Covering the Foothills & Mountains West of Denver

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How Spring Weather Affects Treatment Timing For Weeds, Insect Pests & Tree Diseases

Have you ever wondered why weeds, bugs, and plant diseases seem to appear earlier in spring some years but much later other years?

Unlike holidays and birthdays, the emergence of weeds and insect populations isn’t determined by a date on your calendar. Instead, timing is based on the weather. While there may be a general timeframe during which we typically start to notice plants growing and insects moving about, that timeframe can sometimes vary dramatically.

A cold, wet spring slows down the growth and development of plants and insects, while warmer temperatures speed things up.

As a result, the appropriate timing for weed, pest and disease prevention and control treatments will also vary from year to year.

Determining When to Apply Weed, Pest & Disease Treatments

Setting a specific date for treatments without knowing the exact weather conditions leading up to that date will often lead you to apply treatments at the wrong time.

Apply treatment products too early and the problem you’re trying to address hasn’t yet appeared. Apply it too late, and you’ve missed the window of opportunity during which the treatment will be effective. Either way, it’s a waste of time and money, as the treatment won’t have the desired effect.

So how do you know exactly when to apply treatments for maximum effectiveness?

It’s not a simple matter. The most important determinant is the number of Growing Degree Days (GDD). But weather, such as rain or snow, also plays a role. And when it comes to noxious weeds, the fertility and nutrient content of the soil also affects growth rates.

How Growing Degree Days Affect Treatment Timing

Growing Degree Days is a calculation based on the number of days (since January 1 of the current year) during which the average temperature was higher than the minimum temperature needed for a plant, pest or disease organism to develop.

Each noxious weed, destructive insect and disease pathogen has a minimum temperature below which it cannot grow. That temperature is called the base temperature or lower developmental threshold. Once the average daily temperature goes above the threshold for that organism, it starts to grow.

For most GDD calculations, 50F is used as the base temperature because most woody plants and insects start to grow or develop at that point. So each day we look at the average temperature (the maximum temperature plus the minimum temperature, divided by 2) and subtract 50 to get the GDD for that day.

If the average temperature is below 50F then we have 0 GDD. On the other hand, if the average temperature is 70F, we would have 20 GDD (i.e., 70 – 50).

Or, to make things easier, just use this handy GDD tracking tool put out by the Colorado Climate Center at Colorado State University. It will show the accumulated number of Growing Degree Days for your location.

Each pest and pathogen has a range of GDD during which treatment is effective. Outside that range, there’s little point in applying biological or chemical controls.

An Example …

The Cooley spruce gall adelgid can be successfully treated on spruce trees between 22 – 92 GDD, and also at 1500 – 1775.

To show you how this affects the timing of Cooley spruce gall adelgid treatment, we looked at the GDD here in Evergreen, CO on May 23 each year for the last five years. On that day, GDD ranged from a low of 22 in 2015 to a high of 81 in 2018.

This means that on May 23 in 2015 we would only just be starting to schedule treatments, whereas in 2018 it would be getting almost too late to treat for it.

leafy spurge

Leafy spurge is one of many noxious weeds to keep watch for.

How Spring Weather Affects Pest, Disease & Plant Problems

Wet, cool spring conditions encourage pests, like aphids, that feed on juicy new leaves. We also see the rapid development of fungi, such as powdery mildew, juniper hawthorn rust, and aspen leaf fungi.

In contrast, warm, dry spring weather reduces the likelihood of fungal problems and aphids, but increases populations of pests that thrive in those conditions, such as spider mites.

The difference in precipitation levels can also have a significant impact on insect activity, such as feeding, mating and travel, as well as the development and vigor of host plants. For example, after a wet spring bugs enjoy feeding on the lush, green vegetation but the trees are better able to ward them off (for example, by producing a plentiful supply of resin).

It’s a similar situation when it comes to noxious weeds, such as leafy spurge and yellow toadflax. Plentiful spring moisture helps weeds (as well as grass) grow and mature more quickly, particularly when they’re growing in nutrient-rich soil. However, when the dog days of summer arrive that just means there’ll be more fuel for fires.

Finally, a late snowfall, ice storm or hard freeze can kill or slow down the development of pests, fungi, and weeds that have already reached (or would soon reach) the GDD baseline.

The Bottom Line

Plan your spring gardening, planting and landscape/tree treatments based on the weather, not on the calendar.