I mulch as soon as the plants are 4 or 5 inches tall. Mostly for weed suppression, but also so the ground doesn't dry out so much. (I see you are in Chicago, I live in the western burbs)

I'm from zone 9. I always have mulch, but I don't do veggies. I mulch my beds, I mulch containers. Chokes out weeds and keeps the ground from drying out. I mulch in the winter to keep the temperature more steady.

I'm never without mulch! LOL

Sorry that I have no potato advice to give...

Chicagoland Gardening magazine said in its garden calendar section for the month to wait until the end of May or until the ground is well warmed up. I know I have more fungus if the ground is wet and cold under the mulch, but I don't know the exact reason for waiting.

My neighbors put on mulch a long time ago, though. I usually wait until June. I do have to pick a few weeds in the thin spots by waiting until the ground is warmed up. Once I mulched all over in the fall, and that works good, too. It's all ready to go in the spring and not too thick.

I grow potatoes in northern Wisconsin in my garden as well. I like to use straw to mulch around mine and it works to keep the weeds out and makes for easy digging. I've dug holes and put my pieces in and just plugged the hole with straw and not much dirt. It works great. I put straw over right after planting but you could wait til weeds sprout, hoe em down then cover too! Just be careful if you get hay that its not full of seeds! did that and had my very own crop of weed seeds sprout! You still need to hill them eventually or put more straw on as they put out spuds.

good luck!
Keeker

I mulch as soon as the plants are large enough to have all their leaves above the mulch (so at different heights for different plants). I mostly do it to prevent weeds and molds.

I mulched this Spring but I think I went overboard because a lot of my plants aren't doing well. My coreopsis and cones got mulch overload, I think. The plant guru at one of my favorite nurseries said a lot of plants don't like mulch in Spring and it can actually smother tender new growth. That was news to me. I think I'll go back to fall mulching/Spring fertilizing. Although my weed situation is vastly improved thus far.

This is kind of an old post, but thought I might mention that early mulch for cool crop plants like potatoes works fine...I have planted, and mulched them, in January..they just come up when ready.

Coreopsis, on the other hand is a heat lover, so should not be mulched until it's quite hot out there, if at all. It's a roadside plant in the wild. Tomatoes, eggplant, peppers, okra (you all know the heat lovers) just don't begin to do well until the soil heats up. You can pretty much tell by when it's safe to plant outside.