Using growing degree days to control Euonymus scale

DEAR JESSICA: Do these spots on my euonymus look like trouble?

— Jim Martin,

Wantagh

DEAR JIM: Why, yes, in fact, those spots do look like trouble. They are the aftermath of an insect infestation.

Let’s backtrack: Your plant has been under attack by scale insects. You might not have noticed the white or brown specks ravaging your euonymus last summer because they tend to hide out on the undersides of leaves. But they also can cover twigs and branches. Those insects spent the better part of last year sucking the plant juices out of your plant’s leaves, which explains the dead, brown sections. And, as with all living things, what goes in must come out: Scale’s excrement is a sticky substance called honeydew, and it coats the plant’s leaves. But it gets worse: that honeydew is a perfect breeding ground for something called sooty mold — and those are the dark spots you’re seeing.

To eliminate the mold, you need to stop the honeydew, which means you need to eradicate the scale. Problem is, the female insects are overwintering on your plant’s stems and leaves as we speak, and they’re preparing to unleash a flurry of eggs in spring, which would start the cycle all over again.

Now would be a good time to inspect your plant and remove affected branches. Discard them by placing them in the trash, where they won’t be able to re-infest your shrubs. And be sure to rake up all plant debris from under and around plants. Next, you should apply a dormant horticultural oil. Timing is important, but I can’t tell you when to do it.

Let me explain: Temperature, which obviously varies from year to year, dictates the development and progression of the insect’s life cycle, so in order to attack scale at its most vulnerable stage, you’ll need to pay attention to the weather and spray your plant at between 35 and 120 growing degree days.

Growing degree days is a system used to determine precisely when different insects reach different stages of maturity. Because different stages of different pests are vulnerable to different control methods at different times, this is important to know.

Here’s how it works: On March 1, the average daily temperature is recorded, and for each degree over 50, one point is assigned. On each subsequent day throughout the season, the number of degrees over 50 is added to a running tally. If the temperature is exactly 50 degrees or below 50, then the day is assigned a score of zero. So, for instance, if on March 1 the temperature was 53, the GDD on that day would be 3. If it was 60 on March 2, the GDD would be 3 plus 10, or 13. Let’s say that on March 3, the temperature was 49. That would add nothing, and our accumulated growing degree days would remain at 13.

The next thing you need to do is figure out what the GDD is today. But don’t panic, the Cornell Cooperative Extension of Suffolk County can help. They post an updated accumulated GDD report every day, and you can find it here: ccesuffolk.org/agriculture Find the town closest to yours and go with that score. When it reaches 35, you can apply your dormant oil, and you can do so on any day until the GDD hits 120. This window typically falls around mid-April.

Hopefully, that will do the trick, but if you see signs of the insects in early June, apply horticultural oil (not dormant oil) at that time and again in midsummer (no need to track days for the summer applications).

When applying liquid pesticides, do not use a hose-end sprayer, as it will not mix, dissolve or apply the product correctly. Instead, use a hand pump or a power sprayer.

DEAR JESSICA: Can you help me with my Endless Summer hydrangeas? If I ask 12 people when and how to trim them, I get 15 answers. When do I trim them and how much?

— Betsy Casser,

Port Washington

DEAR BETSY: Endless Summer hydrangeas are similar to Hydrangea macrophylla, except they bloom on both old and new growth, so they’re even easier. That means there isn’t really a bad time to prune them because if you remove buds during the growing season, plants will just make more. Technically, they don’t even need pruning unless stems are broken, dead or unruly, or if plants are overgrown.

In spring, you’ll want to remove dead stems left over from last year, but it’s best to wait until the plant leafs out and you can be sure of what’s actually dead versus what’s dormant.

When you prune overgrown or unruly mature plants, just remove weaker, misshapen or wayward stems from the base of the plant.