to reappear than leaves further up the stem. This is normal, so don’t let that trick you into pruning too early or you’ll windup removing lots of flowers. Any warm spell in spring seems to encourage bigleaf hydrangeas to break dormancy. Once dormancy is broken, they lose a substantial amount of cold hardiness. A spring frost can therefore kill flower buds that might have made it through winter unscathed.

What we want from hydrangea macrophylla; not always what we get. (Miri Talabac/Handout)

The variety of bigleaf hydrangea you’re growing has an impact on hardiness. Some are forced into early bloom for spring sales since the impulse for fresh color is hard to resist, where dwarfed potted plants can decorate homes like a living bouquet. These are considered “florist” hydrangea varieties because they are not intended to be reliably hardy for long-term use in the garden. Some of them may actually survive in our area and will leaf-out just fine in spring, but their flower buds are less cold-tolerant and the blooms can fail to appear. If a potted hydrangea is not sold in a nursery’s shrub department or isn’t tagged with a cultivar name whose label includes a hardiness rating, that may be a tipoff that the plant in question is not the most cold-tolerant.

If plants are well-established but a nearby shade tree has grown larger over time, the hydrangea foliage might no longer be receiving enough light to support abundant flowering. Given their dramatic wilting response to heat stress, we grow these hydrangeas in a shadier spot in gardens but they actually bloom better with more direct sun. Full-day sun would be hard to get away with, but about half-and-half would probably be a good mix of heat relief with enough light energy to fuel abundant blooms. I wouldn’t prune a healthy tree just to give a hydrangea more light, but you could move the shrub in that situation to adjust how much sun it receives.

Q: Scattered parts of my fescue lawn are looking yellow-brown while the rest of the turf is fine. (Typical summer dormancy stress in heat, but otherwise OK.) Does it have a disease, pest, or just stress?

A: A likely cause for the browning is an aptly-named disease called Brown Patch. Common in Maryland lawns, this fungal infection thrives once warm weather arrives (especially warm overnight temps) and only needs moisture from dew or high humidity to infect foliage, so can be prevalent even in a dry year. If you look closely at the lawn early in the morning when dew is present (or after a rainy night), you may see mycelium, which is the fungus body itself branching its filaments over the exterior of the grass blades next to browned patches. You can see examples of this and the characteristic infection lesion on a grass leaf in our Brown Patch on Lawns page.

Fortunately, this infection does not kill the crowns of tall fescue. The crown is the main growth point from which leaves and roots originate, so that means the plant is able to recover. You do not need to apply a fungicide (it wouldn’t cure an existing infection anyway) and can instead wait for regrowth to improve the lawn’s appearance in autumn. Other tips for reducing the risk of outbreaks, like maintaining sharp mower blades, can be found on our webpage. Summer is a great time to get those dull mower blades sharpened (or replaced if needed) before we enter fall lawn care season. Ideally, you should sharpen them at least once or twice a year. Dull blades tear the grass tips instead of making a clean cut, and those wounds increase the vulnerability of the grass to infection in addition to just making the lawn look a bit more ragged overall.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.