To plant, or to save?

How can you tell the difference between seeds that have been harvested from a plant that need to be planted immediately from those that can be saved until the next season?

I don't think you can "tell" the difference, per say. But there is a very small class of seeds known as recalcitrant seeds, which quickly deteriorate when they lose water, whereas most other seeds are at their best when stored in a dry state. The most common recalcitrant seeds are the willows, hazels, black walnuts, chestnuts, acorns, samaras (sycamore), coconut, coffee and wild rice. These die when they dry out, so they cannot be stored for any length of time. Also, many tropical trees have seeds that are large and fleshy and must not dry out if they're to grow.

Basically, you need to look up the seed in question in a good reference gardening book, and it will tell you whether it can be stored or not. Hope that helps.

Yes, JJ, that does help. I have some wildflower seeds from a plant that has just stopped blooming for the year and I wondered if those were in the "plant now" category. I"ll look around and see what I can find out. Thanks.

Usually seeds with hard coats stay viable longer than those that are very thin and/or soft. It is hard to tell though, since that does not always apply, and there are some seeds out there that really do not fit in either category. For veggies I found this link
http://gardening.about.com/library/weekly/aa022503b.htm
For anything else I pretty much use trial and error until I get it right or I can find out more information about a specific plant. Imagine, I just got a 2 year old Albizia seed to germinate in a few days, never though it would happen :)

Olga...great link!! Thank you!!!