Where I live, in the lower North Island, I have found that the ideal time to plant out most brassicas (except brussel sprouts, which need longer) for winter production is March. If I leave it later, the temperatures become low, and the plants go into a state of semi-dormancy (this happens below 10C), with growth slowed right down, and the size of the productive heads being much smaller, if produced at all - sometimes the plants remain very small and unproductive, and then bolt straight to small flower heads in spring when it warms again. This will depend on timing of planting and just how cold the winter is.
On the other hand, when planting in March, I still need to protect them from white butterflies. One year I did this using micromesh, which is used to against psyllids and other pests. It also had the effect of producing a micro-climate of warmer are in the completely covered bed, and those broccoli were HUGE and fully ready for picking by early June - sooner than I had counted on. I see a similar result if I plant them earlier than March too - still warm temps cause faster maturation.
This past year, as I have written elsewhere, has not gone as intended. But between family deaths and births, I planted some late brassicas - broccoli, kohl rabi, kale and cabbage - in three different places in my garden, at different times (figuring any production better than the none that comes from not planting at all!). The results illustrate my point above, and the ones in the (unheated, open windowed) greenhouse, though planted much later, were WAY more productive, due to the warmer temperatures. Check it out:
Broccoli, red cabbage and kale, planted in greenhouse on 5th June. The greenhouse is made of recycled glass windows. I leave some windows open pretty much year round, though did close them on the coldest nights in the first month of growing these seedlings.
While the greenhouse does get cold at night, it's structure provides protection from all but the severest frosts, and it is very warm in there during the day.
Group 1: Some moderate growth - some nice but small heads forming on the broccoli. I went ahead and picked them, as some were bolting straight to flowers, and it was obvious the others weren't going to get much bigger. Cabbage very small, kohl rabi look ok-ish, but no sign of any decent bulbing.
Group 1: These are looking pretty pathetic. I'm picking small shoots and heads of broccoli as they turn up. The other brassicas are pretty much a waste of space.
Just starting to see the formation of heads in the midst of the biggest broccoli variety. Have been picking kale every day for smoothies for a while now. Cabbages looking good too.
Pretty much given up on Group 1 - taking a few brocc shoots. They're now mostly just sitting there until I get around to clearing the bed and planting something else.
My intentions for 2018: Get brassicas in the ground outside by March, and also plant some in succession, including covering later ones to provide a warmer microclimate. Use the greenhouse for extra plantings when the weather gets colder.
To recap on the broccoli:
Group 1: planted 28th April, picked some tiny heads 4th August, few small shoots there after. No decent production.
Group 2: higher raised beds. Planted 4th May. Picked main heads 24th August, lots of side shoots 5th Sept, pulled plants 14th Sept.
Group 3: Planted in greenhouse 5th June. Picked first group of main heads (large) on 8th Sept. As of 14th Sept, another group of main heads just about ready to pick within the next few days, and side shoots forming everywhere.