Snow is melting, patches of bare ground expanding, and gardeners are chomping at the bit to get started with the growing season. For some of us, that means starting seeds indoors.

I am a newcomer to seedstarting, but I won’t let that stop me from sharing the benefit of my (very limited) experience. Which is this: when it comes to tender and many other annuals, patience pays.

In other words, it is better to start such plants later in the available window of planting time, even though the desire to see sweet little seedlings emerge from their pots is almost irresistible. At least, this is true for those of us with very limited space for our seed-starting activities.

Basically, I just want to reduce the chances of transplanting into cold soil, which would make for a harsh introduction to the outside world for my precious seedlings. This is not necessarily fatal, but it can slow initial growth or even stunt the plant for an entire season.

Now, it’s true that the average frost free date has shifted earlier in our area. It used to be May 15th, and now it’s supposed to be May 4th. Even as this happens, however, spring has become more unpredictable in terms of when the soil is sufficiently warmed. Even without frosts, spring can move slowly and stay excessively brisk and, in my unscientfic opinion, this seems to be happening with greater frequency.

(Actually, I’ve been told that a better guideline than the last frost date is to wait until the nighttime temperatures consistently stay above 50 degrees Fahrenheit (or 10 degrees Celsius).

The sooner you start seeds indoors, the harder it is to avoid planting seedlings too early.

I don’t have any good inside spaces for seedlings that are ready to move out before the outside is ready to be hospitable. Plants left too long under the grow lights become overcrowded, which can make them scrawny and vulnerable to disease. Moving them to an indoor spot with insufficient light is not a good solution, either.

So the upside of waiting is a greater likelihood of transplant success. The downside is I have to suppress my desire to fill all the beds and containers with bloom as early as possible. And depending on the weather, the delay might not be necessary. But still, I prefer to go with a lower-risk approach.

What this means in practice is that I’m using the old May 15 frost-free date, climate change be damned. And if the seed packet says plant indoors 4-6 weeks before last frost, I’m planting 4 weeks out. In fact, I’m planning on starting most of my indoor seeds on April 1 or April 15. A few as late as May 1. The only thing I’m starting in March is Sweet Alyssum (Lobularia maritima). But then again, I may buy a few flats of Pansies in April to ensure that I have some color in my containers.

How do you decide on your indoor seed-starting dates?

33 Comments on “A Question of Timing: When to Start Seeds Inside”

  1. I cannot wait to get my seeds growing in my unheated greenhouse ( here in the UK) I keep a diary, and looking back over the years through this , I find that about the 3rd week in March is my time for sowing. Light levels will be much better,and the greenhouse actually starts to warm up by then ! So one or two weekends to go ,and then it will be all systems go. I also find, that by then, I actually want to spend several hours at a time in the greenhouse.

  2. I count back from the average last frost date for my zone (April 23) the number of days stated on the seed packet. The process of hardening off the transplants can allow for colder temps. You can hold off until conditions sre right. Another strategy for a lot of plants is to do “winter sowing.” There are explanations if you google that term. It works for quite a few varieties.

  3. Very good advice! Since I start mine in my sunroom and not under grow lights, I start them earlier. And, of course, it depends on the types of seeds. Zinnias seem to do very well in the sunroom until early May. I don’t start Hyacinth Bean vine seeds until late April or early May, and sometimes I don’t start them until late May, directly into the ground because they like very warm weather. I never grow tomatoes from seed because I only grow a couple of tomato plants each year (we have a CSA vegetable share and a mostly shade garden). So, it depends on the seeds, the indoor space and growing conditions, and whether I plan to start them indoors or out. But that’s pretty much what you’ve said. 🙂

  4. I try not to start them too early, as space is limited. Just the right amount of light, water and warmth seems to be crucial to a good start. It can still get too cold (and wet) well into May here, so I go by the old Memorial Day planting rule of thumb.

  5. I’ve learned to use Mother’s Day as my date to count back from to plant the seeds. Botanical Interests has a great planting guide chart. I’m ready to go, but just isn’t worth starting too soon. I do have a little greenhouse I put up every year on the porch for hardening off. It keeps things warm enough that last week or so before planting outdoors. And helps keep plants I buy prior to be able to plant.
    Hope your snow is gone! We still have patches.

  6. I always have the best of intentions to start seeds early, but most years I get a late start. I think the plants prefer to get out of under the grow lights and into the garden anyway. This year, I got an early enough start on tomatoes that I’ve had to “pot up” to 1/2 gallon-sized pots! I also started too many tomatoes, but I think I’ll plant half in the garden next weekend and hold half back in case temps drop suddenly. In Austin, we should be up above 50 degrees F at night after next week.

  7. Your post came as a reminder for me not to sow too early! It is so hard having to wait, but you are so right about putting seedlings into cold soil and how they then struggle. I have sown a few things that will be fine in the cold frame later, but must refrain from starting the rest until early April. I used to sow around mid April, but have more windowsill space at our new house… our latest frost date is also mid May, but we often don’t get a constant 10°C overnight until mid June which is when I sow my basil. Thanks for the reminder Jason… I had indeed been wistfully looking at my seed packets just before I read this post! 😉

  8. Hello Jason, we’ve got a whole border to do from seeds this year but we don’t have any indoor space to start the seeds off because the window space it taken up by overwintering tropicals that usually spend the summer outside. We can have frosts into late May here (though very, very rare). I’ll be starting to turf overwintering plants out of the greenhouse and at that point, the spare space in the greenhouse will be used for the seeds. It’s more of a logistical operation that a horticultural one for us.

  9. I agree, it is a mistake to sow seeds too early and then keep them getting ever leggier whilst you wait for the soil to be warm enough to plant them out. The old, reliable way to check this out is to see if it is comfortable to sit in the soil with your bare bottom. I haven’t actually tried this though.

  10. I’ve learned the same thing as you: there are better rewards for patience. In Southern Indiana, the saying is to plant red plants (i.e. tomatoes) on Valentine’s Day and greens on St Patrick’s, but when I tried that, my tomatoes were ready to go outside way before the weather was ready for them.

  11. We are only a few days earlier than you on the traditional date to wait to plant tomatoes and other tender plants outside. I have already started with some of my flower and tree seeds as I feel they are hardier. I will wait until mid-March until I plant my tomato seeds – but I might cheat if the weather is good at the end of April. Amelia

  12. This is completely beyond me, since I can’t remember the last time I planted a seed. But it’s very interesting, particularly since considerations of light, temperature, and such determine when I can expect to see native plants emerging and blooming, too.

  13. This is soooo timely, because I have been dithering for days over whether to start some seeds indoors today or tomorrow. Our average last frost is supposedly April 15th. I did quite a bit of indoor sowing about a decade ago but this will be my first attempt here at the Money Pit. I also have a frost-free sunroom that seems to be calling out to be used as a propagation site, lol!

  14. The Belgians wait for the Saints – specifically ‘les saints glaces’, or Ice Saints, Saint Mamert, Saint Pancrace and Saint Servait, whose saint days are on 11,12 and 13 May, before planting out. You are in tune with them! There’s no way I could wait that long though. In practice, I’ve never seen a frost in Belgium in May, but you may have a point re night time temps. I start chillis indoors in February, but they stay indoors, and tomatoes early March, they seem to do fine.

  15. Last year was the one and only year I’ve seriously started seeds indoors. And, I had mixed success. I tried unusual tomato varieties, none of which appealed at maturity! I was anxious to start tithonia and tropical milkweed which are hard to find locally. Both did quite well. However, I’m in a slump this season. A friend will grow tithonia from seed in her greenhouse and I’ll hunt for Asclepius curassavica. Climate change seems to favor the latter with monarchs in these parts. Good luck with your seedlings.

  16. I don’t have a good indoor spot to start anything, so I don’t. The only time I ever started seed indoors was when I had access to a greenhouse. I remember it was an exhilarating feeling to see those little sprouts emerge – almost maternal – taking credit for a life created. It really was pretty cool…

  17. Oooo seed starting – my favourite topic! For me, it’s all about trial and error. As you said, seed packets often give a range, which is a great start, but I find that your personal growing conditions play a big part in how quickly (or slowly) things grow. So my biggest tip is to take lots of notes ’cause next year, you probably won’t remember the details of how and when you planted those nasturtium seeds so that you can make adjustments (or do the exact same thing if it worked out perfectly!)

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