How cold of air temperatures can cabbage plants endure?
They can tolerate around 30 degrees F. but anything lower than that for extended periods will kill most species. This article will give you more information on growing cabbage: https://www.gardeningknowhow.com/edible/vegetables/cabbage/growing-cabbage.htm
I’m helping my daughter grow a container cabbage for The Bonnie Cabbage Program. The cabbage now appears to have split into two separate plants, but I don’t know if this is normal. We want the biggest cabbage head possible, so I don’t want two plants crowding each other. Should I attempt to split these, or is this the way it’s supposed to look?
If you do attempt to split them, it may cause damage to the root system. It will grow like that, but they will not produce well if left like this. If you do split them up, expect a pretty long recovery before they start to grow again. If you choose to kill one off, then you will want to cut it at the base, and leave the root system there. This will avoid any damage to the plant that you are keeping.
Here is an article that will help you: https://www.gardeningknowhow.com/edible/vegetables/cabbage/growing-cabbage-in-pots.htm
Why are my cabbages not forming heads?
Usually, warmer temperatures are to blame, but there can be several causes. This article will help you get started to figuring out what is happening: https://www.gardeningknowhow.com/edible/vegetables/cabbage/cabbage-head-formation.htm
Question details...What Causes Imense Vegetative Growth Without Production? Specifically In Cabbage And Tomatoes.
Likely too much nitrogen.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/too-much-nitrogen-soil.htm
I have been growing the cabbage for about 10+ weeks from seed. They have had a bit of a rough life. First they were slow to grow - temp was in the low 30's C. They were in the shade for about 4-5weeks. Then I planted them out. We then had heaps of rain - 4" the last day of March - 16" in March in total (QLD Australia time - sub tropical climate.) I gave them another hit of fertilizer then, 15N 4.5P 14K. Now they are just producing more and more leaves. An old round ball variety - both red and green. I had savoy cabbage like this last year. The sugar loaf produced big heads but the savoy just great big leaves. Bigger than a tennis racket. Temp now is 14-17 C at night and 24-27C during the day - coming into winter. Never had this problem in 30 years. I think my soil is just too rich at the moment.
Goodness, they have had a rough start!
This article will help you pinpoint the issue.
https://www.gardeningknowhow.com/edible/vegetables/cabbage/cabbage-head-formation.htm
For the past several years, my cabbage plants died. Close inspection shows tiny black ants approx 1/8 - 3/16" long congregating at the base of the plant. This year they killed all my cabbage plants, our watermelon plants, some of my kale and certain annuals in my flower garden. My plants come from many sources so they are not all started in the same growing medium. What are they after? Is there a nature-friendly way to treat the problem? I am an experienced gardener. Usually, if I loose a crop I hope for better success the next year but this problem is escalating. These ants are showing up everywhere. If I see a colony of them I pour boiling water into the nest but that doesn't work at the base of a plant.
This collection of article will offer some relief: https://www.gardeningknowhow.com/special/organic
Can I plant cabbage, broccoli, and cauliflower in my greenhouse with tomatoes?
You can... I would not recommend planting in the same container, but they can, certainly, be in the same greenhouse.
My only concern will be that all of these veggies prefer cooler weather. As long as you can keep the temperature manageable then there will be no problem with this.