Starting Seeds Indoors and Moving Them Out
With a frost-free season of only about 110 days, many favourite crops need a head start indoors. Here’s when to start each one for a Lethbridge spring, how to grow sturdy seedlings instead of spindly ones, and how to move them outside without losing them to wind, sun or a cold night.
On this page
What to start indoors, and what not to
Start indoors
Crops that need a long, warm season: tomatoes, peppers, eggplant, onions and leeks from seed, celery, and the cabbage family (broccoli, cabbage, cauliflower, Brussels sprouts) for an early crop. Slow annual flowers like petunias, snapdragons and lobelia also need a head start.
Start indoors or sow outside
Cucumbers, squash, zucchini, pumpkins and melons grow fast but dislike cold soil. A three- to four-week head start indoors in pots gives them a jump, or sow them outdoors once the soil is warm in early June.
Sow straight into the garden
Root crops and quick growers don’t like being moved: carrots, beets, parsnips, radishes, peas, beans, corn, spinach, lettuce, and potatoes (from seed potatoes). See the direct-sowing list below.
When to start: a Lethbridge seed calendar
Count backward from the date the plants will go outside. For frost-tender crops in Lethbridge, plan on planting out around May 25 to June 1 (see frost dates). Hardy cabbage-family transplants can go out a couple of weeks earlier, in mid-May.
| Crop | Weeks before planting out | Start indoors around | Plant out around |
|---|---|---|---|
| Onions and leeks (from seed) | 10–12 | Late February to early March | Early to mid-May (hardy) |
| Petunias, lobelia, snapdragons | 10–12 | Early March | Late May |
| Celery | 10–12 | Early March | Late May |
| Peppers and eggplant | 8–10 | Mid- to late March | Early June |
| Tomatoes | 6–8 | Early April | Late May to early June |
| Marigolds, zinnias, cosmos | 4–6 | Mid-April | Late May |
| Broccoli, cabbage, cauliflower | 4–6 | Early April | Mid-May (hardy once hardened off) |
| Cucumbers, squash, zucchini, melons | 3–4 | Early May | Early June |
The most common mistake is starting too early. Tomatoes started in February are tall, leggy and pot-bound by late May, and they often do worse than ones started in April. When in doubt, start later.
Every spring, Environment Lethbridge holds Seedy Saturday at the downtown library, a free seed swap with workshops. It usually falls in early April, right in the middle of seed-starting season.
What you need
- Seed-starting mix, not garden soil. It’s light, fine, and free of the fungus that kills seedlings.
- Clean containers with drainage holes: cell trays, small pots, or yogurt cups with holes poked in the bottom. Wash reused trays in soapy water.
- Light. This matters most. Even a sunny Lethbridge window isn’t enough in March; seedlings stretch toward it and flop. A simple LED shop light hung 5 to 10 cm above the seedlings, on a timer for 14 to 16 hours a day, makes all the difference.
- Warmth for germinating. Peppers, tomatoes and eggplant sprout fastest at about 24 to 29°C. A seedling heat mat helps, or the top of the fridge. Once they’re up, take them off the heat.
- A small fan on low for an hour or two a day. Gentle movement makes stems thicker and stronger, which is good practice for our wind.
- Labels. You will not remember which tray was which tomato.
Growing strong seedlings
- Sow shallow. A good rule is to plant seeds about twice as deep as they are wide. Tiny seeds like petunias and lettuce go on the surface, pressed in lightly.
- Keep the mix moist, not soggy. Water from below by setting the tray in a pan of water for 20 minutes, then let it drain. Soggy mix and still air cause “damping off”, where seedlings collapse at the soil line.
- Get them under light right away. As soon as the first green shows, move them under the lights and off the heat mat.
- Keep the lights close. Raise the light as the plants grow, keeping it a few centimetres above the leaves. Tall, pale, floppy seedlings are asking for more light.
- Pot up. When seedlings have two or three sets of true leaves, move them into bigger pots. Tomatoes can be planted deeper each time; they grow roots along the buried stem.
- Feed lightly. Seed-starting mix has few nutrients. Once true leaves appear, water every week or two with a weak, half-strength fertilizer or compost tea.
Hardening off: the step people skip
Seedlings grown indoors have never felt sun, wind or a cool night. Put them straight into the garden and many will scorch, wilt or snap. Hardening off means getting them used to the outdoors gradually, over 7 to 14 days. In Lethbridge, wind is as big a shock as sun.
| Days | What to do |
|---|---|
| 1–2 | One to two hours outside in a shaded, sheltered spot, out of the wind. Bring them in. |
| 3–4 | Three to four hours, with an hour or two of gentle morning sun. |
| 5–7 | Most of the day outdoors, with more sun and a little breeze. Water well, as pots dry fast outside. |
| 8–10 | All day outdoors. Leave them out overnight if the low is above about 10°C for tomatoes and peppers, or above freezing for hardy crops. |
| 11–14 | Out full-time, ready to plant. |
Missed a day? That’s fine; just don’t jump ahead. If a day is very windy or cold, skip it and bring them in. A cold frame or a sheltered corner against the house makes hardening off much easier.
Planting out
Not sure if it’s time? Answer three quick questions:
Is it safe to plant out yet?
1. What are you planting?
2. Were the plants grown indoors or in a greenhouse, and have they been hardened off?
3. Squeeze a handful of soil. What happens?
Uses Lethbridge frost odds (Environment Canada, Lethbridge CDA) and Environment Canada's forecast lows for the next few nights.
- Check the forecast first. Look at the next five nights, not just tonight. A run of mild nights after planting gives roots time to settle in.
- Plant on a calm, cloudy day or in the evening, so the plants aren’t fighting sun and wind on their first day.
- Water the pots first, then water the plants in well. A little compost in each planting hole helps in our clay.
- Bury tomatoes deep, up to the first set of leaves. Peppers and most other plants go in at the same depth they were growing.
- Shelter them for a few days: a board, a bucket with the bottom cut out, or a ring of row cover on the windy side.
- Keep covers handy. A late-May or early-June frost is always possible here. See protecting plants on a frosty night.
Soil temperature matters too. Tomatoes, peppers, squash, beans and corn sit and sulk in cold soil, even after the frost risk is past. Wait until the soil is at least 10°C at planting depth (a cheap soil thermometer settles it), or warm the bed for a week under black plastic first.
Crops to sow straight into the garden
| When | What to sow |
|---|---|
| As soon as the soil can be worked (often late April to early May) | Peas, spinach, lettuce, radishes, onion sets, carrots, beets, parsnips, Swiss chard, sweet peas. These sprout in cool soil and can take light frosts. |
| Early to mid-May | Potatoes (seed potatoes), more lettuce and radishes, kale, turnips. |
| Late May to early June, once the soil is warm | Beans, corn, cucumbers, squash, zucchini, pumpkins, nasturtiums, sunflowers. |
| Every two to three weeks until mid-July | Small sowings of lettuce, radishes and spinach, for a steady harvest instead of a glut. |
“When the soil can be worked” means a handful squeezed in your fist crumbles apart rather than forming a sticky ball. Working wet clay ruins its structure for the whole season.
Sources
- Environment and Climate Change Canada, Canadian Climate Normals 1981–2010, Lethbridge CDA, frost probabilities.
- Seed-starting timings are common horticultural practice for short-season prairie gardens, adjusted to Lethbridge frost dates.
- Lethbridge News Now, “Seedy Saturday returns to Lethbridge”, April 2026.