Follow ground temperature, not the calendar, to determine when to plant

soil-thermometer-crop Garden soil thermometer in the ground. It is showing a temperature of just over sixty degrees, perfect conditions for growing most vegetables. Soil is covered in composted material.

Most Alaskans plant by the calendar. We start seeds based on a set number of weeks before outdoor planting and we plant outdoors based on when Memorial Day Weekend occurs.

It is one thing to calendar indoor seed starting. The environment is controlled. There are no frosts. The fact of the matter, however, is that soil temperatures should be our guide when it comes to planting outdoors, not the calendar. If soil is too cool, plants will be stunted for weeks and even the whole season.

This year is a good case in point. It has been unseasonably cool, at least as of this writing, and the soil has not warmed up enough. Hopefully, the week will have had lots of warm, sunny weather. And, hopefully, you pulled the mulch off your gardens to expose soil to the sun so it can warm up.

Ideally, all Alaska gardeners really should have a soil thermometer to test soil temperatures. You can usually buy one where they sell plants for under $10. And, since all thermometers work the same way, a meat or kitchen thermometer will work provided its scale gets down to 40 degrees.

To take the soil’s temperature, the probe needs to go in two inches for seeds and four inches for transplants. Leave it in the soil for 5 minutes and take readings in a few different places. Night temperatures are what really keep the soil from warming faster, but you will get adequate results if you take your readings early in the morning.

Different crops require different minimum soil temperatures. Lettuces and spinaches can be planted when the soil is a chilly 35 degrees. Peas need 40 degrees, while radishes require 45 degrees. Beets and chard do fine at 50 degrees and beans, be they snap or bush, do best when the soil is at least 55 degrees. Tomatoes, corns and peppers need 60 plus, which is why a greenhouse is usually required.

Of course, it doesn’t help to dump cold water on your soils while waiting to plant and actually planting. Check out your water’s temperature with the soil thermometer. Most outdoor faucets serve up really cold water, around 40 degrees. This is all the reason you should start keeping water in a large container out by your gardens. Water can warm up fast if kept in a black garbage can and won’t chill the soil when you use it.


I like to water the soil in my containers and gardens well before planting. This way, I only need to sprinkle a little bit of water on things after planting to settle them into the soil and I can use a watering can with warm water for that. A bit of diluted fertilizer in the water won’t hurt, but I prefer to fertilize my transplants while they are in their pots, as it is more efficient.

And, once things are planted, I water in the morning on sunny days. This way the soil temperature can recover, which it might not do if watered in the late afternoon.

I know there is a rush to get things in the ground. Frankly, Memorial Day this year is late and if you are planting by calendar, this year that is a good thing. Don’t rush. Wait for warm soil. You might even consider laying out a clear or black plastic to warm the soil for next week’s planting. The clear actually works better.

Jeff’s Alaska Garden Calendar

Alaska Botanical Garden: The nursery is open! Join now for discounts.

Lawns: Water, water, water. Don’t fertilize. Don’t thatch.

Delphiniums: Hand-pick caterpillars and apply Bt formulas.

Equisetum: It is never to early to pull the above-ground portion of these weeds.

Jeff Lowenfels | Alaska gardening and growing

Jeff Lowenfels has written a weekly gardening column for the ADN for more than 45 years. His columns won the 2022 gold medal at the Garden Communicators International conference. He’s authored several books on organic gardening, and his latest book, "Teaming With Bacteria," is available on Amazon. Reach him at