How to Calculate When to Start Seeds Indoors

It would be really nice if there were a resource that outlined the exact day to start the different varieties of seeds indoors.  Unfortunately, there is no one-size-fits-all answer.  We need to take into account frost dates (which determine the length of our growing season) and the climate.  This will be the same for all our seeds, but the seeds themselves have factors to take into account.  How long does it take them to germinate?  How long before they outgrow their starter home?  How big do you want them before transplanting?  When do they go into the ground?  And how much space do you have for them inside?

As you can see, the answers will be unique to your own garden and needs. 

For me, I have learned that January 1 is the day to start onion seeds, both those I grow for green onions as well as the bulbing onions.  At the end of March last year, some of my Walla Wallas were nearly the size of the onion plants sold in nurseries.  The seeds were a small fraction of the cost of the plants or sets, and they really didn’t take much time at all.  As a bonus, they haven’t been stunted by sitting in a nursery for weeks.  They were ready to grow now.  And as onions are very cold hardy, they are among the first to go into the ground.  I’ve usually got them all planted by April 1.  

The usual recommendation, however, for starting vegetable seeds is to allow 6-8 weeks of growth before planting outdoors.  My experience has showed that calculating for 8 weeks of growth really is best.  Vegetables at this point display optimal root development without having overgrown and become compacted. 

In calculating exactly when you want to start your seeds, however, you can’t just count back 8 weeks from the time you want them to go into the garden.  You also have to take into account how long it takes the seeds to germinate.  This year, my tomato seeds took 5-7 days to pop.  Per the usual, the pepper seeds took about 18 days.  They also grow so much slower.  So instead of following the directions on the seed packet that say to start seeds 8 weeks before the last frost date, you really have to allow the full 8 weeks plus another 2-3 weeks for germination for peppers.  Allow an extra week or two for tomatoes. 

The next consideration to factor in is what level of protection you will be providing your veggies.  Row covers provide from 2 to 8 degrees of protection, depending on the type of material used, and the season, with greater protection provided in the fall since the ground is warmer.  Wall of waters (apparently not called that anymore—they’re now Kozy Coats) protect tomatoes down to 20°F—as long as the tomatoes aren’t touching the walls.  At very low temperatures, the channels of water may freeze, and any part of the plant touching the frozen channels will be killed.  Heat sinks, raised beds, tunnels, and hoop houses will all vary in the amount of protection provided and should be factored into how early you want to start your plants. 

Of course, it also depends on how big you want your plants to be before they go into the ground.  I’ve got a fairly short growing season here.  (Actually, according to https://www.almanac.com/gardening/frostdates, I am at risk of frost year-round.)  But I also am well aware of our distinct microclimate.  It gives us something of an advantage most of the time.  For example, yesterday when my husband left for work at 4:40AM, the temperature at our house was 50°F.  In the valley just a couple of miles away (and where the official temperatures are recorded), the temperature was 32°F.

So because I want my tomatoes to be about 12-18 inches tall when I put them in the ground with a Wall of Water for protection on April 15 (taller if they are going in later), I start them at the end of January.  Based on their progress this year, I think the timing was just about right.

I started the broccoli and cabbage in mid-January so that they would be ready for planting in mid-March.  Some of the cabbage was a super vigorous variety.  Wow, it grew like something out of a B-grade science fiction horror movie and was more than ready for the garden 8 weeks later.  Unfortunately, the garden was not ready for it.  Freezing temperatures and all that.  The cabbage and broccoli may be hardy, but I most definitely am not. 

So unfortunately, I can’t provide any precise dates for when to start vegetables, but hopefully you understand what factors to take into consideration when planning out your planting schedule.