I know of course this is a problem for annuals but young perennials newly acquired from a nursery and put in the ground can survive a frost or two?

(Or alternatively maybe any that I do purchase in the next week I should just put in the garage until it is 'safe' i.e. no more frost?)

The problems are if it gets very cold. Around 25F, or so, even the hardy plants can get damaged. Depending on where you are, those temperatures may be normal for the next week or so. Here, we usually don't start planting growing plants until the last week of April, first week of May.

Generally, there is a world of difference between simple frost for hardy plants and cold enough to cause damage.

==>>> depends if they come out of the hothouse .. or were previously hardened off as per above ...

let me put it this way ... in z5 MI .. we always went out on mothers day.. and bought the flats of flowers and went home and planted mom a flower garden ... and if i am not mistaken.. that is mid may ... just because this spring has been warmer.. does not mean that you should be planting ...

all that said ... here is the rule: JUST BECAUSE YOU BUY IT.. DOESNT MEAN ITS TIME TO PLANT IT ...

i used to have a wagon in the garage ... with a 3 foot square piece of wood on it.. and could hold quite a few plants/flats.. and they went in and out of the garage .. until near the frost free date ... and it was also a function of the hardening off process ...

and that is what you should be doing ... if you want to avoid all the worry .... and that is the key .. whether or not a certain plant can take the cold is not relevant to the worry quotient ...

ken

all that said ... here is the rule: JUST BECAUSE YOU BUY IT.. DOESNT MEAN ITS TIME TO PLANT IT ...

Yup, thanks for the confirmation. And that is why I had included in my original post that although I may buy perennials in the next week (in fact I will likely pick up two "Bobo" hydrangeas tomorrow) BUT I will probably put them in the garage for a bit and so not plant them until May 1st. ...and perennials not for a couple of weeks after that.

Although a very experienced gardener I know plants his petunias quite early as he says that they can put up with some cold.

Obviously the plants have to be hardened off first if they came out of a greenhouse but if you have a trustworthy nursery that you shop at just ask them where the plants came from or how long they have had them outside.

When I was living in the northeast I found the best time to plant perennials was about six weeks before the last frost date.

So for me in zone 5b that would correspond to anytime now.