I haven't had much luck getting lavender cultivars to winter over in my zone, even when I've grown them from seed via winter sowing & planted them out in well-drained sandy loam in full sun. However, even in Z5 the ground isn't likely to freeze for quite a few more weeks despite the much cooler daytime temps.

The general rule of thumb is if it blooms in spring, plant it in fall; if it blooms in fall, plant it in the spring. Most everything else can be planted whenever soil/moisture/sun is optimal.

If you get the urge to have a go at growing lavender from seed, the link below will bring you to Swallowtail Garden Seeds where I bought my Munstead lavender seeds. Can't say I had luck growing mature, blooming plants but the seeds sprouted right enough.

Considering lavender thrives on the rugged moors of Scotland, I'm guessing it doesn't like to be pampered in our more temperate perennial beds.

Here is a link that might be useful: Lavender seeds from Swallowtail

i couldnt either ...

and i THINK i remember.. its management of moisture in the soil ...

too wet.. and the roots rot ...

temps are irrelevant ... you are on the wrong track ....

the issue is.. which place.. pot or garden.. is going to be easier to manage moisture ... and this is one plant.. where i would question how easy winter in a pot would be.. either too dry.. or too wet ...

last time i ranted about this.. there were a couple peeps who have succeeded beyond most other belief.. hopefully they are still around.. if not.. use the GW search engine for old posts ...

ken

IME, lavender doesn't particularly care about soil moisture, drainage, heaviness or anything else people talk about. It does care immensely about soil pH. So it is much happier in alkaline clay than acidic sand.

mad_gallica - thanks for that soil info which would explain my utter failure to grow lavender where I am. Sounds like the fact that my soil is lovely, well-drained acid sandy loam is probably a dead giveaway why I've had zero luck growing it here except in containers. It always helps to accept what you can change and what you can't.

I'm going to disagree with the above.......it is ALL about drainage and winter moisture conditions with only a slight impact from soil pH :-) First, most plants tolerate a pretty wide range of pH and lavender fits into this tolerance range as well, although not as wide a range as many other plants (6.5-7.5 is preferred). The PNW is one of the largest commercial lavender growing areas in the US and we have naturally acidic soils. Lavender thrives here, despite having rather wet winters. The key is to ensure good drainage - I can't imagine lavender dealing with clay soils at all regardless of pH. The only time hardiness is at issue here is if fast drainage is compromised, like in clay soils. (and excessive fertility and too frequent irrigation, but that's another issue).

As to transplanting now, in zone 5 I'd opt to wait until spring if possible. Lavender is marginally hardy inground in your zone and planting this late in the season does not provide sufficient time for the plant to become established before cold weather sets in and increases the chances for cold damage/death over winter.