Growing Brassicas

I want to start growing my cabbages.kale,cauliflower for example but according to the instructiona on the seeds packets the they should be sown in March so would it be ok to plant the seeds now in my unheated greenhouse.I use heated trays for things like chillies's Tomatoes etc which are on a shelf in my big shed.Thanks

 

I'd treat them slightly different to tomatoes and chillies.
Ok to give them some heat during germination but after that they need to be much cooler.
But having said that frost free, still possible to get frost in a cold greenhouse for the next month or so.
They will also need full light, so depends on how much light you get in your shed.

 

When I sow my brassica and leek seed for next winter the pots will simply go onto some staging outside where I can keep an eye on them.
If you have a bit of space you could sow direct into a nursery bed: make a shallow drill, sow seed fairly close together, cover over with soil water if dry, when plants are big enough lift and plant out in final positions.
All brassicas used to be done like this, even commercially, until the 90's when there was a move to growing in modules. Nowdays it's all done in large glasshouses.
Why the change?
1) Modern seed, F1, F2 is expensive.
2) Under cover there is more control of the environment and pests.
3) Large parts of the process can be automated.
4) Plants are available when required; for example grower can select varieties, submit dates he wants to plant out. Then seeds are sown x weeks before delivery, 90+% germination, allows n thosand plants to be available on schedule.
5) Less shock to the plant compared to the old method, modern planters can plant the modules with little damage to the plant and few people.

 

Yeah, if you bought them "in" you could get a bundle of bare root cabbage plants, (general term), 25 or so for a shilling, piece of string and wet newspaper around the roots.

 

Remember to protect outside plantings from pigeon attack, they love brassicaes and unless you are near farms with canola (rape), they will find them.