Quick answers
Can I ferment potatoes?
No. Their starch, low water release and the off-flavors that follow make them a poor fit.
What about tomatoes?
Only unripe green tomatoes, in a 4% brine for about 10 days. Ripe tomatoes are too soft and turn to mush.
Is the slicing cucumber from my grocery store okay?
Usually not. They are often wax-coated to last longer on the shelf, and the wax keeps the brine out. Use Kirby pickling cucumbers, or unwaxed cucumbers from a farmers market.
Can I ferment kale, chard or other greens?
You can, but the texture softens a lot. Use them as less than 10% of a mixed ferment rather than on their own.
What about fruit, like apples or peaches?
Ripe fruit has a lot of sugar and tends to turn alcoholic or vinegary. Some traditional Korean kimchi uses a small share (under 10%) of firm, tart fruit, but it's an edge case.
Are onions good for fermenting?
Yes, but they're strong. Use them as about 20% of a mixed ferment. Fermented onions on their own work, but they're an acquired taste.