Growing Degree Days: The Heat Your Garden Banks

What a growing degree day is

Think of a garden as a bank account for heat. Every warm day makes a deposit. A cool day makes a small one, and a cold day makes none at all. A crop is ready when the account reaches a certain balance, whether that takes 60 calendar days or 100.

Growing degree days (often written GDD, and also called heat units) are the unit that account is kept in. The idea, as Alberta Agriculture puts it, is that plant growth depends mostly on temperature, as long as nothing else is holding it back, like drought, hail or poor soil.

The key detail is that most plants barely grow at all below a certain temperature, called the base temperature. Heat below that line doesn’t count. Only the warmth above it goes into the bank.

How to calculate them

The calculation Alberta Agriculture uses is short enough to do in your head:

1. Take the day’s high and low. Any thermometer that records both will do, or use the readings from the forecast page.
2. Average them. Add the high and low, then divide by two.
3. Subtract the base temperature. Use 5°C for most vegetables, flowers and grains, or 10°C for corn, beans and other heat-lovers.
4. If the answer is below zero, call it zero. A cold day can’t take heat back out of the bank.
5. Add each day to a running total through the season.

In one line: GDD = ((high + low) ÷ 2) − base, never less than zero.

A worked Lethbridge example

Here’s how a mixed week might add up, with temperatures typical of a Lethbridge spring and summer. The numbers are for illustration, not a record of a real week.

Look at what this shows. A pleasant 14°C May afternoon gives the tomatoes almost nothing, and gives corn literally nothing. One typical July day banks more heat than a whole week of cool spring weather. And even a balmy-feeling chinook in January adds nothing, because the night is so cold.

Notice the low temperature matters just as much as the high. Our dry air and elevation of about 910 m let heat escape fast after sunset, so Lethbridge nights cool off sharply even after hot days. Cool nights are lovely for sleeping and for sweet carrots, but they slow the heat account down for corn, squash and peppers.

Growing degree days so far this season

Calculated from weather data for Lethbridge, using Alberta Agriculture’s method described on this page.

Why “days to maturity” can mislead here

A seed packet that says “Sweet corn, 72 days” is making a promise based on someone else’s weather. Days to maturity are worked out in trial gardens, often in places with warmer nights and a longer, steadier summer than ours. The plant doesn’t really need 72 days. It needs a certain amount of heat, and in that trial garden, 72 days happened to deliver it.

In Lethbridge, several things can stretch that number:

• Cool nights. A summer night here often drops into the low teens or below, which pulls the daily average down.
• A slow, cool spring. Late May and early June can bank only a few heat units a day, especially for corn and beans with their 10°C base.
• Wind. Steady wind cools leaves and soil, and stressed plants grow more slowly than the thermometer suggests.
• Cold soil. Seeds and transplants sit and wait in cold soil even when the air is warm.

The days count from different starting points, too. For crops usually transplanted, like tomatoes and peppers, the days are counted from setting the plant out, not from sowing the seed. For corn, beans and squash, they’re counted from sowing.

How farmers use heat units

Growing degree days aren’t a gardener’s curiosity. They’re how much of prairie agriculture plans its year. Alberta Agriculture’s Agroclimatic Atlas gives a few examples:

• Alfalfa is ready for a cutting at about 475 degree days, when cut at 10 per cent bloom.
• Argentine canola needs about 1,040 to 1,100 heat units to mature, while the shorter-season Polish types need about 860 to 920. That difference decides which kind can be grown where.

Corn heat units

Corn gets its own, fancier system called corn heat units (CHU), which came out of Ontario. Instead of averaging the high and low, it scores the day and the night separately, because corn responds to them differently. The standard Ontario formula, which Alberta Agriculture’s Agroclimatic Atlas refers to, assumes corn does essentially no growing at night below about 4.4°C, or during the day below 10°C, and that daytime growth peaks at around 30°C, then falls off when it’s hotter.

Every corn hybrid is rated by how many CHU it needs to mature. Farmers compare that rating with the CHU their area usually gets. Alberta Agriculture’s work on grain corn notes that the southern part of the province has enough heat for hybrids rated below about 2,400 CHU, and that areas west of Lethbridge get fewer corn heat units than Lethbridge does. Heat drops off toward the foothills, so where you garden in the region matters.

Timing pests and harvest

Insects are cold-blooded, so they develop on a heat schedule too. That’s why the same pest can show up two weeks apart in two different years: it wasn’t watching the calendar.

The Prairie Pest Monitoring Network, a group of field-crop insect specialists from Agriculture and Agri-Food Canada, the prairie provinces including Alberta, and universities, uses temperature-based models every summer to predict when crop pests will hatch and develop. For example, they use a degree-day model for bertha armyworm, a canola pest, whose pupae need about 352 degree days above 7°C to develop. They run similar models for grasshoppers, diamondback moth and alfalfa weevil.

Notice the base temperature there is 7°C, not 5°C. Each insect has its own base, so a pest model and a crop model won’t use the same numbers.

Using the idea in your own garden

• Keep a garden diary. Write down the date you first see cabbage worms, flea beetles or aphids, alongside the season’s growing degree day total from the tracker above. After a few years, the GDD number will predict the pest better than the date does.
• Watch nature’s own thermometers. Plants bank heat the same way insects do, so the timing of flowering shrubs and trees in your neighbourhood shifts with the season. Many gardeners note what’s in bloom when a pest first appears, and use it as a cue the following year.
• Plan harvest. In a hot summer, plan to pick earlier than the packet says. In a cool one, give crops more time before frost and be ready to cover them.
• Plan covers. If heat units are running well behind by midsummer, that’s your warning that winter squash, melons and late tomatoes may need frost protection in September to finish.

Cool summers and hot summers

No two Lethbridge summers bank the same heat. A cool, showery June followed by an average July can leave the season well behind. A hot, dry summer can put it well ahead, though heat above 30°C, wind and dry soil can stress plants enough that the extra heat doesn’t help much.

Here’s a simple way to see the difference. Imagine two Julys, with every day averaging the same:

That’s about one and a half times the heat for most crops in the hot July, and twice as much for corn and beans. It’s why corn can be ripe by mid-August one year and still be milky when the September frost arrives the next. The difference isn’t your gardening; it’s the bank balance.

Sources

• Alberta Agriculture, Agroclimatic Atlas of Alberta: Agricultural Climate Elements (growing degree days, base temperatures of 5°C and 10°C, upper threshold of 30°C, alfalfa and canola heat-unit examples, corn heat unit thresholds).
• Alberta Agriculture, The Potential for Grain Corn in Alberta (corn heat units in southern Alberta and around Lethbridge).
• Prairie Pest Monitoring Network, weekly insect pest updates, 2026 (degree-day and phenology models; bertha armyworm pupal development).
• Environment and Climate Change Canada, Canadian Climate Normals 1981–2010, Lethbridge CDA (frost-free period).