I'm beginning to make plans now for a small-scale tomato breeding project aimed at getting varieties well adapted to zone 4 Vermont. Goals include a long, productive season; ability to reseed, sprout, and make productive fruit without needing to be pre-started inside; some frost tolerance, able to handle ~30F without cover or mid-20s F with cover; and ability to set fruit when nights are below 50F and days are cool but not cold; and tolerance for growing and fruiting in less than full sun.
I've been gardening in this location for almost 10 years, including tomatoes for most of that time. I've tried a number of varieties, including Sub Arctic, Siberian, Celebrity, Sungold F1, Amish Paste, Roma, Brandywine, Black Cherry, Isis Candy, and a few others. Generally any of these produce good tomatoes in my location, growing in excellent soil (not my native clay, class 6w) in well-drained raised beds. They get good sun for some hours a day, but there is no place in my yard surrounded by forest that truly gets full sun. Last frost is historically around May 23, first frost around September 23, though it's not unusual for last frost to be a week or two either way and first frost often has several weeks of Indian summer afterward with little frost. Winter here lasts about 5 months, typically has 1 - 2 feet of snow on the ground from late December through March, and temperatures are routinely below zero F at night during the coldest months.
I have a healthy pollinator population of wild solitary bees, made up of bumblebees or a similar relative. The garden is mostly organic, with no pesiticides used, no major pest issues except snails and slugs (beer traps and hand-picking deal with them), and an occasional use of 10-10-10 to supplement compost.
I can put about three 4x4 beds, 48 square feet, into tomatoes after allowing space for the rest of my garden. I grow intensively, so each bed could support anywhere from 9 plants with plenty of space each to as many as 25, more tightly packed.
One year early in my Vermont gardening life, I tossed all the unused, unripe, split, going bad, etc. tomato fruits back in the bed at the end of the season expecting them to compost. Many of them sprouted and produced tomatoes the next year, though later than ideal. So I know it is possible to start tomato seeds directly in outdoor soil at this location.
This year I am growing Amish Paste (started from seed myself), Sungold F1 and Brandywine (both purchased as plants locally). These are not part of the project, which I have only started thinking about in the past week. In previous years I have struggled to start tomatoes from seed indoors, but after upgrading my weak fluorescent lights to proper grow lights, this year it worked extremely well. I use tomatoes both fresh and for tomato sauce, and planning salsa for the first time this year, so a mix of paste and "eating" tomatoes is best.
Right now I'm trying to figure out the best varieties to use for a breeding project beginning in 2023 so that I can order them this fall. A few obvious candidates are:
Glacier
Sub Arctic
Jagodka
Siberian
Kimberely
Black Cherry
Matt's Wild Cherry
Roma
Sungold F1
for a good range of genetic material. I don't believe any of these have true frost tolerance, so I think it would be a good idea to include Solanum hirsutum, S. habrochaites, and/or S. huaylasense. I don't want too much beefsteak genes in the mix because those tend to be late, which would defeat the purpose.
I'm unsure to what extend this should be treated as a "promiscuous landrace" (thank you, Joseph Lofthouse, for that great term) and to what extent I should push it in the desired direction with controlled hand crosses. If I do succeed in getting a good landrace going, I may eventually try to isolate some homozygous lines with particularly good traits from it.
I see a number of posters on these forums are doing similar projects in different locations, though most of them seem to be in zones warmer than zone 4. So far my online research has not turned up any cultivated tomato varieties that claim true frost tolerance, even those from places like Russia where it might be expected. I did see some mentions on these forums, complete with photos, of a few plants that did survive a frost while most died. How much of this is from Mendelian genes vs. epigenetic switches vs. microclimate doesn't seem to be well understood.
Suggestions are very welcome!