Growing Tomatoes in Zone 5b/6

STARTING Start indoors: mid-March to early April

Tomatoes need 6–8 weeks indoors before transplant — no more. For a late-May transplant date in Durham, sow seed mid-March to early April.

1. Sow in small pots of seed-starting mixPlant seed about 0.5 cm deep, keep the mix warm (20–25°C) and moist. A heat mat speeds germination from two weeks to under one.
2. Light is everything once they sproutSeedlings need 14–16 hours of strong light — a sunny south window alone usually isn’t enough in a Canadian March. A simple LED or fluorescent fixture a few centimetres above the plants keeps them stocky instead of leggy.
3. Pot up onceWhen the first true leaves are out and roots fill the cell, move each plant to a 10 cm pot, burying the stem up to the first leaves. Tomatoes root along buried stem — this builds a stronger plant.
4. Feed lightlyHalf-strength balanced fertilizer once a week after potting up is plenty. Overfed indoor tomatoes get soft and sappy.

PLANTING OUT Transplant after frost: late May

The May long weekend is the traditional marker, but the forecast decides. Tomatoes are set back by cold even when it doesn’t kill them — soil below 10°C stalls growth for weeks.

1. Harden off for 7–10 daysMove plants outside for a few hours of shade the first day, adding sun and time daily until they’re out full days. Skipping this sunburns leaves white and checks growth.
2. Check nights, not daysTransplant when overnight lows are reliably above 10°C and no frost is in the 10-day forecast. Cloudy late afternoons are the kindest planting time.
3. Plant deepSet the plant so a good portion of stem is buried (up to the first strong leaf set) in a trench or deep hole. Roots form along the buried stem, anchoring the plant against July wind.
4. Space generously and mulchGive each plant 60–90 cm. After the soil warms (early June), mulch 5–8 cm deep with straw or shredded leaves to keep soil — and soil-borne disease spores — off the foliage.
5. Water at the base, deeplyTomatoes want consistent moisture: deep watering that keeps the root zone evenly damp. Wild wet-dry swings are what crack fruit and trigger blossom end rot.

SUPPORT & CARE Support them before they need it

An unsupported tomato becomes a tangled mat that rots from the inside. Choose one system at planting time.

1. Cages for determinate (bush) varietiesCompact varieties set fruit in one main flush and suit a sturdy cage. Cheap conical cages are too small for most — size up.
2. Stakes or a trellis for indeterminate varietiesVining types grow until frost. Tie the main stem loosely to a tall stake every 25–30 cm as it climbs.
3. Prune suckers on indeterminate plantsPinch the shoots that grow in the angle between stem and leaf branch while they’re small, keeping one or two main stems. Better airflow, bigger fruit, less disease.
4. Watch for the big three problemsBlossom end rot (dark leathery patch on the fruit’s bottom — inconsistent watering, not usually a calcium shortage in Ontario soils); early blight (yellow leaves with dark spots starting low — mulch, airflow, remove worst leaves); cracking (splitting after heavy rain follows dry weather — steady watering prevents most of it).

WATCH OUT Common mistakes

1. Transplanting in early May ‘to get ahead’Cold soil and one frosty night erase any head start. A late-May transplant routinely out-produces an early-May one by August.
2. Watering the leaves with a sprinklerWet foliage is how blight spreads. Water the soil, in the morning, and mulch to stop splash-back.
3. Crowding plants for more tomatoesCrowded plants share disease and shade each other’s fruit. Four well-spaced plants out-yield eight crammed ones.
4. Letting indeterminate vines sprawl unprunedBy August it’s a jungle you can’t pick from or spray air through. Ten minutes of sucker-pinching every week or two prevents it.

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