I'm in zone 5, SW Wisconsin. I think they say it's best to wait until 5/15 because there have been freak cold snaps in late April, early May in the past. The majority of the time, we're done with all the frosty stuff by the end of April but every once in a while...

Anyway, I only wait until May for things like Tomatoes and Peppers. They prefer the kind of warm weather we don't usually get until May and later anyway. It won't kill them if you transplant out in late April (unless there's a major frost) but they also won't grow much until the weather warms up a little more.

I plant everything else (peas, beans, lettuce, broccoli, brussels sprouts, etc.) in late April and they seem perfectly happy. As long as we don't get a freak cold snap, they're fine. I play the odds so to speak...wagering that it won't be one of those years that we get a freak frost in May. Nine times out of ten, that bet will pay off where I am.

Do you direct seed your Broccoli and Brussel Sprouts or do you start them inside?

I asked a vendor at the farmers market how they got such wonderful tomatoes since mine at home were barely breathing and the told me they plant in early April. When I asked them what do they do if there is a frost they said replant everything. It must be nice to be able to do that but in a small garden like mine it wouldnt be too hard, just start extra seeds and with some luck you wouldnt need the extra plants and could trade them for garden hogging zucchini from the neighbors. This trickery is all in NE Illinois

I grew up in N. Illinois and my mom who is 78 now, always said to plant after Memorial Day. She learned this from her grandfather who had a small farm in Wauconda.

"the average is 4/30 but the farmers almanac site recommends not planting out until 5/15 this year. Why is that?"

Because an early May night can bring 28 degrees in McHenry & Kane counties while only 35 degrees along the lake/downtown.

Watch for the lilacs around Mother's Day. Usually safe afterward.

I live in north central Ill and it can frost in early May. I plant transplants of broccoli, cabbage, cauliflower and onions in April as a light frost doesn't bother them. I also direct seed radishes, peas, beets,and spinach towards the end of April or when the ground is fit. Some things like hot weather. As megansarah said tomatoes and peppers which I don't plant until about the first of June. The same for sweet potatoes. I repot my tomatoes in May so they have a healthy root structure.

If I plant a couple of early tomatoes I can always cover them with a five gallon bucket overnight. I am more concerned with how warm the soil is getting. Last year was a bad year to get things going early. Cold temps and rain just didn't let the soil get warm.

Frost dates vary in the spring and fall. I have seen it not frost until almost Nov and in 1974 we had a hard killing frost on Sep 18.

An advantage of sucession planting the cool season stuff is that the older plants will be a little more cold tolerant if a cold snap comes along. I usually plant everything when it is at least 2 weeks "too early" and leave myself with an insurance policy of plenty of extra transplants in the waiting at various stages of growth. That way if we don't get any killing frost my season will be that much longer in the spring and I will have wasted very little in the way of plants.