Our last frost date is usually in May, seeds started indoors in March will be ready for transplanting outdoors once the weather warms up. March is an important time to get organized and prepare for the summer garden. By now, the plants we started earlier in the season are growing nicely in the grow tent, and it’s time to make room for the rest of the plants that we’ll start this month.
If you’re in zone 5b and missed the late February start date, don’t worry, there’s still time for you to plant in March. This guide will walk you through what seeds to start indoors this month, including some tips for growing each vegetable to help your seedlings thrive.
Why start seeds indoors?
Zone 5b has a relatively short growing season, with the last frost date typically occurring in late April or early May. Starting seeds indoors in March gives your plants a head start, ensuring they are strong, healthy, and ready for transplanting once the weather warms up.
Many vegetables, especially warm season crops, need this early start to produce a bountiful harvest.
Seeds you can start indoors in March
Here’s a detailed list of some vegetables you can start indoors in March:
Tomatoes
- Why start now: Tomatoes need a long growing season and cannot tolerate frost. Starting them indoors in early to mid March ensures they are strong enough to transplant in late May.
- How to grow indoors:
Fill seed trays with your seed starting mix.
Sow seeds about 1/4 inch deep and lightly cover with soil.
Keep trays in a warm spot (70–75°F/21–24°C) until germination.
Provide bright light (grow lights) once seedlings emerge.
Transplant to larger pots when they develop at least two sets of true leaves.
Peppers
- Why start now: Peppers, like tomatoes, require a long growing season and need consistent warmth. We usually start our pepper seeds in January but if you never got the chance to start early, there’s still time.
- How to grow indoors:
Sow seeds 1/4 inch deep in seed-starting mix.
Maintain a temperature of 75–85°F (24–29°C) for germination. A heat mat can help.
After germination, provide 12–16 hours of light daily.
Transplant to larger pots when seedlings have 2–3 sets of true leaves.
Eggplants
- Why start now: Eggplants are another warm season crop that thrives with an early indoor start.
- How to grow indoors:
Sow seeds 1/4 inch deep in seed trays.
Keep the soil consistently warm (70–80°F/21–27°C).
Use grow lights to provide 12–16 hours of light daily.
Transplant to larger pots as seedlings grow.
Asian greens (bok choy, tatsoi, gai lan)
- Why start now: These fast growing cool season crops can be transplanted outdoors as soon as the soil is workable.
- How to grow indoors:
Sow seeds 1/4 inch deep in seed trays or small pots.
Keep the soil moist and provide bright light.
Transplant outdoors after hardening off when temperatures are consistently above 40°F (4°C).
Luffa
- Why start now: These squash varieties benefit from an early start indoors to ensure they mature in our zone 5b short season.
- How to grow Indoors:
Sow seeds 1 inch deep in individual pots.
Provide consistent warmth (75–80°F/24–27°C) and bright light.
Transplant outdoors after the last frost.
Bitter melon
- Why start now: Bitter melon is a tropical vine that needs a long, warm growing season. Starting indoors in March is ideal.
- How to grow indoors:
Soak seeds in warm water for 24 hours before planting.
Sow seeds 1/2 inch deep in seed-starting mix.
Keep soil warm (80–85°F/27–29°C) with a heat mat.
Provide 12–16 hours of light daily.
Okra
- Why start now: Okra thrives in hot weather, so starting indoors in March gives it a head start.
- How to grow indoors:
Soak seeds in warm water overnight before planting.
Sow seeds 1/2 inch deep in seed trays.
Keep soil warm (75–85°F/24–29°C) and provide bright light.
Transplant outdoors once the soil is warm.
Broccoli
- Why start now: Broccoli takes roughly 6-8 weeks to grow from seed to transplant stage. Starting in early March ensures you can plant them outdoors by mid April.
- How to grow indoors:
Use a seed-starting mix and small containers or seed trays.
Plant each seed about 1/4 inch deep and lightly cover with soil.
Keep the soil moist but not soggy, and maintain temperatures around 70°F (21°C).
Once the seedlings emerge, provide 12-16 hours of light daily using grow lights.
Transplant outdoors when seedlings have 4-6 true leaves and daytime temperatures are consistently above 50°F (10°C).
Cabbage
- Why start now: Cabbage is a cold-hardy crop that matures in 70-100 days. Starting indoors early gives it a head start.
- How to grow indoors:
Sow seeds 1/4 inch deep in seed-starting trays.
Keep the soil temperature between 65°F and 75°F (18-24°C) for germination.
Thin seedlings so they’re 1-2 inches apart once they sprout.
Harden off plants by gradually exposing them to outdoor conditions before transplanting.
Kale
- Why start now: Kale grows quickly and thrives in cool weather, making it perfect for starting indoors in March.
- How to grow indoors:
Sow seeds in small pots or trays about 1/4 inch deep.
Maintain a soil temperature of 65°F-75°F (18-24°C).
Provide strong light and water consistently.
Transplant outdoors when plants are 4-6 weeks old.
Some herbs require a longer growing period, while others are hardy and thrive in cooler weather. Here are a few great options:
Parsley
- Why start now: Parsley is slow to germinate and benefits from an early start.
- How to grow indoors:
Soak seeds in warm water for 24 hours before planting to improve germination.
Sow seeds 1/4 inch deep in seed trays.
Keep soil moist and maintain a temperature of 70°F (21°C).
Germination may take 2-4 weeks, so be patient!
Basil
- Why start now: Basil grows quickly but is sensitive to frost, so starting it indoors ensures it’s ready when the weather warms.
- How to grow indoors:
Sow seeds 1/4 inch deep in seed trays.
Keep the soil temperature between 70°F-75°F (21°C-24°C).
Provide 12-16 hours of light daily.
Transplant outdoors after the last frost or grow in containers indoors.
Asian herbs (shiso, thai basil, lemongrass)
- Why start now: These flavourful herbs are slow growers and benefit from an early start indoors.
- How to grow indoors:
Shiso: Sow seeds on the soil surface and lightly press down; they need light to germinate.
Thai basil: Sow seeds 1/4 inch deep, keep soil warm, and provide bright light.
Lemongrass: We start by using rooted stalks in water to set roots, later to be transplanted into the soil.
Tips for success if you’re late starting seeds
If you missed the late February start date for certain crops, here’s how to maximize your time:
Choose faster maturing varieties: Look for seeds labeled as “early” or “fast maturing.” For example, some broccoli or cabbage varieties mature in as little as 50-60 days.
Use heat mats: Warm soil speeds up germination, especially for slow starters like peppers or eggplants.
Provide light: Invest in grow lights to ensure seedlings are strong and healthy. Natural light from windows alone may not be sufficient in March.
Consider direct sowing: Some crops, like leafy greens, can be direct sown outdoors in late March or early April if the soil is workable.
Key dates for transplanting
Cool season crops: Transplant outdoors 4-6 weeks before the last frost (mid to late April in zone 5b).
Warm season crops: Depending on the weather, transplant outdoors after the last frost (late April to early May) Keep in mind frost dates have shifted over the years.
For us, May 24 weekend has always been our go-to timeframe. It’s a popular and generally safe time to plant outdoors in Ontario, zone 5b especially for flowers and vegetables. You should consider local frost dates, soil temperatures, and weather conditions to ensure the best results. If the weather is still cool or unpredictable, it may be wise to wait a little longer.
With March being a pivotal month for some cold climate gardeners, it’s time to sow seeds for a variety of vegetables, from tomatoes and peppers to Asian greens to bitter melon. With proper care and planning, this early investment in your garden will pay off with a lush, productive harvest come summer.
It’s an exciting time knowing that the new gardening season is just around the corner and we can’t wait to start some of these seeds indoors this month. Let us know if you’re starting what seeds you’re starting this month! We’d love to hear from you.
Let’s grow more together! 🌱
