Leaves on my Hydrangeas - Ask Extension
Is there something wrong with the leaves on my Hydrangeas? They look different than other hydrangea leaves.
Knowledgebase
Leaves on my Hydrangeas #873739
Asked June 19, 2024, 8:34 AM EDT
Is there something wrong with the leaves on my Hydrangeas? They look different than other hydrangea leaves.
Montgomery County Maryland
Expert Response
Yes, we can see a number of thrips on the foliage, and this is damage characteristic of their feeding. They feed somewhat like aphids, sucking plant juices, and can cause distorted growth and pale coloring (and eventually burnt-looking dry areas) where the tissues were damaged. Unfortunately, they can be hard to eradicate. Thrips are more often seen on annuals, perennials, and houseplants, but they can occur on trees and shrubs as well.
There are a few options for managing them, though none are perfect and a few might persist, though at that point, natural controls (predatory insects, fungi that infect insects, etc.) may be enough to keep their numbers below a damaging level.
If you aren't bothered by the potential to sacrifice flowers on the hydrangea this summer, you can cut back the plant drastically to physically remove most of the thrips population right away. Regrowth (or the current growth, if you don't prune) can be treated with a spray of either horticultural oil, insecticidal soap, or the active ingredient spinosad. (These are all lower-toxicity or organic options.)
Follow all product label directions and be aware that some foliage "burn" damage from the spray itself might occur in areas where insect damage is heavier, which is normal. If that happens and it's too much of an eyesore, just trim those individual leaves off. Sprays will need to be thorough, since direct contact with the pest insect needs to occur for the ingredient to work well. This means spraying all foliage surfaces possible, upper and lower, and repeating the treatment as often as the product label states. Avoid spraying while temperatures are above about 85 degrees or foliage damage may result (independent of the potential for "burn" from the injured tissues having those chemicals applied).
If the plant is growing near a wall, solid fence, or any other obstacle to good airflow (to keep foliage cooler), that might explain the outbreak, as such conditions can promote spider mite population booms as well. If not, it is not unheard of for some years to just have abundant thrips, though their populations won't necessarily repeat high levels every year thereafter. After the hydrangea goes dormant and frost kills the foliage, rake-up shed leaves and dispose of them in case they harbor pests or disease spores for next year. It's not a foolproof measure, since they could always blow/fly back into an area, but it will help reduce risk.
Miri
There are a few options for managing them, though none are perfect and a few might persist, though at that point, natural controls (predatory insects, fungi that infect insects, etc.) may be enough to keep their numbers below a damaging level.
If you aren't bothered by the potential to sacrifice flowers on the hydrangea this summer, you can cut back the plant drastically to physically remove most of the thrips population right away. Regrowth (or the current growth, if you don't prune) can be treated with a spray of either horticultural oil, insecticidal soap, or the active ingredient spinosad. (These are all lower-toxicity or organic options.)
Follow all product label directions and be aware that some foliage "burn" damage from the spray itself might occur in areas where insect damage is heavier, which is normal. If that happens and it's too much of an eyesore, just trim those individual leaves off. Sprays will need to be thorough, since direct contact with the pest insect needs to occur for the ingredient to work well. This means spraying all foliage surfaces possible, upper and lower, and repeating the treatment as often as the product label states. Avoid spraying while temperatures are above about 85 degrees or foliage damage may result (independent of the potential for "burn" from the injured tissues having those chemicals applied).
If the plant is growing near a wall, solid fence, or any other obstacle to good airflow (to keep foliage cooler), that might explain the outbreak, as such conditions can promote spider mite population booms as well. If not, it is not unheard of for some years to just have abundant thrips, though their populations won't necessarily repeat high levels every year thereafter. After the hydrangea goes dormant and frost kills the foliage, rake-up shed leaves and dispose of them in case they harbor pests or disease spores for next year. It's not a foolproof measure, since they could always blow/fly back into an area, but it will help reduce risk.
Miri
So I cut the larger hydrangea back to very short stems and then completely removed a smaller hydrangea next to it. Should I also spray the soil from the removed plant just in case the thrips are in the soil?
Arlene
Hello Arlene,
While some species of thrips can pupate in the soil (the life stage between juvenile and adult), there is no need to treat the soil, nor would application of an insecticide to the soil for the pupae provide much control. If any treatment will be needed, it will be the foliage of any remaining hydrangea plants that were originally infested so any thrips that rebound will be suppressed.
Miri
While some species of thrips can pupate in the soil (the life stage between juvenile and adult), there is no need to treat the soil, nor would application of an insecticide to the soil for the pupae provide much control. If any treatment will be needed, it will be the foliage of any remaining hydrangea plants that were originally infested so any thrips that rebound will be suppressed.
Miri
