Most medieval houses made it a priority to use even a small bit of exposed earth to make a garden. Deep in cities, people who lived in rooms and flats didn’t even have this much; but country people certainly did, and every city dweller with even a small patch of land out the back door planted a garden.
They didn’t carefully distinguish between vegetables, herbs and flowers. After all, some vegetables (which are really fruits) have beautiful flowers, and some flowers have edible roots. It hadn’t been so many centuries before that Europeans had been domesticating wild plants, which after all grow root, stem, flower and fruit for their own purposes and how man uses them is just a matter of preference and toxicity.
The most commonly grown plants were garlic, onion, and parsley. A modern distinction calls onions vegetables, suggesting that size alone makes the difference. The plant mallow was considered a vegetable, not an herb.
Roses were flowers, but rose hips were edible, and both roses and violets boiled with sugar to create flavored syrup. Violets and some other flowers went straight into salads and desserts. Crocus pollen was known as the herb saffron; native to Europe, it was still very valuable, since it came in such tiny amounts per flower. Chives, then as now, were used for strong, sharp flavor, but the purple flowers were also beautiful enough for cut flowers.
The vegetables we recognize were mostly of three types. Cabbages tended to be loose-leaf; lettuce, kale and cabbage looked much the same, before hybrids created differentiated shapes and sizes. Root crops, too, were less differentiated. Carrots weren’t orange; they were more like (white) parsnips or (purple) beets. Legumes were invariably dried for winter, not eaten with pods the way we eat green beans or sugar snap peas.
There’s some evidence for vine plants that grew bulbous fruits like cucumber, eggplant and zucchini, but these may have been restricted to the Mediterranean zone. Of course, they did not have New World plants like potatoes, maize and tomatoes. “Corn” meant seed and referred to wheat or rye.