Forget the calendar: This method reveals the ideal time to tackle garden pests

We know that most plants and insects emerge in spring, advance through summer, and, in cold regions, go dormant over winter. But beyond those general seasonal cycles, there’s no way to predict precisely when, for example, seeds will sprout, garlic will be ready for harvest or pests will be at their most vulnerable.

Those developmental stages vary from year to year because they’re dependent on the weather.

That’s where growing degree days come in.

GDD is a cumulative system used to determine exactly when plants and insects will reach certain stages of maturity. And with so much variability — with different pests having different life stages that are vulnerable (or resistant) to different control methods at different times — it’s a helpful tool for gardeners.

The system, introduced by the French entomologist René Antoine Ferchault de Réaumur in 1735, is a measurement of heat accumulation that accurately predicts when seeds will germinate, crops will mature and specific pest populations will hatch and reach advancing stages of maturity.

The progression of an insect’s life cycle, like much of the natural world’s biological activity, is dictated by environmental factors like temperature. And because not every life stage will succumb to treatment, weather is the only thing that can truly accurately inform the timing of many control methods.

Here’s how it works

The average daily temperature is typically recorded on March 1, and for each degree over 50 degrees Fahrenheit (10 degrees Celsius), one point is assigned. Then, every day throughout the remainder of the season, the number of degrees over 50 is added to a running tally. If the temperature is exactly at or below 50 degrees, the day is assigned a score of zero.

Let’s say the temperature was 53 degrees on March 1; the GDD on that day would be 3. If it was 60 on March 2, the GDD would be 3 plus 10, or 13. If the temperature was 49 on March 3, that would add nothing, and the accumulated growing degree days would remain at 13. The tally continues to grow through a regionally specific date in the fall.

Seems complicated? The good news is you don’t have to crunch the numbers yourself.

Your local cooperative extension office likely tailors and tracks this information for your region, and many post the GDD, as well as pest- and plant-specific GDD guidelines, on their websites. In addition, the agricultural company Syngenta offers a handy tool among its GreenCast online resources that quickly generates the GDD for your zip code.

What does this mean for your garden?

As an example, Colorado potato beetle eggs and pupae are not susceptible to pesticides, so attempting control during those stages would be pointless. But their larvae are vulnerable to the natural biological control Bt from the time they emerge until they reach 1/4 inch (6 millimeters) in length. Rather than heading out into the garden with a ruler — or spraying every day and hoping for the best — you can time the application for between 65 and 185 GDD.

Similarly, the first generation of euonymus scale insects are most vulnerable to Neem oil and horticultural oil treatments between 400 and 575 GDD.

And for pre-emergent crabgrass controls, like corn gluten meal, to be effective, they should be applied just before 200 GDD. That’s why I recommend a treatment window that coincides with the time between when the first forsythia blooms and the last of the lilacs fade: It’s a visual cue tied to nature’s reaction to temperature.

Depending on the variety, most tomatoes, which love the heat, are ripe for the picking between 1,000 and 2,000 GDD. But, for the record, my tomatoes always let me know when they’re ready.

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Jessica Damiano writes weekly gardening columns for the AP and publishes the award-winning Weekly Dirt Newsletter. You can sign up here for weekly gardening tips and advice.

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