When can I set out cabbage and brussels in the Denver area? Also, will hardening off be any different than summer crops?
You can set your plants out after danger of frost is past. You will need to harden off your plants.
Here are several links to help you.
https://www.gardeningknowhow.com/garden-how-to/seeds/how-to-harden-off-your-seedlings.htm
https://www.gardeningknowhow.com/edible/vegetables/brussels-sprouts/how-to-grow-brussel-sprouts.htm
https://www.gardeningknowhow.com/edible/vegetables/cabbage/growing-cabbage.htm
I basically have the info I need. I just need to know what to use cut the cabbage head off from the plant so that it will grow again. What tool do I use?
Use a clean sharp knife to cut the cabbage.
Your plant will not grow another cabbage head.
Here is a great article about harvesting cabbage.
https://www.gardeningknowhow.com/edible/vegetables/cabbage/harvesting-cabbage.htm
Do cabbage plants regrow? My son got a cabbage plant from school last spring. We planted it and harvested it late summer. This summer it regrew. I don't know if it is forming a new cabbage again or not though. It has got tall flowering shoots out the top. Thanks.
If some of the root end remained in the soil it is possible it can regrow.
I would watch it grow!
Do you need to dust cabbage plants? When is best time? I thought you had to dust plants for insects but not really sure!
I grew up with my grandparents and parents dusting the cabbage with fresh powered lime. It will deter those insects and worms that want to dine on your cabbage. Dust in the morning when the plants are dewey. If you need to reapply, wait at least 48 hours.
I personally do not dust my plants, but many gardeners regularly do so.
https://www.gardeningknowhow.com/edible/vegetables/cabbage/growing-cabbage.htm
When to know if cabbage is ready for transplanting into garden? How many leaves or how tall should it be?
This article should help: https://www.gardeningknowhow.com/garden-how-to/seeds/when-to-transplant-a-seedling-plant-into-the-garden.htm
What tool is used to cut cabbage off when ready, and does it grow another head of cabbage after cut in that same spot?
They normally produce one head per plant, which are best cut uing a sharp knife.
I have some black leaf cabbages in the garden. They are growing up-ways and have large purple/green leaves on them with no heads. Please could you tell me when they are ready to eat, if possible?
They need cool weather in order to form heads. You can help keep them "cool" by placing mulch around the base to regulate the soil temperature.