Starting Plants from Seed

Many Canadian gardeners start seeds indoors in January and February — then we’re ready to get out there by the time May long weekend rolls around!

When to start seeds indoors

windowsill1. Check the estimated last-date-for-frost here: last frost dates for Canada and US. Below are Alberta’s first and last frost dates.

2. Seed packets will tell you how many weeks indoors the plant will need, so with these two pieces of info, you’ll know when to start.

Seeds generally need about 6 – 8 weeks indoors. Some seeds, (for example sweet peas and zinnias) will be ok with less and geraniums and pansies need more time indoors — about 14 or 15 weeks.

Dahlia, coleus and petunias need 10 – 11 weeks indoors, start in late February. Start dianthus, phlox, sweet alyssum and African marigold in mid March, they’ll need about 9 weeks indoors. Starting seeds early gives young plants time to develop sturdy roots before planting in the garden.

albertafrostdatesMore specifics at the Alberta Ag and Forestry website.

Containers for planting seeds indoors

Flats with individual compartments (cells) work well and they’re found at the nursery or greenhouse. Flats should have small drainage holes in the bottom and a larger solid tray for support.

transplantingPots made from shredded wood, peat, newspaper or other organic material work well and may be placed directly into the ground to save transplanting stress particularly for plants with tender seedling roots. If you’re using trays saved from previous years, clean well before reusing.

Cover plant trays with clear plastic domes to allow light in and keep moisture contained, creating a small greenhouse effect. When the seedlings are a few inches tall the covers may be removed.

Potting mixture for planting seeds indoors

Organic potting mix works for both indoor and outdoor plants and contains no synthetic plant foods or chemicals. After placing potting mix into the trays, water and let settle — when the water is absorbed into the mix the cells should be nearly full.

How deep to sow the seeds

Plant each seed deep enough that you could set 3 more of the same seed above it and don’t forget to tag seeds — you’ll thank yourself when it’s time to transplant.

Packets will indicate whether the seed needs light or darkness to germinate; seeds that need dark may be covered with newspaper or a dark plastic bag until they’ve germinated.

How many seeds in each compartment

If the seeds are old or there’s some doubt, plant two or three together just in case a couple don’t germinate. If they all germinate, cut off growth on the extras at ground level rather than pulling them out so that newly forming roots will be undisturbed.

The best place in the house for growing seeds

Shelves from parkseed.comThe windowsill may not always be the best location for your seedlings, at night they are usually the coldest spots in the house and in the afternoons they may be the hottest. Seeds will produce strongest roots in consistently warm soil.

The average room temperature should be about 15C to 16C or higher.

There’s a nice selection of indoor growing cabinets, trays and lights  at

What to use as indoor plant lights

Seedlings grow well under fluorescent lights or “grow lights,” designed specifically for new plants. If you don’t want to invest in grow lights, cool white fluorescent or a combination of cool white and daylight tubes will work. Lights should be 2 to 4 inches from the tops of the seedlings and allow plants 12 to 16 hours of light per day. With too little light, seedlings will develop long thin stems as they reach for a light source. An auto-timer is handy for setting routine hours of light and dark.

Providing heat to the roots

Many garden centers sell mats to be used as a source of bottom heat for seedlings; they’ll germinate sooner and form healthier roots in warm soil.

Transplanting to larger pots indoors

indoorgarden2If your seedlings outgrow their little cells and it’s too early to plant outdoors, move them into larger containers, for example peat pots or Styrofoam cups; don’t forget to punch holes in the bottom for drainage. Scoop the seedlings out of their cells with a spoon or using the plastic ID tag as support, trying not to grab the stem or leaves; if it snaps off at the base the plant won’t survive. Adjust your light source to accommodate taller seedlings.

The hardening off process for seedlings

About two weeks before transplanting seedlings outdoors, begin the process called ‘hardening off’ or gradually getting them used to the outdoors. This minimizes transplanting shock and after all the time and care you’ve invested, it’s worth the effort.

Start by setting young plants outside in a shaded, protected area for a few hours a day during the afternoon. Bring them inside before the temps begin dropping in late afternoon; each day adding a bit more time outdoors. After a few days, let them sit in direct sunshine and gradually increase their time in the sun daily. After a couple of weeks the seedlings should be ready to stay outside until transplanted into the garden. Check the forecast for possible risk of frost.

Transplanting seedlings into the garden

To reduce transplanting stress, pick a cloudy day or late afternoon, direct sun may cause the plants to wilt at first but they should recover within a day or so.  If you’re putting newspaper or peat pots directly into the ground, ease open the bottoms of the pots to allow the roots to start spreading quickly and trim back the tops to soil level. Then water and tend and enjoy your plants for the season!




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