Folklore in southern Tasmania says that you must wait till Hobart Show Day to plant your tomatoes. This is supposed to signal that we wont get frosts after this date, which would potentially damage newly planted seedlings. So naturally this local wisdom is a generalisation.
If you live in a frost free area, you can plant tomaotes much earlier, even in late September. Young tomato plants like warm soil and frost free conditions, so if you can provide this (eg in a polytunnel or hothouse) then you don't need to wait.
For the rest of us, its good to a) keep records of frosts at your place, b) watch for local indicators like the lilac trees finishing their blossoming, and c) keep an eye on the weather forecast for the next few weeks. In Hobart this year we have a few pretty cool and windy days on and just after show day, so we recommend holding on to your seedlings (if they're ok in their pots) for another week or so until things warm up. One of the best tomato growers I know in Hobart always plants his on Melbourne Cup day. Generally small seedlings planted a few weeks later will catch up to large leggy seedlings planted too early, since the small ones suffer much less transplant shock.
Its very important to 'harden off' your seedlings by exposing them to outside conditions gradually over a week or two. Otherwise they have to cope not only with root disturbance, but the shock of going from a sunny windowsill to a cold and windy garden spot.
Of course, some of you would have bought seedlings at the Botanical Gardens sale, and eagerly put them in the ground weeks ago. No problem, just keep an eye on them and consider protecting them from very cool nights and the wind.
Happy planting!
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