How to Pot Up Tomatoes Like a Pro
It’s that time of year again—the sun is trying to stay out longer, we’re tiptoeing past frosty nights, and if you’ve started your tomatoes indoors (as most of us in Zones 5 and 6 do), it’s officially time to pot them up.
If you’re new to growing tomatoes or you’ve always wondered why your seedlings get leggy or stall out before the big garden transplant, this post is for you. We’re diving into the why, how, and when of potting up tomato seedlings—and I’ll also share this season’s surprise experiment that just might change how I start all my tomatoes from now on.
🌱 What Does "Potting Up" Mean?
Potting up is the process of moving your seedlings into larger containers as they grow. If you’ve ever started seeds in those tiny little cells (or reused yogurt cups or egg cartons—no shame!), you’ll know they can outgrow their pots fast.
Tomatoes, especially, are vigorous growers. And unlike a lot of other garden plants, they love being buried deep, which helps them develop strong roots and sturdy stems.
📆 When Should You Pot Up Your Tomatoes?
If you’ve started your seeds indoors around mid-March (give or take, depending on your frost dates), you’ll likely need to pot up at least once, maybe twice before they make it into the garden around early June. I personally wait until June 1st, because my local frost likes to play sneaky little tricks—and nothing hurts more than losing beautiful young tomatoes to an unexpected cold snap.
Here’s a quick guide to knowing when to pot up:
Your seedlings are 3–5 inches tall
They have their first set of true leaves (not just the seed leaves)
Roots are starting to peek out of the bottom of your tray
The leaves look crowded or the stems are getting leggy
🧪 This Year’s Experiment: Rockwool Cubes
Okay, let’s talk Rockwool. This season, I decided to try something new: starting my tomato seeds in rockwool cubes instead of my usual soil-based seed starting mix.
And let me tell you—I’m very impressed.
Here’s what I noticed:
Germination was nearly 100%. Like, freakishly good.
The seedlings have been stronger, with thicker stems and healthier roots.
The cubes promote excellent air pruning, similar to soil blocks, which means better root structure and less transplant shock later on.
I’ve used soil cells for years, and while they’re tried-and-true, this side-by-side comparison has me thinking I’ll be incorporating more rock-wool into my seed-starting moving forward. Anecdotally, the difference in plant health is already visible—and I love having a little data point in my garden nerd journal!
🌿 My Tomato Line-Up This Year
What’s a tomato blog without a tomato brag?
This year, I’m growing a mix of tried-and-true favourites and a few new-to-me varieties that I’m very excited about. Here’s what’s in the grow tray:
Green Zebra – A tangy, stripey stunner. Mild, sweet, and gorgeous in a salad.
Great White – Huge, creamy tomatoes with a mellow flavour. One of my favourite slicers.
Sweetie Cherry – These are prolific little sweet bombs, and perfect for snacking straight from the vine.
Northern Wildflowers Varieties – I’m carrying these seeds in my shop now, and I’m testing them this season. They’re hardy, Canadian-grown, and full of potential. Stay tuned!
Final Thoughts
Growing tomatoes is what started it all for me, over 15 years ago. There’s just something about the smell of the leaves, the satisfaction of picking that first juicy fruit, and the way a tomato plant can absolutely take over a sunny corner of your yard that makes me come back to them year after year.
If you’re just getting started, or if you’re in the thick of potting up your own seedlings, I hope this post gave you a little guidance—and maybe even a new method to try out.
And remember: Even if you only grow one thing this year—let it be a tomato.
