Legend has it at April 15, but the weather has become unpredictable.

Ed Hume's site says average is April 3, but April 26 is the 'safe' last-frost date.

Here is a link that might be useful: last frost date

These last frost dates are a little vague, please remember they are averages too so earlier one year and later the next.

The dates for Vancouver not the same as for Portland, and I suspect not even the same for one side of town as the other. My last frost date is supposed to be Apr 10 according to the Hume list...in 2006 we had significant frost on May 2, 3. :) If you are talking safe Spring temps for very tender annuals i.e. impatiens, green house grown, you might consider easy to remember Mother's Day as your safer date.

A nippy 24F this morning - My 24" geranium pots are decorated with table cloths on my front porch