There is no tomato that will give you a crop before the snow flies in zone 5 in Ontario, unless you have a heated greenhouse to grow them in.

Subartic is the fastest growing tomato that matures in 48 days from the time you set it out. That means you need about 2 months of warm or hot weather to get them to the fruiting stage. Even if there was the time, there is still the issue of shorter days with cool weather

Thanks Oilpainter. I do get tomatoes as early as mid-August because I start my plants indoors under lights in April. By the time I plant them out they are good-sized, healthy plants. I have never had success winter sowing tomatoes, but just last week, I had a ripe tomato on a 'volunteer' plant from last year's Livingston's Golden Queen. I did try Sub Arctic Plenty this year, but I am not a fan. The first ripe fruit were on August 11, but they are too small and not as tasty as I expected. I posted this question on the Tomato forum and have lots of suggestions which I will mull over during the winter. Enjoy the Fall.

I get Subarctic Plenty producing reliably well in Saskatoon, and compared to Saskatoon, zone 5 Ontario is a tropical sauna. In fact, my wife and I would like to move there to get away from this province with cold, unpredictable summers. All kinds of cherry tomatoes also do well here. Having never grown slicer tomatoes, I can't help you there, but yes a greenhouse will help for the cultivars that are more finicky. Just make sure it can be cooled (ventilated) well in your hot summers.
Sorry, I just find it funny when Ontarians complain about their growing season! You guys have it EASY.

Hey dr horticulture--That's a big whoops. You don't want to reserect that whole Ontario versus the prairies battle.

For a prairie Canadian you should know not to lump all one province into the same growing zone. Did you forget about northern Ontario.

We grow slicers and you can too. Just look for those which have a fast maturity date. Applause( a new one for us this year and one I will be ordering again) has a maturity date of 66 days. Some are even earlier. I think that's close to your cherry tomatoes. If frost threatens then pick then green. We put the green ones in 6 quart baskets about 3/4 full in the basement and they ripen just fine without losing their taste. You do have to sort through them every couple of days, but that works for me.

If you don't get it already I suggest you order T+T seeds catalogue--you can do it on line. They're in Winnipeg and have a lot of varieties of flowers and veggies bred for the prairies

hey oilpainter,

Thanks for the suggestions. I will look up T+T for next year.
No I didn't forget northern Ontario - I think you folks have it even worse than in the Prairies, sometimes. I was just referring to the Toronto-Ottawa-Mtl banana belt where people have never experienced the misery of a July frost, or a summer where half the nights can't crack 10 degrees :( It is the greatest myth that the Prairies have hot summers...only localized areas do, like around Medicine Hat.