For indoor seed starting, timing matters

Spring arrives March 20 — 58 days from today, and as you see, I’m eagerly counting the days! My seed catalogs, dog-eared and worn, have been perused intensely since the first of the year; seed orders are arriving and my fingers itch to dig into the soil.  

The truth is, I’ve already started sowing indoors. But wait, it’s February — isn’t that too soon? Yes and no. It is too early for warm-season vegetables but the perfect time for others.  

Unfortunately, too many gardeners make the common mistake of starting all their seeds mid-winter. We’re desperate to start growing and think we’re getting a “jump” on the season.  

Sadly, this is not the case. By spring planting, many of our early-sown plants are stressed, spindly, weak and struggle once planted outdoors.   

Know when to start 

Timing is everything when seed-starting, especially for seeds started indoors. When to start depends on what you are growing and your area's last average day of frost.  

Your garden literally lives and dies by two dates: the last average frost date in the spring and the first average frost date in the fall. These dates also mark the start and end of your frost-free growing season.  

Frost dates are critical to determining when to sow seeds indoors and outdoors and help ensure seedlings are planted when soil temperatures and daylight hours are optimal for their growth. 

The last average day of frost (32 degrees) means a 50% chance of frost occurring on that date. This date typically signifies when it is safe to begin transplanting or sowing many warm-season crops outdoors. Similarly, the average first day of frost indicates a 50% chance of frost in the fall.  

For the Coeur d’Alene area (including Hayden, Post Falls and Rathdrum), the last and first average frost dates are May 6 and Oct. 5, respectively. However, in Athol, just a few miles to the north, the dates change to May 23 and Sept. 16.  

While these locations are in the same region, a few miles or feet in elevation can make a big difference. You can find your frost dates here: https://garden.org/apps/frost-dates/.

My years of experience gardening in Coeur d’Alene have taught me to wait until at least May 15 to plant outdoors — at that point, there’s about a 20% chance of frost.   

Still, it’s important to note that the dates are averages and not guarantees since many other variables, such as structures, windbreaks or open land, can affect your landscape’s climate. 

Going back to start 

Depending on the plant variety, indoor sowing usually begins four to 16 weeks before the last frost date. Check the seed packet; it contains a wealth of information, including indoor and outdoor sowing recommendations and the number of weeks to start before the last frost.

Count back from your last frost date to determine when to start indoor sowing. For example, tomatoes are sown indoors six to eight weeks before the last frost; for the Coeur d’Alene area, that corresponds to mid-March to early April (not February!). 

Frost dates also align with the plant’s “days to maturity,” or how many days it takes to achieve a harvest. This allows you to select varieties that produce a crop within your frost-free growing period.

Consequences of starting too soon 

As gardeners, we aim to grow a vibrant garden filled with healthy and productive plants. Unfortunately, sowing seeds at the wrong time does not achieve this. It wastes time, effort, and money and results in dismal transplants. Here’s what to expect when seeds are sown too early in the growing season:   

• Spindly and weak seedlings result from insufficient light. Seedlings require 12 to 16 hours of light each day, which a south-facing window in winter cannot provide. Grow lights help, but plants often outgrow lighted shelves.  

• Crowded and rootbound seedlings. Seedlings rapidly outgrow their initial containers, requiring repotting into larger containers, often more than once. Plants held in small containers become stressed and rootbound. The competition for resources like water and nutrients is also more significant, resulting in stress and nutrient deficiencies. 

• Physical space considerations. Unless you grow in a commercial greenhouse, you will likely run out of growing space before transplanting outdoors. One plant flat can hold up to 1,000 seedlings. As they grow, so does the space required to keep them, including the need for sufficient light and heat. 

• A greater risk of disease and pests. The longer you keep plants indoors, the greater the occurrence of fungal diseases, like dampening off, or pest infestations, like aphids. It can be challenging to manage or eradicate these situations.  

• A greater risk of transplant shock. Finally, stressed or weakened seedlings will take longer to establish when moved outdoors and are at a much greater risk of transplant shock. 

So, what can you sow now? 

If you’re itching to sow something, you’ll be delighted to know that many cool-season vegetables, herbs and flowers must be sown 10 to 12 weeks before the last frost.  

That means starting seeds of cool-season vegetables like broccoli, cauliflower and cabbage or alliums such as onions, leeks, shallots and chives in late winter is perfectly OK. 

Indoor sowing in February also benefits slow-to-grow herbs and flowers like thyme, sage, parsley, snapdragons, pansies and petunias.  

Just steer clear of sowing any warm-season veggies and flowers before their time.  

As much as we yearn to dive into seed-starting as early as possible, patience and our frost dates are key to growing healthy, thriving plants. Starting at the right time ensures sturdy seedlings are ready to transition seamlessly to the garden, saving you time, money and heartache. It also prepares your garden for a productive and flourishing season. 

• • •

Candace Godwin is a certified Idaho Master Gardener, Garden Consultant, and owner of The Coeur d’Alene Coop (thecoeurdalenecoop.com) that offers seasonal plant sales and articles on gardening and raising backyard chickens.

    These tomatoes were sown out of season in October. By early March, they were leggy, weak, and rootbound.
  

    Seed packets contain lots of information, including when to start seeds indoors, like this assortment of cool-season vegetables, herbs and flowers.
  

    Sowing seeds indoors before the appropriate time results in leggy seedings, weak, stressed, and rootbound plants, and increases the risk of pests and disease.
  

    Many cool-season vegetables, herbs and flowers must be sown 10 to 12 weeks before the last frost — meaning you can start these seeds now!
  

    Candace Godwin