I think the odds of success here are best if grown in the ground unless you have a pot the size of a 4' or 6' diameter stock tank.

To grow a giant cabbage, I'd think the following would be important:

Very fertile, very well-enriched soil that both drains well and also holds moisture well. Moisture is going to be a key factor in growing a giant cabbage, but so is fertile soil. All the moisture in the world won't grow a giant cabbage if the soil isn't rich and fertile too. If I were doing this, I'd add a lot of compost---probably a bag or two if using purchased compost.

More than adequate moisture. This is going to be hard in our climate. An inch of water a week would be the bare minimum and it may take a lot more than that to keep the soil moist at all times....at ALL times...and moist but not soggy, and never, ever dry.

Full sun. Cabbages have to have at least 6 hours of sun a day and 8 would be better. In our climate, more than 8 might not be good once hotter temperatures arrive. If necessary, once the hot temps arrive, y'all could erect a shade structure lean-to type thing just west of the plant to shade it in the afternoon.

Regular feedings with a balanced fertilizer, probably once every 10 to 14 days.

Very wide spacing with nothing, nothing, nothing around it competing for water or nutrients. I wouldn't put anything within 3 or 4 feet of it, and would mulch heavily, probably with cardboard on top of the soil and some form of organic mulch on top of that. Don't allow any weeds to become established.

As for protecting it, I'd start out with a cutworm collar and would spray the plant with Bt from day one and with great regularity to prevent cabbage moth damage. I'd sprinkle organic Slug-Go or Slug-Go plus on the ground to keep away the snails, slugs, pillbugs and sowbugs. I'd cover it with a lightweight floating row cover---the kind meant to protect plants from insects, not the heavier weights intended to protect plants from frost.

I'd check it carefully every morning and every night.

If it is not in a fenced in garden, I'd build a smallish fence in a circle around it, probably 4-5' in diameter and you could use woven wire fencing or chickenwire, but you'll need a deer netting or chicken wire top that can be easily removed by humans but not easily removed by clever wildlife. (I'd probably use clothespins to attach a bird-netting or deer-netting cover to the top, so I could easily remove the top to weed the plant, check the undersides of leaves for caterpillars, etc.) . If you have gophers, voles or moles, I'd sink hardware cloth into the ground as far as possible before planting, or plant in a huge container or in a raised bed with a bottom lining of hardware cloth. I wouldn't want to have to explain to a 3rd-grader that a rodent ate the plant!

Have you been over to GardenWeb's "Growing Giant Vegetables" forum? I'd be surprised if this has not been discussed there before, although finding it via the search feature may be difficult. You could post your request for advice there and get expert answers from serious growers of giant vegetables.

I think it will take 2 to 3 months to get a huge cabbage, and maybe longer. Do they have to finish this up before the school year ends?

If they only gave the kids one plant, I'd find and plant seeds of the most gigantic cabbage variety I could find and grow a couple of those too as back-ups. I'd hate seeing a kid pin their entire hopes and dreams on one single plant.

I've linked the Giant Vegetable forum below.

Good luck.

Dawn

Here is a link that might be useful: Giant Vegetable forum at GW

Severial years ago I read about a man trying to grow a large (pumpkin I think), his son was a nurse and they rigger up some kind of I. V. drops for it. I dont remember much about it, except that they really wanted to win.

Larry

Sounds to me like this cabbage is going to be very expensive, in terms of supplies, labor and angst. Wishing you the best! You know I never do anything the easy way!

Seedmama

Well, so far....we've taken the children's wading pool that is intended for sweet potatoes, poked holes in it, lined it with weed-block fabric and now have filled with a mix of bagged "garden soil", peat, and some dirt from our garden. It's nestled in a corner of the backyard area right by a water hydrant. I'll be able to put deernetting around it to protect it and there's a small fence panel on the west to help with the shading. I've got out my BT and 10-20-10 left over from onion planting. We're going to keep notes on it and take her picture with it every week or so. We have 10 weeks to grow it.

Thank you so much Dawn!! We've got so many new, young gardeners here with children that some may run into this project down the road. From what I understand, this is Bonnie's 9th year for this. How often should I spray the BT? and what the heck is a cutworm collar? Is that the same as driving something down beside your tomatoe plants to impede a cutworm? (I haven't been speaking gardenese long but I think that's what you're referring to, right?)

Larry - That's an interesting idea on the IV!!! May have to rig something like that up when it gets warmer just to keep it hydrated!

It will be on display for anyone to see at the Spring Fling. heh!

Paula