Use the Google, by copy and pasting the following into Google search, with "searchterm" being your search term:
searchterm site:http://forums.gardenweb.com/forums/load/salvia

Or just go to the bottom of the page and use the search box. You have the option of selecting just the salvia forum.

You may have figured this out by now but there are two different search boxes. If you want to search just the forum that you are in, use the first search box that's about 2/3 of the way down the page, right before the box where you would type a new message. If you want to search ALL the forums, use the box at the very bottom of the page.

My mistake. Most of the other forums have the search link above its title