You can buy or make wire hoops for supporting row cover. Work out the length of wire you need for the height and width you want. Alternatively you can use plastic piping such as is used for water pipes. You'll also need ground pegs, either bought or diy.

Just FYI, Bacillus thuringiensis (Bt) is an amazing substance that kills cabbage moth caterpillars very effectively. I started using it a few years ago, by recommendation here, and it rocks. A lot easier than tunnels. Spray it on every week or two.

Not sure I understand. Bt kills the caterpillars on the plant. The moths are hatched from those caterpillars. So if you get rid of the caterpillars, you get rid of the moths, and the potential for more. I use Bt, and I NEVER see any caterpillars. If you have caterpillars hatching on your brussels sprouts, and you have them under a row cover, you aren't going to protect them from more infestation. The moths that hatch are locked in. A row cover doesn't kill the things, and you have to kill them to avoid them.

The row cover will protect against other pests, too. Plus add some protection from excess sun, heat, cold, hail, and the like.

That may be, but the OP wants a row cover specifically to protect from cabbage moths. Maybe the OP could comment on whether protection from excess sun, heat, cold, hail, and the like is an issue. My point is that unless you kill the critters, a row cover won't stop them if they're already on the plants.

The row covers need to go on before the butterflies are out and about. Since it is only March that shouldn't be a problem.

A couple of points about BT.

I don't know the cost in the USA but here it is expensive and given our rainy climate would need repeated applications. Row cover is fairly cheap and mine has lasted years.

The Sierra Club recommends that B.T. should only be used as a last resort. It is not completely benign. https://www.sierraclub.ca/national/programs/health-environment/pesticides/bt-fact-sheet.shtml

You put row covers on your brussels sprouts with cabbage moth caterpillars inside, and the butterflies will be out and about under your row covers. Why is this so hard to understand?

Bt in the US is cheap. 8 oz of the concentrate, making many many gallons of spray, is $7 at Home Depot. I just got some last week.

From your fact sheet - "B.t. is of low threat to humans because it does not persist in the digestive tracts of mammals. Classified as 'slightly toxic,' the most common side-effects are skin and eye irritation". Everything is slightly toxic by that measure. It's not as if you're going to drown yourself or the environment in it.

I would get the tunnels set up at or before the time of transplanting -- mid to late May. Not sure but don't think any cabbage moths would be flying about at that time up here. Don't know enough about BT to have an opinion one way or the other, but I would be concerned about potential harm to other beneficial insects, and let's face it I'm lazy and probably wouldn't spray -- good intentions and all that.

It's not hard to understand. But it's not relevant to this question. It's March, Mxk3 is asking for future reference. These plants won't even be sown yet. Plenty of time to get the row cover on. You can do it at the same time you plant out the seedlings. Row cover is a no brainer imo given doubts over long term use of BT and the relative cost. Row cover isn't even 'slightly toxic'.

The Sierra Club's conclusion:

"Although Bt is less toxic to mammals and shows fewer environmental effects than many other synthetic insecticides, it is not ‘safe’. B.t. should only be used after all other methods of prevention have been used, only when entirely necessary, and in the smallest quantities possible."

I've had good luck using rebar and PVC. Pound some rebar into the ground and slip 10-12' long pvc pipes over it. Put a "hoop" every 4-6' - but play around and see what works for you. I build cold frames this way, so they have to support the weight of fabric and plastic, but if you are just using a light row cover, you might be able to space the hoops further apart. The rebar and PVC can be used many years.

I use this low tunnel design and then swap out fabric for the plastic in summer. In zone 5 I can grow all winter, and you don't have to mess with putting the fabric on and off every time you water, harvest, etc.Hinged Low Tunnel

Thanks for the replies!