Hi Vicky and welcome to GoY. Sounds like you've overwatered the compost, either that or it is a bag you've had open for a while. You don't say where you live in the UK but it is unlikely that either carrots or cabbage will grow to any size at this time of the year, they need to be sown in spring when the lengthening days encourage growth. Not sure what 'parasites' you are attempting to keep out, did you mean pests?

Agree with Mg - the situation is too wet. Only the soil should be kept moist, not the leaves. It should also be well draining with good ventilation. If your cabbage is covered in mold, I would think twice before eating it. Carrots & cabbage are both cool weather crops so you have about 2 months of growing time left. You can keep the carrots in the soil and just harvest them as you need them.

B. I 'think' Vicky has only just sown the seed - can you advise us please Vicky.

No hope of growing carrots outdoors in the UK from seed sown at this time of year. If you want to grow cabbage now it would be best to buy some young plants from a garden centre.

Stera, why can't you grow carrots in the UK this time? I just reseeded my carrots last week and the young plants look fantastic. They love the cooler weather and will continue growing up through November with few pests around if any. That's plenty of time for nice looking carrots. Danvers only need 75 days for large sweet carrots. These are very hardy plants and will survive freezing temperatures and actually be come more sweet.

We grow carrots all year, here in the desert. While our frosts are light, they do exist. Is the limiting factor gray, chilly days, or does the soil freeze early in the UK?
We are planting cabbage seedlings now, even though the heat is stressful, so they get good sized rosettes before the chilly nights trigger heading.

Both Tug. in the UK you start your carrots growing as the days lengthen and the soil is getting warmer... then they will romp away and you harvest them about now or a little later, unless you are a commercial grower who covers them in straw and fleece to keep in the ground. The light simply isn't good enough to encourage germination at this time of the year.

Well, by the soil freezing early, I meant even earlier than New York, where Bathgate lives.
Vicky, in my experience, white mold on the compost is usually caused by overwatering, poor air circulation--if what you mean by mesh is the spun-bonded floating row cover--poor light--same cause--and/or too coarse, unfinished compost.

I've gone out in the snow to harvest my carrots. The snow preserves them perfectly as long as they're in the ground. I think they taste sweeter. However, you gotta have the rays. Ain't nothing gonna grow without no rays! :)

Hey Tug, did you see my skipper? I thought of you when I posted this. Check out my skippers.

http://www.growsonyou.com/photo/slideshow/333430-skipper/all

Got it, Bathgate! I need to check the pictures more often, obviously!

Bathgate I've never troubled about why. All the advice on line and in books and on seed packets recommends spring or summer sowing. The RHS recommended last date for sowing is July and they know a lot more than I do..
There's no reason at all why you shouldn't sow in September if you really want to but don't expect much of a result.

Thanks Stera Mg already answered my question a long long time ago. Thanks anyway.

Tug - lol!

So she did. Not to worry.