For vegetable gardeners, winter begins as a welcome break from the overwhelm of late summer and fall harvesting, preserving and clean-up. Then, each year, like clockwork, winter's relief gives way to impatience; a desire to get back into the dirt. It’s tempting to start your vegetable seeds as early as possible, but getting your timing just right is important. Start too late and your vegetables may not mature in time for harvest; start too early and your stressed and overgrown transplants may fail to thrive. It’s a Goldilocks problem. Conventional advice to new gardeners often comes in the form of easy but frustratingly vague and sometimes inaccurate answers - from lists of what to plant each month to advice that relies only on frost dates and garden zones.
Expected transplant dates, determined by the soil and air temperatures of your garden, are more reliable indicators of when you should start seeds than frost dates.
The best answer to the question of when to start your seeds is one no one wants to hear: it depends. And what factors, specifically, it depends on will be unique to you and your garden: the climate of your region, the air and soil temperatures and microclimates present in your garden, the season-extension techniques, if any, you plan to use, the specific vegetables you want to grow and how much of your own food you want to produce. Buying transplants from a garden centre is certainly easier, but if you’re determined to grow your own vegetables from seed, you will need to tailor your timing to the conditions of your garden by developing your own seed-starting calendar over time.
Conventional Gardening Advice is your Jumping-off Point
Look first at the information provided on your seed packets. Use the details they provide, especially if you bought your seeds from a local company that produces their seeds near where you live. But know that you will likely need more information. Seed packets can be vague (does “days to maturity” mean from seed or from transplant date? Who knows? “Plant out when ground has warmed.” But how warm is “warmed”?) and the information they provide is inconsistent between seed companies and varieties.
A lot of sources tell you what to plant each month. I don’t know why. What you should plant in March depends entirely on where you live - how far North or South, coastal or inland etc. Unless you know this sort of advice is coming from someone who lives near you, it isn’t very useful.
The most common type of advice on when to start seeds tells you to use your first and last frost dates to figure it out. This method is a good jumping-off point for your seed-starting calendar (though it isn’t perfect, and I’ll get to why below). To sow spring seeds using this method: use a farmer’s almanac or google search to find the last frost date for where you live. Start your seeds however many weeks before or after that date that each crop needs to reach its ideal size for transplanting. You will need to know:
- Your last frost date.
- How many weeks of indoor growing each vegetable needs before transplanting (see chart below).
- Which vegetables can withstand frost (and can therefore be put out before your last frost date) and which cannot.
Seed Starting Example 1: Say you live in Calgary, Alberta and want to grow kale and tomatoes. Your last frost date is between May 11th and 20th. You split the difference and assume May 15th will be your last frost date.
- Kale (cool-season crop): Since kale does well with frost and cool weather, you plan to transplant it outside two weeks before your last frost date, on May 1st. Since most leafy greens need about 4 weeks to reach ideal transplant size, you start your kale seeds on April 17th.
- Tomatoes (warm-season crop): Most conventional advice says to start tomato seeds 6-8 weeks before your last frost date. You once again split the difference and start your tomato seeds on March 27th, planning to transplant them outside on May 16th. For some people, this will work well. For others, it won’t.
Most seed starting advice that relies on frost dates is actually telling you to start seeds x-many weeks before the date you can safely transplant seedlings outside, not x-many weeks before the last frost date. It’s confusing because most advice assumes that these dates will be the same. But, depending on where you live, they may not be. A more reliable way to know when to start your seeds is to figure out when the soil temperature in your garden is likely to be optimal for transplanting what you want to grow. Then, count back the number of weeks of indoor growing time those vegetables will need before transplanting from that date.
Seed Starting Example 2: My garden is in a cool coastal region in USDA plant hardiness zone 6b. My last frost date is somewhere between April 21st and May 6th. However, where I live, Spring is long, cool and wet. After my last frost date there are still several weeks before nighttime air and soil temperatures are reliably warm enough for planting warm-season vegetables, usually around mid-June.
Most seed starting advice that uses frost date is actually telling you to start seeds x-many weeks before the date you can safely transplant seedlings outside, not x-many weeks before the last frost date.
When I’ve followed conventional advice and started my tomatoes 6-8 weeks before my last frost date one of two things happened:
1) I transplanted them out early-mid May. They rarely died outright, but they spent weeks struggling and suffering in temperatures they did not like. And they never caught up in size, yield or health to transplants that weren’t subjected to those conditions.
2) I kept them inside for 4-5 additional weeks waiting for ideal temperatures. They became root-bound and overgrown in their pots, more susceptible to disease and nutrition deficiencies, and struggled after transplanting to make up for lost growing time.
Instead, I’ve learned that what works for my garden is to start my tomatoes relative to my optimal transplant date - about 4 weeks later than I would start them relative to my last frost date - so that they are 6-8 weeks old for transplanting in mid June, when conditions in my garden are likely to be ideal for tomatoes.
How to Calculate your Seed-Starting Dates: Step-by-Step
Seeds need warmer temperatures for germination than transplants need for growing. If you want to direct sow, you will need to start later than if you start indoors.
Grouping vegetables with similar soil temperature preferences and growth habits simplifies the process of figuring out what seeds to start when. You can do this in many ways, with varying degrees of precision and detail. The goal is to figure out what works for you. But a good place to start is by dividing the cool season crops from the warm season crops.
- Frost hardy crops can be transplanted out several weeks before your last frost date. Just how many weeks will depend on your garden’s growing conditions. Aim to plant less hardy cool season crops, like lettuce, on or just before the frost date. Both frost hardy and frost tender cool-season crops benefit from some sort of season extension method such as a row cover or cold frame.
- Warm season crops need to be transplanted well after all danger of frost has passed. But some are able to handle cool weather better than others. Tomatoes need lots of sun and warmth, but reserve the warmest locations in your garden for crops like eggplant and hot peppers.
- Some warm season crops will spend months with you indoors, needing to be potted up into larger containers. Others, like cucumber and squash, should only be started a few weeks before you plan to transplant them out.
Ideal Growing Temperatures for Cool and Warm Season Crops/Vegetables
COOL SEASON VEGETABLES:
Minimum soil Temperature:
Heavy Frost Ok
Broccoli, Brussels Sprouts, Cabbage, Green Onions, Kale, Mustard, Radish, Spinach
Minimum soil Temperature:
4C - 7C (39-44F)
Light Frost Ok
Beets, Carrots, Cilantro, Dill, Lettuce, Parsley, Peas
WARM SEASON VEGETABLES:
Minimum soil Temperature:
10C (50-F) Nighttime
Beans, Tomatoes, Cucumbers
Minimum soil Temperature:
15C (59F) Nighttime
Peppers, Eggplant, Melons
Ideal Transplant Size/ Indoor Growing Time:
COOL SEASON VEGETABLES:
Broccoli, Cabbage, Cauliflower
Leafy Greens, Peas, Radishes
WARM SEASON VEGETABLES:
Tomatoes, Hot Peppers, Eggplant, Basil
Cucumbers, Squash, Melons, Peas, Beans
Days To Maturity
Days to maturity is another key variable for determining when to start your seeds. Aim to transplant cool season crops when your soil temperature is right and it’s early enough for them to reach maturity before the weather gets too hot. Transplant warm season crops when your soil temperature is right and it’s early enough for them to reach maturity before the last frost in fall. If your cool spring or warm summer season is too short for this, you may be able to get around it by choosing quick growing or hot/cold tolerant varieties.
Gardening Techniques that Affect your Seed-Starting Calendar
Lots of techniques exist to extend the period of time a crop can grow in your garden or speed up the time it might otherwise take to grow, so you can grow more or grow earlier. If you plan to use any of these techniques, you might need to adjust your seed starting schedule accordingly.
- Row covers are an easy way to raise the temperature around your transplants, allowing them to go out weeks earlier. Low tunnels and cold frames have the same effect, but require more effort.
- Covering vegetables with shade cloth extends the period of time cool weather crops can be in the garden without bolting and protects heat-loving crops from the hottest days in warm climate gardens.
- Adding a heat mat to your indoor seed starting setup speeds up germination significantly.
The weather conditions of your garden are largely determined by your garden zone and other features of your region (wet, dry, windy, temperate etc). But home gardens also contain microclimates - smaller areas within your garden that, for various reasons (relation to house and other structures, direction and path of sun, wind conditions), are warmer or cooler (or wetter or drier) than the general conditions of your garden zone or region. Microclimates can allow you to transplant (and therefore start) seedlings earlier or grow vegetables that otherwise might not be suited to your garden zone or region. You can work with or create microclimates by constructing above-ground beds, using planters and pots, placing heat-loving crops in sheltered locations against south-facing walls or behind wind-breaking hedges or fences.
Yes, you can grow almost anything, besides carrots, indoors as transplants. It’s a great idea for cool or short season gardeners who want to get vegetables started before it’s warm enough outside.
I described above the example of how, because of the cool springs in my region, I plant my tomatoes later than conventional frost date advice suggests. However, I have also had success starting tomatoes a few weeks earlier when I choose early “short season” tomato varieties and plant them in a warm microclimate - a large deep planter pushed against a south-facing wall.
Succession sowing refers to a category of growing strategies that involve sowing seeds at multiple times throughout the year to produce vegetables more efficiently and over a longer period of time. From sowing small amounts of lettuce every few weeks in spring to starting a whole garden full of seeds in midsummer for a fall crop, succession sowing adds more elements to your seed starting schedule, but may be worth the extra effort for you if:
- You want a consistent supply of your favourite vegetables throughout the growing season rather than big gluts of produce you need to harvest all at once and cook or preserve.
- You want to grow an additional crop of vegetables in the Fall, after your summer produce is harvested, when no transplants are available at garden centers.
The most important thing to remember about starting vegetables from seed is that almost every guideline and piece of advice, including what I’ve recommended here, is provisional. Treat seed-starting advice as guidelines and suggestions, not set-in-stone rules. The key is to use the best general guidelines you can find, keep good records of your progress and, over time, customize your seed starting schedule to the conditions present in your garden and your preferences as a gardener. Learning to grow food from seed is about attitude, process and practice, not just acquiring a list of dates. Developing your own seed starting calendar is a multi-year process of trial and error and experimentation and you won’t get everything right the first try. And that’s fine, it’s all part of the process of learning to garden.