When to Start Seeds Indoors - With Free Printable Seeding Guide

Updated: Mar 23

If you are like me, you are anxiously anticipating and counting down the days to when you can begin your 2021 garden! For fellow Albertans or others with similar frost dates, I will share with you when to start seeds indoors in Alberta. If you live in a warmer climate these dates can be adjusted. Starting plants from seed indoors is not only fun but it ends up costing a fraction of the price that it would to buy these plants already started at a greenhouse!

Find your free printable guide to when to start seeds indoors below.

Vegetable seeds with a seed tray for the seeds to be started indoors in Calgary Alberta, Canada zone 3 growing zone.

To begin with I will discuss what supplies I use for starting plants and then I will share with you the dates I like to plant my seeds.

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Useful Supplies for Starting Seeds Indoors

Soil - It is helpful to have a good quality potting soil mix like this Pro-Mix (note, this is a large compact bale that will be enough to fill a large garbage can). If you attempt to use any compost or soil from outdoors you will most likely end up with soil gnats, aphids and possibly more problems. I do not suggest this rout. (Trust me, I have learned this the hard way!) Surprisingly, good quality potting soil mixes are actually soilless, containing no natural soil. They are great for retaining moisture but also not becoming waterlogged (which can cause seed and/or root rot). Potting soil is not good for vegetable garden beds (unless mixed with 1/3 compost and 1/3 top soil) but is wonderful for growing plants in containers! Be mindful of the fact that although this soil can retain nutrients, it does not contain many nutrients. Your plants will benefit greatly from fertilizing once every week or so. Check out your local garden center or hardware store for more potting soil options.

Grow Lights - It is not necessary to use grow lights if you have a sunny window but you will end up with less leggy, stronger and all around healthier plants by using grow lights. I started my seedlings indoors without grow lights in a sunny window for 2 years before I made the leap and purchased grow lights. These grow lights from Amazon are the grow lights I am using now.

Indoor Greenhouse - An Indoor greenhouse set up is also not necessary but does make germination quicker and the layered shelves enable you to grow more in a smaller space. I purchased one for the first time last year and love it!

Plug Trays/Plant Pots - You will also need a variety of plant plug trays or plant pots. You can get creative here if you would like. Some people use things like toilet paper rolls, egg shells, disposable cups or clear clamshell containers (the ones things like lettuce and grapes come in) to grow their plants. The type of container is not important as long as you have water drainage. If you use disposable cups, make sure you put holes in the bottom of them to allow for sufficient water drainage.

Labels - Use popsicle sticks, re-write on old plant labels or purchase blank labels so that you can keep track of what you are growing. I like to include the seed brand on my labels so I can make notes on what ones grow best. I bought these ones from Amazon and have enough to share with all my friends and to last me years to come!

Timer - Ideally you will have your grow lights on for 14 to 16 hours a day. I am not very consistent with turning my lights on and off manually and just purchased these programmable timers from Amazon to help me out! This way I can set the timer and forget about the lights.

Dates for Starting Seeds Indoors in Southern Alberta

Now that you have your supplies we get to the big question; when do we plant our seeds? I will share dates with you for when I start different varieties of plants for my zone 3 growing location in Alberta. Our last hard frost in the spring tends to be around the 3rd week in May but we often get one last light frost in early to mid June. I try to keep an eye on overnight forecasted low temperatures and cover sensitive plants during any dips near freezing. (Check out what plants are most sensitive and need to be covered here.)

It is important to realize that seeding dates can vary based on your lighting and growing environment. Adjust for how much sooner or later your last expected frost date is to make this list work for your location. (Find out frost dates for your location here.) Take notes on how your plants fare and whether you think it would be beneficial to start them sooner or later for next year. By doing this you will end up with your own perfected seeding dates.

Seed packages often state how many weeks before the last frost you should start your seeds indoors. This is a great guideline, but something you will want to adjust depending how big you want the plants to be by the time you plant them outside and how quickly they grow in your house. This is especially the case for flowers. Professional greenhouses likely seed earlier than my dates, but they have more space to pot plants up as they grow. If you have less space you may need to seed a few weeks later so you plants don't outgrow their smaller pots prior to moving outdoors.

I have included some of the flowers I like to start myself in this list. I find I am able to save so much money by growing my own flowers from seed!

Basil seedlings being grown from seed in a plant pot indoors in Alberta, Canada growing zone 3.

Indoor Seeding Dates for a Last Expected Frost date of June 1st to June 10th

January 14th - 21st










(If planting herbs in January, expect them to be in full sized pots by June. I like to start my herbs early so that I can begin harvesting them sooner, even while they are still indoors!)

February 14th - 21st

Brussels sprouts



Lobelia (Tip: Seed in a seeding tray and transplant when half an inch. Place 3 - 5 plants in each hole with holes 1.5 inches apart.)




Determinate Tomatoes (Expect these to grow and ensure you have room to transplant them as they grow out of their pots. Tip: transplant so that the leggy part of the stem is under the new soil.)

March 7th - 14th



Indeterminate Tomatoes (Expect these to grow and ensure you have room to transplant them as they grow out of their pots. Tip: transplant so that the leggy part of the stem is under the new soil.)





March 21st - 28th





April 1st - 7th





April 21st - 28th

Corn (Can also be direct sewn at the beginning of June)

Cucumbers (Plant in fiber pots that can be planted into soil with the bottom of the pot removed. They do not like their roots disturbed during transplant.)

Sunflowers (Can also be direct sewn around May 21st.)

Squash (Plant in fiber pots that can be planted into soil with the bottom of the pot removed. They do not like their roots disturbed during transplant.)

Click here to get your free printable zone 3 vegetable indoor seed starting guide!

If you live in a warmer climate and are looking for start dates more applicable to the warmer zones, I recommend checking out these lists.

Make sure you harden off your plants prior to transplanting to prevent them dying from shock.

To find out when I direct seed my garden, read When to Plant Your Vegetable Garden.

Flowers started indoors from seed in zone 3 Calgary Alberta, Canada.

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