[Grubbypaws]

On the T&M seeds site it states that one should grow Spring Onion White Lisbon next to carrots as the smell helps deter Carrot Fly.

Does this actually work? If so how close should they be? I am growing my carrots in containers; should I be mixing the spring onions in with the carrots in the same container or just putting a pot nearby?

Many Thanks

[JayG]

The theory is that the strong onion smell masks the smell of the carrots, which is the way the carrot fly homes in on its target.

The reality is that although you might put a few of them "off the scent" it only takes a couple to find and then wreak havoc with your crop.

Much better to cover with enviromesh or similar which should be quite easy growing in a container.

[Nikkithefoot]

I tried growing onions near my carrots one year, and it did seem to lessen the problems with carrot fly. I might try it again, but it really means I have to think extra hard about where I plant things to make succession easier. Also the later sowings of carrots won't be protected as the onions will be out by the time the carrots are ready.

[Trillium]

You also need to grow an awful lot of the onions to repel determined carrot flies. It's a waste of space, really, if you don't eat that many of them. Easiest is to simply use enviromesh, as already mentioned. It'll cost a bit but can pay off by saving space to grow other crops you really want.

companion planting does offer some help when paired right, but it's not an ultimate defense.

[digalotty]

its a help but there will always be a risk involved, you can raise pots up over 2ft to help aswell

[Mazzie]

Carrot flies can't (supposedly!) get above 18in so raise your pots and you should be fine. 
I believe the flies are put off by any strong scent so garlic chives, marigolds, strong herbs etc will all do a similar job.  And remember to thin them out when the day has cooled off.

Im going to jinx my year but ive not done any of the above and never had a problem but then again ive only grown in pots until now.

[Benandbill]

Just after reading this y'day I was watching Claire' Allotment on YouTube and she was planting a row of onions then a row of carrots etc.  I might give it a try.  A bloke on the adjoining plot to mine grew carrots last year and they were fine.  Straight out of the ground.  They were a bit small, but no onions, no pots, no growing them over 21 inches, I'll ask him his secret and pass it on 

[Kristen]

I've grown carrots and had no problem.  All that means, I'm afraid, is that I had no carrot fly in this area.

If you do get Carrot Fly then it won't be stopped by Onions, or thinning at dusk, or a 2' high barrier. And their "eggs" will over-Winter in the ground to give you a problem next year too

It may very well be reduced significantly, but it doesn't take many Carrot Fly to wreck a crop.

Here's what Aunty said:

"I subscribe to that theory every year and build a 3 ft high enviromesh wall around my carrots.

Every year I get carrot fly"
http://chat.allotment-garden.org/index.php?topic=40129.msg475584#msg475584

Cover them with enviromesh (or old net curtains, or fleece)

Enviromesh available from:
http://www.gardening-naturally.com/acatalog/Enviromesh.html#aEM001