Better safe than sorry

Walk through any garden center or home store today and you’ll find rows and rows bursting with a beautiful array of summer flowers, vegetables and herb plants. Spring has finally arrived, and gardeners in North Idaho are eager to start planting everything from petunias to peppers.

Whoa, Nellie! Hold on a minute (or a few weeks). Just because these warm-season plants are available doesn’t mean it’s time to plant them.

Know Your Frost Dates

If you have gardened in our region for any length of time, you’re probably familiar with two old adages about when you can safely plant your warm-season vegetables and flowers.

One is “When the snow is off Rathdrum Mountain;” the other is “Plant on Mother’s Day.” Depending on the weather, some years that might be true. But for the most part, following this advice puts your warm-season plants at risk of succumbing to our unpredictable spring weather. It has snowed on Mother’s Day in Coeur d’Alene in the past. Brrr!

A more accurate method for safe planting is to go by your area’s last average day of frost in the spring. There are two frost dates; one for when you can expect a first frost in the fall and one for a final frost in the spring. This is an average date based on historical weather data for your area and on probability.

For Coeur d’Alene, Hayden, Post Falls and Rathdrum, the probability of seeing 32 degrees on May 13 is 30%. Not bad. But, if you want even better odds, wait a week and plant around May 22 when there is only a 10% chance frost.

For areas to the north or those at higher elevations, last frost dates are pushed even further out on the calendar. For example, Athol still has a 30% chance of frost June 1; it drops to 10% on June 14.

Frost dates for towns in Kootenai and Shoshone counties can be found on the University of Idaho Kootenai County Extension Master Gardeners website: www.uidaho.edu/extension/county/kootenai/garden. Scroll to Resources and click Resources for New Gardeners.

Warm-Season Plants Require Warmth

Warm-season vegetables and flowers (tomatoes, cucumbers, peppers, basil, petunias, marigolds and zinnias, to name a few) are highly susceptible to cold temperatures and will quickly die off in cold, wet soil. To grow and thrive, these plants require warm soil (at least 55 degrees) and warm air temperatures.

When warm-season plants are put into the ground before these temperature thresholds are met, at best they will remain static with no growth until conditions change, or worse case, they will perish.

Plan to Hold Tender Plants for a Few Weeks

In case I haven’t been clear… early May is not an optimal time to plant warm-season plants in North Idaho.

If you’ve already purchased warm-season plants and they have been hardened off (acclimated to the outdoor environment) you can hold plants outdoors for a few more weeks until weather conditions improve. Most plant vendors will have hardened their stock before selling, but it’s always a good idea to ask.  

Place young transplants outside during the day in an area protected from intense sunlight and wind. In the evening, move plants inside a garage or other protected area out of the danger of frost.

Avoid bringing hardened-off plants into the house for an extended period, as the temperature in your home is likely to be too warm for plants that have already acclimated to the outdoors.

Keep Frost Protection at the Ready

If your warm-season flowers or vegetables have already been planted, you’ll need to have some form of frost protection ready for any late frosts, cold rains, or wind.

A row cover is perhaps one of the best items a gardener can have at the ready. Row covers are made of lightweight polypropylene or polyester fabric that allows rain and sunlight to pass through. Most importantly, they help to insulate plants from cold and wind.

Row covers can also be used later in the season to keep insects off plants and to serve as a shade cloth to keep plants from overheating.

Another inexpensive item to protect plants is a clear plastic storage tub. When placed over plants, these tubs will create a mini-greenhouse environment. Recycled items like plastic milk jugs or plastic lettuce containers can also be used to protect smaller seedlings. Just make sure to anchor the container so it doesn’t blow away.

Low tunnels require a bit more effort to construct but are excellent forms of protection for larger growing areas. A simple low-tunnel can be made by using PVC pipe and 6 mil plastic sheeting.

It’s worth the small investment, as low-tunnels can extend your growing season in both the spring and fall. And, once the weather warms the plastic can be removed and the structure left in place to hold a shade cloth when summer’s heat soars.

So, before you grab your trowel and head out to the garden to plant your tomatoes, check your last average frost date and have some frost protection handy. And … if you really feel the need to dig in the soil, now is a great time for weeding!

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Candace Godwin is a certified Idaho Master Gardener and the owner of The Coeur d’Alene Coop (thecoeurdalenecoop.com), offering seasonal plant sales and advice on gardening and raising backyard chickens.