While blooms are the most noticeable parts of hydrangea shrubs, it’s the leaves that often run into problems.
Leaf problems are aesthetically undesirable and often signal the presence of a disease. Holes on hydrangea leaves, however, are most likely caused by pests.
Even though hydrangeas aren’t particularly attractive to pests, some pests are still going to cause trouble when it comes to hydrangea leaves.
I’m going to cover the most common hydrangea pest problems, how you can protect your hydrangea shrubs from them and what to do with damaged leaves.
Pests Eating Hydrangea Leaves
There are three common pests that can eat holes into your hydrangea leaves, or they can eat entire leaves, leaving hydrangeas partially or completely defoliated.
Slugs are one of the culprits when it comes to holes on hydrangea leaves. While slugs primarily feed on decaying materials, when not available, they will not shy away from hydrangea leaves.
They prefer cool, wet conditions, so the moist soil of hydrangeas is a welcoming environment for them.
Slugs are mostly active at night, so if you suspect that the holes on your hydrangea’s leaves are caused by slugs, you should check the shrubs at night to confirm.
But even in daylight, you can identify them by the trail of shiny mucous they leave behind on the leaves.
Slugs will devour leaves by working inward from the edges. Young growths can be chewed down all the way to the stem.
They can hide in crevices, under mulch, dense vegetation, and reproduce rapidly, so they can and will cause trouble if not properly dealt with.
Caterpillars are another threat to hydrangea leaves. Of all the caterpillars, the leaf-tier or celery leaf-tier moth’s larvae are especially fond of hydrangea leaves.
These primarily attack Annabelle’ hydrangea (Hydrangea arborescens ‘Annabelle’) and other smooth hydrangeas.
Besides munching on the leaves of hydrangeas, they also have the habit of gluing together hydrangea leaves. This is a smart defense mechanism from potential predators.
These caterpillars morph into a chrysalis in 10 days or so, then turn into adult moths after a week or so.
The moth then lays eggs again in late fall on the stem of hydrangeas, where the eggs overwinter, and in spring the whole process starts anew.
– Rose Chafers
These are a type of beetle that you’ll usually find on roses, but they also attack hydrangeas and many other flowering plants.
Unlike the other pests I discussed, rose chafers take to the blooms of hydrangeas particularly, but they also eat leaves, causing holes on them.
You can identify rose chafers (Macrodactylus subspinosus) by their tan bodies and long orange legs.
You’ll find them munching on the blooms and leaves of your hydrangeas, leaving behind large irregularly shaped holes.
Now that you can identify the pest causing holes on hydrangea leaves, let’s see what you can do about protecting hydrangeas from these pests or treating an ongoing infestation.
Protect Hydrangeas from Pests
The three main culprits causing holes on hydrangea leaves and bloom are slugs, caterpillars and rose chafers. Here’s how to get rid of each and prevent them from coming back.
How to keep slugs away:
- The soft bodies of slugs easily get hurt by sharp fragments of crushed oyster shells, sharp sand, sawdust, or diatomaceous earth (this is even lethal to them). Create a barrier of about 2-foot wide around your hydrangeas and keep it dry by replenishing it to keep slugs and snails away.
- Avoid mulch in your garden as they can attract slugs
- You can set traps (cups of milk, beer or yeast dissolved in sugar water), although some of them may attract even more slugs
- Keep ducks in your garden, they’re a natural predator of slugs
- Avoid using metaldehyde, which is a poison. It’s not very effective and can be dangerous for birds and other slug predators
- Removing slugs by hand is the easiest way to deal with an ongoing infestation, but inspect your hydrangeas at night, when slugs are most active
How to keep caterpillars away:
- Because the leaf-tier moth lays its eggs on the stems of hydrangeas in late fall and because it picks hydrangeas that grow on new wood, it’s a good idea to cut back your hydrangea in early spring as close to the ground as possible (leave only about 4 inches of the stem), thus removing any eggs that are still on the stems
- Alternatively, you can spray the stems with a biological insecticide that contains Bacillus thuringiensis kurstaki. Spray as soon as you see new growths on your hydrangea. The insecticide will kill caterpillars emerging from the eggs
- You can deal with an ongoing infestation by opening stuck leaves one by one and crushing the caterpillar
How to keep rose chafers away:
- Avoid using sandy topsoil in your garden because they lay their eggs in sandy soil
- They enjoy sunny spots and dry soil, so adding more trees and shrubs that help shade your garden also helps
- Growing a lawn with a high clover content will also help keep these pests away
- Introducing nematodes into your garden will help destroy rose chafers larvae
- An ongoing infestation can be controlled by hand-picking rose chafers or vacuuming them with a hand-held vacuum and putting them in soapy water
Hail Damage on Hydrangea Leaves
A hailstorm can leave the soft leaves of your hydrangea all torn up and shredded, making it easy to mistake for insect damage.
Therefore, a hailstorm can also cause holes on your hydrangea leaves. Unfortunately, you can’t do anything to prevent hailstorms.
Most plants, including hydrangea, will survive hailstorms, it’s just that their leaves will be damaged.
What to do With Damaged Hydrangea Leaves?
Whether from insects or a hailstorm, hydrangea leaves that are already damaged will stay damaged.
You can remove severely damaged leaves or simply allow them to fall off naturally. They’ll simply grow back healthy again.
Hydrangea leaves can suffer all sorts of damage from insects, fungi, and other natural events. Some of these leaf problems can be prevented (insects and fungi can be kept away), others are not preventable (e.g., hailstorms).
Holes in the leaves of hydrangeas are almost always caused by insects, and the three types of insects I discussed — caterpillars, rose chafers, and slugs — are the ones that cause the most damage.
Focusing on the prevention methods I described can significantly reduce their incidence in your garden.