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Here’s why a soil thermometer is a must for your Alaska garden this year

  • Author: Jeff Lowenfels
    | Alaska gardening
  • Updated: January 23, 2020
  • Published January 23, 2020

The traditions that govern how we garden are losing their rationale. Despite the severe winter temps of late, our springs and summers really are warming too fast to ignore. In fact, they are warming here two and even three times faster than in Lower 48 gardens.

So, this week I am encouraging all Alaska gardeners to really re-examine how we garden. Things are getting warmer, fast, and we need to quickly adjust when and even how to do things in the Alaska garden.

One area that we all should concentrate on is soil temperature. While germination temperatures have not changed, the dates when our soils warm up have. If you don’t have a soil thermometer, your favorite search engine will help you locate one if there aren’t any left at local nurseries. They go for as low as six bucks.

Why do we care? Well, lettuce, onion, parsnip and spinach can germinate when soil temperatures reach 35 degrees. (That’s why they don’t need starting indoors.) We are now seeing this temperature in our soils by early April and even late March in some micro-climes.

The new norm should be to buy enough seeds of these for at least two additional plantings. We can easily plant successive crops into late August and even early September.

Next, when the soil warms to 40 degrees Fahrenheit, things like beets, broccoli, Brussels sprouts, Chinese cabbage, cabbage, carrots, cauliflower, kale, collards, kohlrabi, leeks, parsley, peas, radish, rutabaga, Swiss chard, celery and turnips will germinate outdoors.

Soil temperatures of 40 degrees were the main reason Memorial Day weekend worked as the time we used to traditionally plant outdoors. With all this warming, we need to establish a new date.

Then there are tomatoes, green beans, dry beans, cucumbers, squash, corn, pumpkin, eggplant, melons, cantaloupe, okra and peppers — all vegetables or fruits that require soil temperatures to be 50 degrees before they will germinate. This is the main reason why soil warming techniques such as cold frames or clear plastic mulch and greenhouses were traditionally mandated for growing them here.

Do your soils heat up enough, early enough to now sustain these crops in your Alaska garden? Probably, for many of them. I know several gardeners who now grow pumpkins and squash without help. I mentioned a pot-grown okra last month. Which of the listed 50-degree plants should be added to the “these grow here planted directly in the soil without help” list?

Of course, successfully growing more than one harvest of crops is also influenced by air temperature. Tomatoes need above 55 degrees to set fruit. We are now achieving this in many areas of Alaska.

Some crops we traditionally grow do not do well in hot air and soil temperatures. Cabbages, spinach and chard, for example, will bolt into seed or develop a spindly stalk. If you do multiple harvests, you now need to have special areas in the shade, where the temperatures can actually be kept cooler. I guess you will need a good outdoor thermometer to accompany that soil thermometer you must get.

Jeff’s Alaska garden calendar

Valentine’s Day at the Alaska Botanical Garden: The garden will be lit up with Valentine-themed additions to guide you along the path. Hors d’oeuvres from Fromagio’s, drinks, hot cocoa, live music and tasty desserts await in the greenhouse. $35 for non-members, $25 for members, $12 for children (includes a craft), free for kids 6 and under.

Orchid Society: I failed to note that if you are even remotely interested in orchids, you should look into the Alaska Orchid Society,, as for all manner of useful information. Next meeting is at the BP Energy Center, 6:30 p.m. Tuesday, Jan. 28, with a program on Paphiopedilum and Phragmipedium beginning at 7 p.m.

Seeds to buy: Sweet peas, celery.

Pelagoniums: If you have some growing indoors, now is when you should start to trim them up. Let cuttings callous over for 48 hours and then root in damp sand or soil.