Author: Rebecca Last, Gardening at Last, Master Gardeners of Ottawa-Carleton; published with permission

Browsing seed catalogues and imagining my garden for the coming season are among my favourite activities for these cold winter days. Growing new plants from seed is easy, fun, and economical. But first, you need to know when to start seeds.

To answer this question, you need to know:

  1. When you want this plant to be ready to plant out in the garden (aside from the restrictions of our grow zone, gardeners have some discretion here), carefully reading the seed’s catalogue listing or package will usually provide the other information you need. However, for more unusual perennial seeds, you may need to do some research online to find out.
  2. How long will the seeds take to germinate and grow to a large enough size for planting outside?
  3. If there are any specific requirements for breaking dormancy,?

For example, suppose I have seeds for swamp milkweed (Asclepias incarnata), a lovely native perennial that is helpful for monarch butterflies. I want to plant the milkweed after our last anticipated frost on Victoria Day weekend (or the following week for very tender plants). So, here’s my calculation for when to start the seeds:

  1. Plant outside on May 23rd. Back up one week for hardening off the seedling (May 23 ← 7 days = May 16)
  2. The seeds will take 10-15 days to germinate, so we count back another two weeks (May 16 ← 14 days = May 2). I also know that my newly sprouted seedling will need about a month to become large enough to transplant. (May 2 ← 30 days = April 2).
  3. Being a native, my swamp milkweed requires a period of cold stratification to break dormancy. This means I’ll have to store the potted seeds in the fridge for about a month before they germinate. So, we back up another month. (April 2 ← 30 days = March 3).

Therefore, I will need to pot up the seeds no later than March 3. Of course, seeds don’t have calendars or clocks. All my timing calculations are approximate. Actual plant growth depends on us, as gardeners, providing the right conditions and a measure of luck.

There is Flexibility

Sometimes we can accommodate seedlings that don’t perform as expected by being flexible when we plant outside. For our milkweed example, I only need to have the seedling ready when the first monarch butterflies arrive in my garden, which is usually the second or third week of June. Thus, I have a good month of flexibility to get my milkweed seedlings ready for butterfly visitors.

When you’re planning an entire garden of annual vegetables, things can become more complicated. The calculations are the same, but you need to run the numbers for each species and for each planting that you’re planning. Some market gardeners use a planting calendar, as shown in the illustration.

Table on When to Plant Seeds in Hardiness Zones 4b to 5a

You need to consider whether your seeds are for warm- or cool-season plants.

Seeds for Warm Season Vegetables

Warm-season vegetables cannot be planted outside until the soil has warmed and all danger of frost has passed. Typically, these are the veggies we plant outside on or after the Victoria Day weekend. Warm-season veggies include much of the nightshade (Solanaceae) family (tomatoes, peppers, eggplant, tobacco, ground cherries, etc.), the cucurbit (Cucurbitaceae) family (everything from acorn squash to zucchini, including melons), beans, corn, okra, and sweet potatoes. Most warm-season plants need to be started indoors as they require a longer growing season than we have in Ottawa. Exceptions are those that don’t transplant well, such as beans, ground cherries, and sweet potatoes.

Seeds for Cool Season Vegetables

Cool-season vegetables, on the other hand, can be sown directly as soon as the outdoor soil can be worked. Cool season veggies include: many root veggies (beets, carrots, parsnips, potatoes, radishes, etc.); brassicas (broccoli, cabbage, kale, etc.); leafy veggies like chard, lettuce, and spinach; peas; and a lot of the allium (Amaryllidaceae) family, such as onions and leeks. Root vegetables should be directly sown, as they are very difficult, if not impossible, to transplant. Personally, I like to start my leafy greens indoors to give them a head start. I can trim the surplus plants and eat them as microgreens before I
harden them off and transplant them outside.

Many cool-season veggies will mature early, leaving a hole in your garden. Lettuce, for example, will bolt and become inedible after the summer heat arrives. Radishes typically form and can be eaten within 4-5 weeks of planting. Savvy gardeners plan to fill these holes in the garden with a second crop—a practice called succession planting.

Planning for a Second Crop

Planning your second crop requires knowing how long it will take your plant to finish growing and be harvested. “Days to maturity” is standard information in seed catalogues and on the seed package. The only difference between this mid-season calculation and the spring one is that we are counting back not from the first frost-free date in spring but from the last frost-free date in fall.

Ottawa typically sees its first frost about the second week of October—let’s say October 10. If I harvest my garlic on August 1 and immediately plant another crop, it needs to be one that will mature in the 70 days before frost threatens. I need a crop that will tolerate cooler fall weather, and to avoid problems with pests and diseases, it shouldn’t be in the same family as the plants I’ve just removed. Garlic is an allium, so I don’t want to plant anything related to onions. Some varieties of tomatoes mature in two months, but these heat lovers won’t appreciate the cool autumn nights. Leafy greens, like kale or spinach, would be a good choice. I could plan a couple of harvests of radish and lettuce. Carrots or beets would also work well.

Now you can calculate when to plant what seeds, and you should also have an appreciation for cultural and climate considerations. Happy growing!

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