Hi and welcome to permies!
I bet there are a
many encyclopedia-like permaculture and plant databases floating around. Here are just three:
https://permacultureplantdata.com/plant-database
https://www.appropedia.org/Welcome_to_Appropedia
https://pfaf.org/user/default.aspx
That said, I can't personally verify the
usefulness of the above sites though, as I have never used them. If folks are telling you that 'a plant must serve many purposes in a permaculture design' then that probably stems from Mollison's big black book, the Permaculture Designers' Manual. He writes the following...
Mollison, in Chapter 3, METHODS OF DESIGN on page 55. wrote:The Basic Energy-Conserving Rules
Every element (plant, animal or structure) must be placed so that it serves at least two or more functions.
Every function (e.g. water collection, fire protection) is served in two or more ways.
That rule, we see, is about conserving
energy. But it's also helpful for promoting resilience, space savings, and cost savings.
Now, the context of the section Mollison wrote is about zones and sectors, which are two awesome design concepts which everyone enthused about permaculture can hold near and dear to their heart and homestead. So the rule is not just about the plant itself, what the plant can do -- whether it's
medicinal, Nitrogen-fixing, mulch producing, useful for timber or fuel
wood, fruit or nut bearing, pollinator friendly, shade, coppice/pollard, cash crops, dynamic accumulator, etc. -- but it's about
patterning.
All that is to say, you may find it is better to have just a single book of 100 useful plants local to your climate, that you can source and use, than a world-wide database at your fingertips. It's not just knowing about plants, but actually using them, by placing the right plants in the
right relationship to other elements, to meet your (and other living things') needs.
Keep the questions coming!
P.s. When you get a chance be sure to check out "
how permies works".
Edit to add: If you find yourself in a Temperate climate, looking for useful plants, be sure to check out Dave Jacke and
Eric Toensmeier's
Edible Forest Gardens. It's a two volume set, and the first has 100 recommended species for forest gardens, and the second is chock full of information about useful plants.
https://www.edibleforestgardens.com/ (
Amazon link)