How to Grow Tomatoes in Ontario

Growing Tomatoes in Ontario: A Complete Guide

Tomatoes are the crown jewel of Ontario gardens. Whether you are in Ottawa (Zone 5a), Toronto (Zone 6b), or the Niagara region (Zone 7a), this guide will help you harvest heavy, flavourful tomatoes from your backyard or raised beds.

Understanding Ontario's Growing Season

Ontario's tomato season is shorter than many gardeners realize. The last spring frost typically falls between mid-May (southern Ontario) and early June (northern Ontario), while the first fall frost can arrive as early as late September. That gives you roughly 100 to 130 frost-free days depending on your location. Since most indeterminate tomatoes need 70 to 90 days from transplant to first ripe fruit, timing your start indoors is critical.

Choosing the Right Varieties

Not every tomato variety thrives in Ontario's climate. Stick to varieties proven in Canadian conditions:

Early season (55-65 days): Sub Arctic Plenty, Stupice, Glacier, and Manitoba. These are ideal for northern Ontario or gardeners who want the earliest harvest possible.
Mid season (65-80 days): Celebrity, Defiant, Mountain Merit, and Juliet. Celebrity is a reliable disease-resistant hybrid that does well across the province.
Full season (80-95 days): Brandywine, Cherokee Purple, San Marzano, and Mortgage Lifter. These heirloom favourites need the longest, warmest summers. In Zone 5, consider using black plastic mulch and row covers to give them the heat they need.
Paste tomatoes: San Marzano, Roma, and Amish Paste. Perfect for canning and sauces, these determinate varieties ripen their crop in a concentrated window, which is useful for preserving.

Starting Seeds Indoors

In Ontario, start tomato seeds indoors 6 to 8 weeks before your planned transplant date. For most of southern Ontario, that means sowing between March 15 and April 1.

Use a quality seed-starting mix. Avoid garden soil, which compacts in small cells and can carry disease. A peat-free or coco coir-based mix with perlite works well.
Sow seeds 6 mm (1/4 inch) deep in cell trays or small pots. Water from the bottom to keep the surface from crusting over.
Maintain 21-24 C (70-75 F). A heat mat speeds germination from 10 days to 5-7 days. Remove the mat once seedlings emerge.
Provide strong light. A south-facing window is rarely enough. Use a full-spectrum grow light 5-10 cm above seedlings for 14-16 hours daily. Leggy seedlings are the number one complaint from Ontario gardeners starting indoors.
Pot up when the first true leaves appear. Move seedlings to 10 cm (4 inch) pots, burying the stem up to the first leaves. Tomatoes form roots along buried stems, giving you a stronger plant.

Hardening Off

This step is non-negotiable. Ontario spring weather can swing from 25 C to 5 C overnight. Begin hardening off 7 to 10 days before transplanting:

Day 1-2: Place seedlings outdoors in a sheltered, shaded spot for 1-2 hours.
Day 3-4: Increase to 3-4 hours with some direct morning sun.
Day 5-7: Gradually increase sun exposure and time outdoors, bringing plants in at night if temperatures dip below 10 C.
Day 8-10: Leave plants out overnight if no frost is forecast. By now they should be ready for the garden.

Transplanting Outdoors

Wait until night temperatures consistently stay above 10 C (50 F) and soil temperature reaches at least 15 C (60 F). In the Greater Toronto Area, this is typically the last week of May or first week of June. For Ottawa and eastern Ontario, the first or second week of June is safer.

Dig a deep hole and bury the transplant up to the top set of leaves. This encourages a massive root system.
Space plants 45-60 cm (18-24 inches) apart for staked or caged indeterminate types, or 60-90 cm (24-36 inches) for determinate bushes.
Add a handful of compost and a tablespoon of bone meal to each planting hole. Calcium from bone meal helps prevent blossom end rot, a common Ontario problem.
Water deeply at transplant. Give each plant 1-2 litres of water to settle the soil around the roots.

Supporting Your Plants

Ontario's heavy thunderstorms and humid summers mean tomato support is essential:

Staking: Drive a 1.5 m (5 foot) stake 30 cm into the ground beside each plant. Tie the main stem loosely with soft twine every 20-30 cm as it grows.
Caging: Use sturdy concrete reinforcing wire cages (not the flimsy cone cages from garden centres). A 60 cm diameter cage made from 150 cm tall wire fencing is ideal.
Florida weave: For row plantings, drive stakes every 2-3 plants and weave twine on alternating sides. This is the most efficient method for larger gardens.

Watering and Feeding

Tomatoes need consistent moisture. Inconsistent watering is the primary cause of blossom end rot and cracking in Ontario gardens.

Water deeply 2-3 times per week rather than lightly every day. Aim for 2.5-5 cm (1-2 inches) of water per week, including rainfall.
Water at the base of the plant, never overhead. Wet foliage in Ontario's humid summers is an invitation for early blight and late blight.
Mulch with 8-10 cm (3-4 inches) of straw or shredded leaves to maintain even soil moisture and suppress weeds.
Feed every 2-3 weeks with a balanced organic fertilizer (such as 4-4-4) once fruits begin to set. Too much nitrogen early on produces lush foliage but delays fruiting.

Managing Common Ontario Diseases

Ontario's humid summers create ideal conditions for fungal diseases:

Early blight (Alternaria): Brown concentric rings on lower leaves. Remove affected foliage promptly. Mulch heavily to prevent soil splash.
Late blight (Phytophthora infestans): Grey-green water-soaked patches that spread rapidly. This is the same pathogen that caused the Irish Potato Famine. If late blight hits, remove and bag affected plants immediately. Do not compost them.
Septoria leaf spot: Tiny spots with grey centres on lower leaves. Improve air circulation by pruning suckers and spacing plants adequately.

Prevention is your best strategy: rotate tomatoes to a new location each year (3-year rotation minimum), prune lower branches that touch the soil, and choose disease-resistant varieties labeled VFN or VFNT.

Extending the Season

Ontario's first fall frost can cut your tomato season short. Here are proven ways to squeeze extra weeks out of your harvest:

Row cover or frost blanket: Drape lightweight fabric over cages on cold nights to protect against light frosts down to -2 C.
Wall o' Water or water-filled cloches: These protect transplants from spring cold snaps, letting you plant 2-3 weeks earlier.
Green tomato ripening: When frost threatens, harvest all remaining tomatoes. Place green ones in a single layer in a cardboard box with a ripe banana. The ethylene gas will trigger ripening over 1-3 weeks.

Harvest Tips

Pick tomatoes when they are fully coloured but still slightly firm. In the peak of August heat, check plants daily as ripe fruit can crack or attract pests within 24 hours. Store fresh tomatoes at room temperature, never in the refrigerator, which destroys flavour and texture.

With the right variety selection, careful timing, and consistent care, Ontario gardeners can enjoy a tomato harvest from late July through October. Plan ahead, start early, and give your plants the support they need for a bountiful season.