Garden
When to Plant
When to Transplant Seedlings: Two Jobs, One Phrase
Two different jobs share that phrase. Potting up — cell to bigger pot — happens indoors as soon as the first true leaves appear, and sooner is better because a small seedling has less to lose. Planting out is a separate decision weeks later, and it waits on hardening off.
Which move do you mean?
Two things get called transplanting and they are months apart.
Potting up is indoors: a seedling comes out of the cell it germinated in and goes into a bigger pot, because the cell has run out of room. It happens weeks before anything goes outside, and it is triggered by the plant.
Planting out is the garden, and it is triggered by the weather and the calendar. It has its own preparation — hardening off — and its own timing, which is counted back from your last frost.
Almost every page on this subject answers one of them without saying which. The rest of this one is about potting up, because it is the move people delay and the delay costs them something.
Potting up: at the first true leaves, and sooner is better
University of Maryland Extension: "The ideal time to transplant young seedlings is when they are small and there is little danger of setback from root shock. This is usually about the time the first "true leaves" appear above or between the cotyledon leaves."
Small is the qualification, not a description. The instinct is to wait until a seedling looks sturdy enough to survive being handled — and that is backwards. A bigger seedling has more root to disturb and more top to support while it recovers. The moment it is least bothered by being moved is the moment it looks least ready.
Knowing which leaves are which is the whole skill, and it takes one look. Maryland: "the cotyledons or "seed leaves" are the first leaves to appear." They are usually plain, rounded, and identical on every seedling in the tray — a bean and a tomato look much the same at that stage. The next pair look like the plant is supposed to look. That second pair is the cue.
There is a second trigger if you missed the first. Maryland captions a photograph of tomato seedlings that have "outgrown the containers" and must move to larger ones until it is time to plant out. Leaves overlapping their neighbors, or roots matted at the bottom of the cell, mean you are late rather than early — not fatal, just harder on the plant. The overlap reading is ours; what Maryland states is the outgrown-container trigger itself.
Handle it by the leaves
Four steps, all Maryland's, and the first one is the one that decides whether the plant lives.
- Lift it by a leaf, never the stem. Maryland is explicit: handle small seedlings by their leaves, not their delicate stems. A torn leaf is survivable and the plant grows more. A crushed stem is not survivable, and the stem is exactly what an adult hand reaches for.
- Separate gently and do not tear the roots. Lift small groups together to make it easier to separate individual plants, and take the roots apart rather than pulling them apart. Maryland's instruction is to avoid tearing roots in the process.
- Make the hole first, then bury most of the stem. Make a hole in the medium before the seedling is out of the tray, so it spends no time in the air. Then set it so that the leaves and growing point are only slightly above the soil line — most of the stem can be buried, which is what quietly fixes a slightly stretched seedling.
- Firm the mix and water gently. Firm the media around each plant and water gently. Then straight back under the light — Maryland says to continue lighting them 14 to 16 hours per day.
Burying the stem is the counterintuitive one and it is worth knowing why it works, though this reading is ours rather than Maryland's: a stem in contact with damp mix has a chance to root along its length, so a seedling that stretched toward a distant lamp can be partly buried and come out stockier than it went in.
Then turn the heat mat off
This is the step nobody mentions, and it is why a tray that germinated beautifully goes leggy in the two weeks afterwards.
Utah State: after germination, aim for "temperatures between 60 °F to 65 °F" at night, with days about ten degrees higher — and then the consequence, "If temperatures are warmer than this, spindly plants can result." Their instruction is direct: "remove or unplug any heat pads used during germination if temperatures are too warm."
The thing that helped them germinate is now making them stretch. Bottom heat gets a seed out of the mix; after that it is just heat, and warm seedlings grow faster than they can build stem to support.
Maryland's figures overlap rather than match — "a 55-60°F night temperature and a 65-70°F day reading" — and cool-season crops want the cooler end of that. Somewhere in the fifties and sixties is the answer both give. And if your seedlings are already stretched, the other half of the cause is how far away the light is.
And do not feed them for a month
A newly potted-up seedling looks like it wants feeding. It does not.
Maryland: "Hold back on fertilizing your seedlings. Most potting mixes contain enough fertilizer to grow a seedling for 5-6 weeks."
Five to six weeks covers most of the indoor life of most transplants. A seedling started six weeks before the last frost is going outside at about the point the mix runs out — so for a lot of crops, the answer to when to start feeding is that you do not, indoors, at all.
Transplanting questions people actually ask
When should I pot up seedlings?
As soon as the first true leaves appear above or between the seed leaves. Maryland's reasoning is that the ideal time is while they are small, because there is little danger of setback from root shock — so the moment a seedling looks least ready to handle is close to the moment it minds least.
What is the difference between seed leaves and true leaves?
Seed leaves — cotyledons — are the first to appear and are usually plain and rounded, near-identical across different crops. The next pair look like the plant is meant to look. That second pair is the signal to pot up.
Why are my seedlings leggy after they came up fine?
Often the heat. Utah State says to remove or unplug heat pads used for germination once temperatures are too warm, because above about 60 to 65°F at night, spindly plants can result. The other half is light distance — the lamp wants to be two to four inches above the leaves.
Can I bury the stem when transplanting?
Yes, and Maryland says to: set the seedling so the leaves and growing point are only slightly above the soil line, with most of the stem buried. It is also the practical answer to a seedling that has stretched, though how much a given crop will root along a buried stem varies.
Sources
Every planting date, depth, and temperature on this page traces to these university extension guides. Where they differ, the ranges above hold the whole spread.
- University of Maryland Extension — Care of Vegetable Seedlings
- Utah State University Extension — Starting Vegetable Seeds Indoors: IV. Seedling Culture and Transplanting
We are building GardenTrack to keep track of which tray is due for which move, so the step people delay is one you get told about rather than one you remember — the app is in beta now, invite-only.