Have you had heavy rainstorms recently?

The splitting is caused by water issues like heavy rains as gumby said or by very inconsistent watering methods from one extreme to the other. Winter squash normal grows slower than summer but not drastically so. It could be any one of many issues - lack of nutrients, root damage, pests, sun exposure, water, etc. Just because plants are growing right next to each other it doesn't mean they are getting everything one of them might need. Was it direct seeded or a transplant?

Dave

Looks like storm damage, Also looks like something white on the ground maybe some disease or lack of nutrients, Depends what zone your in you could start some from seeds which do better then transplanting.

No heavy rains recently, but I do get very strong winds.

They were transplants.

I'm in zone 10 and we're very hot right now, 80's - 100 F.

Maybe the watering. I've been watering them everyday because they wilt with the hot sun (so maybe inconsistent watering that way?).

So, should I pull them out and start over? They don't look sick, just not very big at the moment. They've been in my bed going on 3 weeks.

If the plants look otherwise healthy, I wouldn't pull them. I've taken straws, sliced them as long as the split stem and then sliced them lengthwise, and then slipped over the stem that is split - kind of like a cast for the plant. I think it helps the plant from disease, insects, etc.

When I've seen split stems like that, it's always been because of watering.

The splits normally heal/scab up all on their own. But you sure shouldn't be watering them every day. Nothing, unless it is a container, should need daily watering. That only causes roots rot and stunted plants - and split stems. And that isn't hot weather for zucchini.

Dave

As Dave and others have said, daily watering shouldn't be necessary. It is common for squash leaves to lose their turgidity and 'wilt' in hot weather.

Usually, by the next morning they pick themselves back up and look healthy. Insect damage will make this condition worse. The Squash Vine Borer is the worst culprit, and even squash bugs can exacerbate squash's natural tendency to wilt.

If they are still wilted in the very early morning, after they have absorbed the dew (assuming there's dew in such a hot climate,) then they may indeed be thirsty. However, try not to let them go that long. Try to follow the 1" of water (plus a half an inch for every 10 degrees over 80) per week rule. Because of the large surface area of their leaves, squash can usually handle a bit more, but give them that extra by regular deep waterings.

Daily waterings can encourage the squash roots to stay at the hottest part of the soil... the surface. Deep waterings encourage the roots down following the water. Weekly (or slightly more often in very hot, very dry or sandy places), deep waterings train the roots to find the cooler, moist soils below.

Thank you everybody!