Weather or Not

By Vince Streano

Anacortes American - May 13, 2009

Aaahhhhhh, spring, when an Anacortesan’s fancy turns to growing tomatoes. Growing tomatoes in Anacortes is like rooting for the Mariners. Each year we’re sure we’ll have a winning team, or garden, and by the end of summer our hopes are dashed once again. Judging from the number of people I saw Saturday buying tomato plants at the Master Gardener’s plant sale, I have to believe hope springs eternal.

If you’ve wondered why some years your garden grows better than others, it probably has a lot to do with growing degree days. Growing degree days is simply a way of measuring how much heat your garden gets during each day. With the renewed focus lately on growing your own food, knowing how to calculate growing degree days can be a big help in deciding what to plant and when.

Each plant has its own requirements as to how much heat it takes to germinate, grow and ripen. Some plants, such as corn and tomatoes, take much more heat to reach maturity than say lettuce or kale. When planting your garden you should know how many growing degree days you’re likely to get during the growing season and plan accordingly.

Growing degree days, also known as heat units, can be measured many different ways, but the simplest and most common is to add together the high and low temperature of the day, divide by two, and anything over a threshold of 50 is considered a growing degree day.

So if the high temperature of the day is 60, and the low is 46, when added together you get 106. Divided by two you get 53, which gives you three growing degree days.

Typically here in Anacortes we don’t get many GDD before the end of April. This year, according to one Web site I checked, we’re at 45 for the first four months of the year. At my house in Dewey Beach I’ve counted only 40.

In 2008 we only had 8.6 growing days by the end of April. For the year, Anacortes averages anywhere between 1,200 and 1,500 growing degree days. Last year was one of our lowest with only 1,197. No wonder my tomatoes never ripened.

To find out more about GDD, I called Bob Hart at La Conner Flats. Bob has been growing vegetables for over 40 years on his farm, and for the past seven years has been conducting vegetable trials to determine which varieties grow best here in Skagit County.

“Most years we get about 1,550 growing degree days here at the farm” Bob told me. “If we get 1,600, we get a better crop.”

Bob told me the crops that do best here are root vegetables and leafy greens such as kale, chard, lettuce, etc. And peas do well, as they only need about 1,200 heat units or GDD.

Bob went on to explain how you can increase the heat units in your own garden. One way it to start plants in a greenhouse, or keep them under cover to give them extra warmth. Bob also said you can increase your heat units by planting beside a south facing wall so your plants get reflected heat, or by building a wind break, or putting a cloche over your plants, to block some of the breezes.

“We have to give our tomatoes a little boost to at least 1,650 to 1,700 GDD to ripen, and melons need at least 1,750”, Bob told me.

Growing degree day measurements tend to be very localized. You may find you get more or fewer heat units in your garden than your next door neighbor, depending on how your garden is sited and how much sun it gets. It can even vary in different locations within your garden. Planting a garden for maximum southern exposure ensures you’ll get the maximum heat available.

In comparing GDD totals in other waterfront towns with Anacortes, I learned that Anacortes is usually highest. Compared to Oak Harbor, Port Townsend and even Sequim, we came out on top in each of the past three years.

One of the interesting things I learned while researching GDD is that dandelions only need 50 growing degree days to flower.

Of course we all knew that because they have already flowered on most of our lawns. I’ll include growing degree days in my monthly weather summary so we’ll know where we stand.

To learn more about growing degree days and how to calculate them for specific plants or pests, go to Be sure to do the five minute online tutorial.

To learn which plant species grow best in our environment, visit Bob Hart’s Web site at