Last Frost Dates and When to Transplant Seedlings

I killed a flat of tomato seedlings in late April once. Just set them outside on a warm afternoon, figured they’d be fine, went inside to make lunch. By the next morning they looked like something had sat on them. Turns out a 38-degree night will do that. Lesson learned, eventually.

If you’re gardening in the Pacific Northwest, knowing when to transplant seedlings outdoors is basically a guessing game played against a sky that has no interest in cooperating. Our last frost date here around Redmond is roughly mid-April, but that number is an average, not a promise. Some years we get frost into early May. Some years March is weirdly warm and you start second-guessing everything.

What the Last Frost Date Actually Means

It’s a 50% probability date. Which means there’s still a coin-flip chance of frost after it. Not exactly the kind of odds I want to bet a tray of pepper seedlings on, and peppers take forever to grow, so I don’t.

Cold-tolerant stuff like lettuce, kale, broccoli, and spinach can go out a few weeks earlier than the last frost date without much drama. These crops handle a light frost, usually down to the low-to-mid 20s without flinching. My kale has survived some genuinely miserable April nights and just kept going.

For warm-season crops, tomatoes, peppers, cucumbers, basil, squash, you want to wait until nighttime temps are reliably above 50 degrees and frost risk is basically gone. In Redmond that usually means late May or even early June if the spring is dragging its feet, which it often is. The soil needs to warm up too, not just the air. Cold soil stresses transplants even when the air feels fine.

Hardening Off: Don’t Skip This Part

Here’s where most people (including past me) go wrong. You can’t just take a seedling that’s been living under a grow light in your basement at 68 degrees and shove it into the ground. It will sulk. Sometimes it will just die. You have to harden them off first, which is a fancy way of saying let them get used to the outdoors gradually.

I start by setting my transplants outside in a sheltered spot for an hour or two the first day. Somewhere they get indirect light and some airflow but aren’t getting blasted by full sun or wind. Then I bring them back in. Next day, two to three hours. By the end of two weeks they’re out most of the day and spending nights outside when temperatures allow. That’s it. Takes some remembering, but the plants come through it a lot tougher than if you skip the whole thing.

My older daughter calls this “plant school.” She’s not wrong.

A Few Cheap Ways to Buy Yourself More Time

If you’re impatient (hi) and want to get warm-season transplants in the ground earlier without gambling them on the weather, there are some low-cost options that actually work.

Row cover fabric is probably the most useful thing in my garden for about nine months of the year. A layer of floating row cover draped over newly transplanted seedlings gives you a few degrees of frost protection and cuts the wind. Cheap, reusable, and I use mine until it literally falls apart.

Wall-O-Waters (or the generic versions at any garden center) are those accordion plastic sleeves you fill with water. They work surprisingly well for getting tomatoes in the ground a few weeks early. I think of them as a sleeping bag for your tomato plant. A very weird, watery sleeping bag.

Cloche from a plastic jug. Cut the bottom off a milk jug, set it over a transplant at night, leave the cap off during the day so it doesn’t cook. Costs nothing. I have approximately forty of these.

The Calendar I Actually Use

For what it’s worth, here’s roughly how I time transplants in Redmond. This isn’t gospel, just what’s worked more often than it hasn’t.

• Early to mid-April: Transplant cold-tolerant crops outdoors. Broccoli, cabbage, kale, lettuce, spinach, chard.
• Late April: Still too early for tomatoes and peppers without protection. Keep hardening them off.
• Mid to late May: Tomatoes and peppers go in, once nights are consistently above 50 degrees. I check the 10-day forecast obsessively during this window.
• Late May into June: Squash, cucumbers, beans from transplant or direct seed. Basil waits until it’s genuinely warm because basil is dramatic.

You can check the historical frost data for your specific zip code with a quick search. It’s free, it’s more useful than the general “mid-April” advice, and it takes two minutes.

Anyway, transplant timing comes down to knowing your crops, watching the forecast, and being willing to cover things up at night when April decides to remind you who’s in charge. Which, around here, is never you. It’s always April.

Lettuce be patient out there. (I couldn’t help it. Sorry.)

Photo by Georg Eiermann on Unsplash