Slow cooking can be a tricky method, as the lengthy cooking time often leads to mushy, over-cooked vegetables. When a lot of liquid is present, it can also leach the nutrients from the vegetables—though if you are consuming the liquid, it is not a problem.
A tip to combat this is to place harder vegetables on the bottom of the slow cooker and more delicate ones on top.
Veggies are cheapest and tastiest when they are considered ‘in season’—so it pays to keep an eye out at your local shop for what’s in abundance.