Around Vancouver, indoor seed starting runs from early December to late February. Onions and leeks go in first, December 3 to December 10, and sweet potatoes come last, February 18 to February 25. Every date counts back from an average last spring frost of March 18. If you garden in a colder or higher part of British Columbia, your frost comes later, so every window below moves later with it. The table, the light and warmth requirements, and the common mistakes follow.
The date everything counts back from
Environment and Climate Change Canada’s climate normals for Vancouver International Airport put the average last spring frost on March 18. That is a mean, and the word matters. In many years, frost arrives after that date. Treat March 18 as the middle of a window with frost possible on both sides of it. Count every sowing date back from the frost window around March 18, and never treat the date itself as the all-clear.
Extension guidance from the University of Minnesota lists how many weeks each crop usually spends indoors before it goes out: 14-15 weeks for onions and leeks, 12-13 for celery, 10-11 for broccoli, Brussels sprouts, cabbage, cauliflower and head lettuce, 9 for eggplant, okra and peppers, 5-6 for tomatoes and 3-4 for sweet potatoes. Subtract those weeks from March 18 and you get the sowing windows in the next section. The weeks are general horticultural practice, so read them as the usual guidance rather than a law. The calendar months in that table belong to Minnesota and have no place on a British Columbia calendar.
Why does the sowing window have two ends?
The range exists because both ends cost you something. Sow at the early end and the plants may be ready before the weather is. Seedlings that wait for weeks in small cells run out of room, stretch toward the light and tangle their roots. A cold, wet March can hold them indoors longer than you planned, and they only get leggier.
Sow at the late end and you give up weeks of growth. A pepper that goes out small has less time to flower and ripen before the cool, wet fall. Long-season crops like onions, leeks and peppers feel that loss most, because the weeks lost at the start come straight off the end of the season. A sound middle path is to aim for the early part of a window if you have strong light and a warm spot, and the later part if you are working with a cool windowsill.
What if you garden north of Vancouver or at higher elevation?
The Vancouver station describes one climate, and a province is not one climate. Northern and upland districts run weeks behind it, with a later last spring frost. The direction is simple: colder ground means later sowing, so push every window in the table later by the number of weeks your frost trails the coast. Starting earlier only produces seedlings that wait.
On its 1991-2020 map, Natural Resources Canada puts much of the Vancouver area in Canadian hardiness zone 9a. That is the Canadian system, built on seven climate variables, and it does not convert to a USDA zone. Canada’s own plant hardiness site says there are no simple conversions between the two. It is also Vancouver’s zone, not the province’s. The March 18 figure comes from the 1981-2010 normals. Environment Canada has also published 1991-2020 normals, and US frost tables use 1991-2020, so compare dates across the border with care. To find your own district’s frost range, see First and Last Frost Dates in British Columbia, and for wider local conditions start with the British Columbia guides.
The calendar, crop by crop
The table below takes the usual weeks indoors for each crop and counts them back from March 18, the average last spring frost at Vancouver International Airport. Both ends of each window are shown. Colder districts sow later than these dates, and milder spots near the coast can sow earlier.
| Crop | Usual weeks indoors before planting out | Indoor sowing window |
|---|---|---|
| Onions, leeks | 14-15 weeks | December 3 to December 10 |
| Celery | 12-13 weeks | December 17 to December 24 |
| Broccoli, Brussels sprouts, cabbage, cauliflower, head lettuce | 10-11 weeks | December 31 to January 7 |
| Eggplant, okra, peppers | 9 weeks | January 14 |
| Tomatoes | 5-6 weeks | February 4 to February 11 |
| Sweet potatoes (slips sprouted from tubers) | 3-4 weeks | February 18 to February 25 |
How to read the table
Onions and leeks lead the calendar because they grow slowly from a thread-thin seedling. Start onions and leeks between December 3 and December 10, and expect the pots to sit under lights through the darkest weeks of the year. Celery follows a couple of weeks later, then the cabbage family and head lettuce at the turn of the year. Peppers and eggplant share a single date, January 14, because both are slow, heat-loving crops that need the full nine weeks.
Tomatoes come much later than most beginners expect. Five to six weeks is enough, and starting them in December produces tall, tired plants by April. The full routine for them, from germination to potting up, is in Growing Tomato Seedlings Indoors. Head lettuce sits in the same window as broccoli, and the specifics of germinating it in cool conditions are in Starting Lettuce.
These weeks count back to planting out at the last frost, and not every crop goes out then. Broccoli, cabbage and head lettuce tolerate light frost once hardened off, so they can often go out ahead of the average last frost date. Tomatoes, peppers, eggplant, okra and sweet potatoes are another matter. They want mild nights and warm soil, which usually arrive after the frost window closes. For those crops, sow toward the later end of the range or a little after it, so the plants are not waiting in pots for the weather. The British Columbia Planting Calendar shows how these indoor starts line up with outdoor sowing and transplanting.
Which crops should you not start indoors?
Root crops such as carrots, beets, radishes and parsnips belong in the ground. They form a straight taproot, and transplanting disturbs it, which produces forked or stunted roots. Beans and peas germinate quickly when sown in the garden and gain nothing from weeks in a pot. Squash, pumpkins and cucumbers grow fast and resent root disturbance, so in most cases direct sowing after the soil warms beats an early indoor start. If you do start squash in pots because the season is short, keep it brief and use a pot that can go in the ground as is.
Light, warmth and hardening off
Why do seedlings on a windowsill get leggy?
A winter windowsill gives weak, one-sided light, and the days in a British Columbia December and January are short and often overcast. Seedlings respond by stretching toward whatever light they can find. The stems grow long, pale and thin, and they flop over before the first true leaves have properly formed. Turning the pot every day only partly helps.
Grow lights solve the problem. Hang them close above the leaves and raise them as the seedlings grow, so the light stays strong without scorching the tips. A plain shop light fixture with full-spectrum or daylight-balanced tubes or LED strips does the job. Leave them on for a long working day and give the plants a dark period at night. That matters most for the December and early January sowings: onions, leeks, celery and the cabbage family usually do poorly on a gray winter windowsill alone.
What temperature do seeds need to germinate?
Most seeds want steady warmth more than they want light at first. A cold, drafty windowsill gives a patchy, slow germination, and the peppers and eggplant that go in on January 14 are especially sensitive. A heat mat under the tray evens out the temperature, or you can use the top of a warm cupboard. Once seedlings emerge, move them under the lights straight away, because they no longer need the extra heat. Cooler air at that stage keeps the growth sturdier. Cool-season crops such as lettuce and brassicas germinate well in a room that is merely comfortable.
How do you harden off seedlings?
Indoor plants have never felt wind, direct sun or a cold night. University of Minnesota Extension advises that all plants be hardened off over the last 1-2 weeks before planting out. Give every seedling the full 1-2 weeks of hardening off before it goes in the ground. Start with a few hours in a sheltered, shady spot, then lengthen the time outside each day and introduce more sun and breeze. Bring trays in if the forecast shows frost or a cold, driving rain. Tender crops such as tomatoes and peppers need a little extra care and should come in on cool nights. Skipping the step is the quickest way to scorch leaves that took ten weeks to grow.
What goes wrong
What happens when you start too early?
Overgrown seedlings are the usual result of eagerness. The signs are stems that are tall and thin, roots circling the bottom of the cell, lower leaves yellowing and plants that flower in the pot. Tomatoes started in December are the classic example. They are far bigger than the guidance calls for by the time the weather allows planting out, and they suffer more at transplanting. Prevention is simple: sow inside the window for each crop and not ahead of it. If plants do get ahead of you, pot them up into a larger container and keep the light close.
What is damping off and how do you prevent it?
Damping off is a fungal problem that kills seedlings at the soil line. A seedling that looked healthy yesterday is lying on its side with a thin, pinched, water-soaked stem. It is most common when the potting mix is cold, soggy and crowded, and when air barely moves. Use clean trays and fresh, light seed-starting mix. Water from below or sparingly, let the surface dry between waterings, and run a small fan on low across the tray for part of the day. Once a seedling has collapsed it cannot be saved, and the neighbors in the tray need more air and less water.
Is it safe to plant out the week after the average last frost?
March 18 is an average, so in many years frost comes later. A tray of tomatoes or peppers set outside right after that date can be killed by a single clear night, and even without frost, cold soil stalls their roots. Wait past the average last frost date before setting out tender plants, and watch the forecast and the soil as well as the calendar. Hardy brassicas and lettuce can take the risk earlier. Cover anything vulnerable with a cloche or row cover when a cold night is forecast. The pace of spring is local, so your local garden club or a nearby nursery can tell you when plants actually go out in your district.
Common questions
When should I start tomatoes indoors in British Columbia?
Around Vancouver, February 4 to February 11 is the window, counting 5-6 weeks back from the average last spring frost of March 18. Colder districts should start later, because a later frost date moves the whole window with it. Tomatoes are tender, so they go out only after nights are mild.
Do I need grow lights to start seeds indoors?
For the December and January sowings, yes. Onions, leeks, celery, the cabbage family and peppers all start in the darkest part of the year, when a windowsill gives too little light and the seedlings stretch. For tomatoes in February and sweet potatoes in late February, a bright south-facing window can work, though lights still give sturdier plants.
What if my garden is colder than Vancouver?
Move every window later. Northern and upland districts have a later last spring frost than the Vancouver station, and some run weeks behind. Find your local frost range and count the same weeks back from it. Starting at the Vancouver dates just produces seedlings that wait in the pots.
Can I start beans, squash or carrots indoors?
In most cases, no. Carrots and other root crops resent transplanting, and beans and squash grow quickly and dislike root disturbance. Sow them directly in the garden once the soil is warm enough for each crop.
Sources: api.weather.gc.ca | planthardiness.gc.ca | extension.umn.edu