You rent a time machine and go forward to sometime in July. Then it's a simple matter to look up the weather data for the last few months and find the last frost.

If that isn't practical, ask locally, or look at the frost maps. The relevant date is when 90% of the time there isn't a later frost. Here that's June 1st. At that point, you can look at a forecast and be pretty sure that's it for the year.

Of course plants that can handle frost can be planted earlier.

Here is a link that might be useful: Frost Free Maps

Memorial Day, silly! Mother's Day if you're in southern New England... but you might just "get busted" by a late frost.

One thing to note is too check the garden soil temperature too. If the soil is too cold tender annual plants, elephant ears, dahlia tubers, glad corms etc. (that all like warm soil) will just sit in the cold soil and not do much. In the case of tubers and corms may even rot. So you have to think about this too if you grow plants that want more tropical temperatures. Perennials on the other hand can be planted any time now even if the soil is cool. Most don't seem to be affected by it.

also a full moon can bring on cold nights. I think there is supposed to be one May 31. One of the weather guys was mentioning earlier this month that May would have two full moons in the same month - making one of them a 'blue moon'.

I'm sure I won't be waiting quite that long to put out annuals and houseplants, but I will be prepared to cover them if necessary. Oh, duh, I put out annuals already!!!

Well, I knew Memorial Day was the "proper" time to do it in our area from what others have told me, but being new to this I asked the question because of the hot days we've been having the past couple of days, and I had a few plants that I was anxious to plant. Since I have a container garden, I did plant them, but will be bringing the plants indoors until Memorial Day.

Are there any wild plants that reliably signal "OK, now there really won't be any more frost?" Something along the lines of "it ain't spring till the mesquite trees bloom?"

Not that it's more useful than "memorial day" but maybe it's more poetic.

I dunno about native plants that signal "frost free"... never thought about it, frankly. It IS more poetic, though!

Every year I want to plant prior to Memorial Day, last year I did (mid-May). Everything survived, but it was basically in suspended animation for the 2 weeks until Memorial Day. I think Sedum hit on the key point for annuals... and that's soil temperature. It needs to be warm for them to really put on the growth.

I will be shopping early, but will probably wait to put them in the windowboxes and planters.

scoopc
I have planted before Memorial Day, "tempting fate," just because I couldn't wait any longer! I always watched the weather and would bring in plants that were in portable containers, and throw pots and sheets over ones that weren't portable, if cold temps re-appeared. I agree with chelone, though, they do tend to be in "suspended animation" during that time.
I usually put something out on Mother's Day, in honor of my mom, who was my gardening buddy! That's the earliest I'll plant out. Then, I keep my fingers crossed...and continue to "plant out" for the rest of the summer!
Enjoy yourself!
Emily