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A few signs that it might be warm enough to start planting outside

  • Author: Jeff Lowenfels
    | Alaska gardening
  • Updated: April 30
  • Published April 30

Due to you-know-what, I am not wandering around the supermarkets like I have in past springs, but I know the number one question I would be peppered with if I was: When can we plant outside?

You can try to look up the answer on the web. You will find all manner of predictions as to when Alaska will have its last frost of the year. Most are wrong. The loyal reader knows why. For Alaskans, there are at least two parts to the answer here. The first has to do with air temperature and the second with the soil.

The air temperature part of the question should be translated to “when are we going to have our last frost?” The only sure answer is: when the birch leaves on your property — or your nearest neighbor’s who has birch— open and grow to the size of a squirrel’s ear. This is the sign that there won’t be any more frosts until September and that it is safe to start gardening in outdoor soils. Let me emphasize “start.”

Then there is the issue of soil temperature. As noted a few weeks back, some seeds can be planted in very cool soils, while others need a much warmer place to germinate and thrive.

A soil or compost thermometer is a useful tool (and perhaps, cooking or yogurt thermometers?). Are there useful phenological events like birch leaf size to determine soil temperatures as well?

It is said elsewhere that when crocuses bloom, cool temperature crops such as peas, spinach and kales can be safely planted. Similarly, soil is warm enough for potatoes when dandelions bloom and carrots can purportedly go in when daffodils blossom. I am not sure about any of these observations and will start taking my own. Join me.

Of course, the exact happy day that the leaves get big enough or soil temperatures warm enough obviously varies by the location of your garden. When it occurs in Sand Lake, gardeners up in the Glenn Alps will still have a week at least. And who knows when it will be warm enough in Nome. Keep an eye on those leaves. The same applies to soil temperatures.

When we can safely plant outdoors gets a bit more complicated for Alaska gardeners because so much of what we grow outdoors is started indoors. Note that this is so whether we grow it ourselves or buy starts from a nursery (Reminder: all are open! Visit them if you want them to survive for next year!). The advantage to growing indoors is that both outdoor air and garden soil temperatures are not critical.

The loyal reader knows the disadvantage to starting things indoors — plants will need to adjust to outdoor exposure to the sun’s UV rays. These do not penetrate glass and resistance needs to be built up in order for a plant to survive outside. In addition, there are drying winds that a plant needs to “learn” to handle. This process of adapting requires about a week.

ANYTHING and EVERYTHING grown indoors has to be “hardened off" before it can be planted outdoors. I don’t use all capitals often in these columns because only Satch Carlson and Susan Nightingale could do it well, but when I do, pay attention.

Regardless of outdoor or soil temperatures, hardening off requires a week or so to accomplish. I used to advise a complicated process of gradually moving plants from a few days of outdoors in deep shade to sunny locations while increasing the amount of time each day thereafter. The other extreme, which I now follow, is to just leave your plants in the shade for a week.

So where does this leave the gardener this weekend? Go ahead and plant peas, lettuces, kales, spinach and Swiss chard outdoors as long as your garden is in a sunny location. Keep an eye on those birch leaves and start making some phenological observations in your own yard to help you make gardening decisions in future seasons.

Jeff’s Alaska garden calendar:

Nurseries: Visit your favorites now. Keep our nursery system alive in these unusual times.

Alaska Botanical Garden: All manner of information, including an online nursery sale, is at Please consider renewing your membership or getting one if you foolishly don’t already have one, as the ABG is hurting just like all businesses in Alaska.

Veggies: Summer squash, cucumbers, pumpkins, edamame

Plant outdoors if you have warm soils: Peas, spinach, onion sets, potatoes, Swiss chard, mustard, kale. Potatoes in containers.

Corn gluten: This is the stuff that stops seeds from germinating and lasts for six weeks. Depending on when your snow went out, these next two or three weeks are the time to prevent new dandelion plants from germinating. It won’t kill existing plants, however. There are several pre-emergent weed killers. Make sure you know what you are buying.

Hardening off: It is a bit too early, but you should be looking for a good spot: One with shade, completely out of the wind where you can leave the plants all night. It is too much work pulling them in and out of the garage or house.