Read on to find out exactly what seeds you can plant in May for a bountiful harvest in zone 3-5. Navigating unpredictable May weather can be difficult for many gardeners. This guide will help you to determine what seeds you should be starting indoors and which seeds can be directly sown outside in the month of May. The goal in planting the following seeds in May is to make sure that they can come to maturity and potentially produce a good harvest before cool weather and frost are on the horizon!
You will notice that many seed recommendations cross over between April and May planting. Which seeds you start and when really depends on your growing space, access to light, and your first and last frost date. For example, some people choose to get a head start on their melons in April and transplant them once or twice before planting them outside while others opt to start them mid-late May so that the young seedlings can go right into the ground.
Plant hardiness zones are used by gardeners, farmers, and other growers to help understand which plants will grow, thrive, come to maturity, and with perennials, survive the winter. Refer to these charts when making a plan about what seeds to plant in May to ensure that you have the most time for your plants to mature in your growing window.
Canadian Plant Hardiness (Growing) Zones
Canadian hardiness scales are more involved than other scales and are calculated using seven different factors including daily minimum and maximum temperatures, frost free days (above 0°C), snow depth, wind, rainfall, and suitability. Click here to find out about your Canadian Hardiness Zone.
USDA Plant Hardiness (Growing) Zones
USDA growing zones are more widely used and are based on the average annual minimum temperature of an area. It is divided into 10°F zones. Click here to find out about your USDA Hardiness Zone.
Tips + Tricks for Planting Seeds in May
Dedicate a warm space in your house, greenhouse or garage (21°C or 60-70°F) that you can start your delicate seedlings. Ensure that your plants have access to lots of light- both grow lights and south facing windows are great for starting plants. At this point, your seedlings will likely be able to go straight into the ground before transplanting or “potting up” so space shouldn’t be a huge issue.
Verse yourself on your own frost dates. I love to write these down for myself over the years so that I can look back at trends in my own garden. Organize the seeds that need to be started each month in separate envelopes or ziplock bags. Packages will often advise you to plant ‘x’ amount of days before last frost. PlantMaps.com is a great resource to help you find your own growing zone and your unique frost dates.
In Northern Alberta our last frost date is typically between May 21-31st and the first frost date in the fall lies between September 1st and 10th. The best reference for when you should plant individual seeds is the back of your unique seed package or the website that you ordered them from.
Use soil blocks to produce healthy seedlings that can be started in close proximity with zero plastic waste or extra cleanup. Check out my newest tutorial to discover how to mix, pack, and press soil blocks of various sizes easily.
Seeds to Start in May Indoors
As summer approaches, the list of seeds to start indoors goes down!
Cucumbers: Start some of your cucumber seeds indoors to get a jumpstart on the growing season. Unlike some other squash, cucumbers transplant relatively well. Start them 2-3 weeks before transferring them outside. I like to start some of my cucumbers early and direct seed some after-this way I have staggered harvesting times.
Beans (Snap Beans, Pole Beans, Dry Shell Beans, Lima Beans): Plant your bean seeds mid May in small pots 2-3 weeks before transplanting them into the garden.
Broccoli: Start Broccoli indoors in April or the beginning of May. It should be transplanted when conditions are consistently above zero and it has 6-8 true leaves.
Brussels Sprouts: Start Brussels sprouts in early May and transplant out when they have 6-8 true leaves and the last frost has passed.
Cauliflower: Start cauliflower in early May to transplant early June.
Celery: Start celery anytime in April or May indoors. Celery can be quite finicky (weak and spindly) as a young seedling; so it is good to give it a strong start indoors.
Kohlrabi: In short growing zones like zone 3, start kohlrabi indoors in early May. Planting outdoors in hotter months like late May and July can cause the plant to mature too quickly, resulting in smaller bulbs.
Pumpkins and other Squash: The appeal of waiting until mid-late May to plant your pumpkins and other squash is that you likely wont have to transplant them into a bigger pot, they can be put straight into the ground in June after the last frost. I recommend keeping a fan blowing lightly on them to help them harden off before exposing them to the elements.
Sweet Peas: Start your sweet peas indoors in early May to give them an early boost. Starting sweet peas early allows them to get strong before releasing them into the wild!
Seeds to Direct Sow in May Outdoors
Cold-hardy vegetables and flowers can be direct seeded into the garden in early May. More sensitive crops should be planted mid-late May depending on your last frost date. May is unpredictable! Be ready to cover the new seedlings in case of harsh spring winds, rain, frost, or snow.
Asparagus Crowns: Plant these in early May… as soon as your soil can be worked! This guide is awesome if you are an asparagus newbie.
Beans (Snap Beans, Pole Beans, Dry Shell Beans, Lima Beans, Soy): Plant your beans mid-late May after the risk of frost is gone. Refer to your package- some beans have different start dates!
Beets: Be sure to plant these 1-2 weeks after the last frost when the soil is warm to ensure that they produce good roots.
Cabbage: Starting very early in May, cabbage seeds can be direct sown into the ground BUT you should add 20-25 days to maturity according to TNT Seeds Cabbage Growing Guide.
Carrots: Carrots are fairly cold tolerant and can be started in May just after the last frost date. Consider leaving room for succession planting so that you have delicious baby carrots and fresh edible carrot tops throughout the growing season.
Cucumbers: Direct seed your cucumbers in late May after the last chance of frost has passed. Alternate between started seedlings and un-started seeds to stagger your harvest times.
Herbs: dill, basil, cilantro, mint, parsley and other herbs can all be seeded directly into the garden in mid-late may. Germinating herbs in cold soil can be hard, it requires a lot of patience and extra time.
Peas: Peas can be direct sown as soon as the ground is workable! Get them in soon.
Pumpkin and Other Squash: Direct seed your pumpkins and squash after the last frost date.
Radishes: Radishes can be direct seeded just after the last frost in your area. Seeds will sprout within 7-10 days and can be successively planted for a harvest that stretches throughout your growing season.
Salad Greens: Arugula, spinach, Swiss chard, romaine, kale, and other salad greens can be planted at this time. Leave room for succession planting every couple of weeks to ensure that you are able to harvest throughout the summer.
Scallions: Scallions should be started just after the last frost date and can successively be planted every three weeks or so for a harvest that lasts into the fall.
Sunflowers: If sunflowers started indoors become root bound or leggy before transplanting outdoors, they will need to be staked. Direct seed
Sweet Peas: Sweet peas can be direct sewn in early may in cool soils and can tolerate light frost.
Turnips: Turnips should be direct sown after the last frost date and can be successively planted. Check out my tutorial to see how you can harvest and use turnip greens in your recipes!
What is Hardening Off + Why Do It?
Hardening off readies your plants that were started indoors to be planted outdoors without shocking, stunting, or *gasp* killing them. Exposing your tender seedlings or other started plants to sun, rain, and wind before they are permanently outside is a must.
Set your plants outside for short intervals if possible every day for 1-2 weeks before planting outdoors to get them used to the elements.
May weather is often unpredictable. In my zone 3 garden, the snow has just melted but it is often still dipping below 0°C (32°F) at night. It is exciting to think of planting cold tolerant crops in early May but gardeners need to be prepared for spring winds, heavy rains, frost, and even snow!! Have a plan for cool nights and wild weather. It is important to protect your precious seedlings and young plants from the elements. There are several options for crop protection including row covers, covering with a blanket or tarp, garden cloches, and pulling mulch up overtop of the tiny seedlings. This year I will be mulching in between my rows and using the mulch as a backup warmth layer when needed for my early plants.