Some fernlike things are not ferns

It's not completely unknown for a fern collection to be handed in containing a rogue Non-Fern. I think they were probably included by students who had left everything a bit late and ran round in a panic grabbing everything fern-shaped. So that won't happen to you, because you'll leave plenty of time for collecting and studying and identifying, and no doubt would then laugh scornfully at such rank amateurism.

But everyone has to start somewhere, and so I will use this page to post any species I think of that might confuse to begin with. There's also some help on this in Merryweather's fern key.

A common flowering plant with ferny foliage is yarrow, Achillea millefolium. The millefolium describes its very divided  'thousand leaves'.

CC image by Sigman

In the right season you'd see a tall stem with flowers, :

CC image by O Pitchard

but without this to give the species away, you might just see a lot of the soft feathery leaves. They are very soft and flexible. If you find yarrow, you might consider some kind of trade with someone collecting its family, the Asteraceae. I would say the leaves have a 3-D look with divided bits going in all directions - and that's what looks decidedly unfernlike to me.  No doubt there are exceptions, but I would expect a divided fern frond to basically be in one plane.

Another species of flowering plant which might cause confusion is Herb Robert, Geranium robertianum. In the right season it'll have lots of pink flowers, and eventually its leaves turn red.  But without those you could initially mistake this leaf for a fern:

CC image by Sigman

But if you look at the plant as a whole, you shouldn't get confused, the leaves are usually sprouting from a stem, not emerging from the ground singly. They have a groove in their base which doesn't look very fern-like.

public domain image

These rogue species aren't going to be a problem for you at all after your initial forays, but I thought they were worth a heads-up.

Of course the best sign that something IS a fern is that it will have little sori on the underside of the "leaf" (- not really a 'leaf', it's called a blade, and the "stem" is a stipe. Ferns have nice words to describe them, as you will discover). And although not all will have sori, really you want to collect the ones that do, as their shape and arrangement will provide extra clues about the species you have found.