Know Your Growing Season

Learn how long your growing season is—your last frost in spring and first frost in fall—so you know when to start seeds inside or avoid growing them.

Understand Your Growing Season

Understanding your growing season is important for so many reasons, but especially in Cold Climates. The average length of the growing season across the United States has increased about 2 weeks. However, note that climate change has also created more extreme weather. So we are occasionally experienced a much shorter growing season due to late and early winter storms. In short season climates you need to plan carefully and in some cases avoid growing certain plants altogether.

How long is my growing season?

Growing anything in the cold northern regions is difficult enough. But aside from sub-zero winters and and cooler summers, what we can grow is often limited by the length of our growing season. Most plants need a minimum growing season of 90 days. Generally a growing season is defined by warm soil, warm days and nights, and adequate moisture. With both late and early snowfalls, freezing temperatures and frozen soil, a northern growing season can be very very short in some years. With last winter frosts occurring in May, and first fall frosts occurring in September, planning for a minimum 90 day window can be difficult if there are late and early deviations from the normal. Once the last frost has passed, it still takes a while for the soil to warm enough to provide good growing conditions. Fortunately when temperatures cool in September, the soil takes a long time to cool. That helps to keep plants growing until a frost nips them. So a growing zone 3 or 4 usually will have 120 frost free days, but the actual air and soil temperatures will affect the potential growth ability of plants in during that time.

How do I select plants appropriate for my growing season?

Fortunately, growers and sellers do a pretty good job of taking the worry out of your decisions. Plant hardiness and growing zones take into account the length of growing season as well as typical high and low temperatures. Plants designated for your growing zone should survive the typical high and low temperatures and have plenty of time to reach growing season maturity. Fruits and vegetables in particular need ample time to mature and ripen. Be sure to select varieties most appropriate for your region.

Do be aware however, that microclimates can exist within growing zones. These small "pockets" of different growing conditions are created by physical conditions in the landscape and can potentially extend or shorten your growing season. You should consider the extra warmth of a building sheltered area, the exposure to cold winds, etc. when selecting plants.

What does my growing season have to do with starting seeds indoors?

Each plant seed requires a different length of time to germinate and mature enough to be planted outdoors. So it is important to know the last frost date and when the soil is warm enough for transplanting your seedlings. Most seeds need about 6 weeks to be ready to go out to the garden.

If you are starting tomato seeds, be sure to select a variety appropriate to the length of your growing season. Note the "days to maturity" on seed packets. Many of them require 90 days until they are ready to harvest, but some can go up to 120 days. If you are in a short growing season, long season fruits may never reach harvest, especially if you have a late spring or an early winter. There are some great tomato varieties that mature in 50 to 60 days. Since tomatoes are annual plants, the growers will not identify appropriate growing zones for them. However your local garden centers will generally sell the best varieties for your area.

Root vegetables and vining melons often require 100-120 days for maturity. Root vegetables will be fine in warm soil after the air temperatures drop, but vine fruits may not make it to harvest in a short season.