Hi Deanna,
It is such a long and convoluted list, you will never find a complete one. Plus some things will come true, but only in isolation while others will come true no matter what. It can be confusing.
Hybrids which could say Hybrid, F1, or have an X in the name will not come true. They could look similar to the parent, or they may look absolutely differnt depending on the lineage of cultivars used to create the hybrid.
Some named cultivars are not hybrids and will come true. Echinacea purpurea 'White Swan' is a good example of this. It has a name just like a hrbrid, but really it is a stable strain. BUT if grown in a normal garden it probably will not come true from seed since the regular Echinacea purpurea and other pink forms are popular in garden. White is recessive and cross pollination will result in pink babies.
Plant that are just a straight species will come true like Echincea purpurea. Though again crossing could occur since it is a popular plant and it will cross with cultivars. Plants that are straight species and you know no cultivars are growing close by will come true like my Veronicastrum virginicum. There's no named cultivars of this plant so you never have to worry about crossing.
I hope that all made sense. If you are still wondering about certain plants, just post and I can help you sort it out.
Remy

That convoluted answer was very helpful--I at least know to look for Hybrid, F!, and X! I will take your kind offer and post on any plants about which I have questions.

Actually, I do have one more question. My basic understanding is that species can only parent with others of the same species. Is this still true for plants? So Rudbeckia hirta can't pollinate with Rudbeckia fulgida? I ask because I handed out seeds from a Rudbeckia hirta, the only Rudbeckia hirta I have. While I know pollen from any neighbor's yard could make it's way here, the chances are much slimmer than the plant cross pollinating itself. So, this year I have a good chance of self-pollination, right? No hirta and fulgida blends? I know this is impossible in the animal world, but I doubt my knowledge of the plant world because there are so many amazing crosses and hybrids out there. Maybe species can intermix?

Next year won't be so good for purity. I've got a lot of wonderful Rudbeckia hirta seeds to plant and put in my garden. If they come to flower, then who knows what I'll get!

Thanks again!
Deanna

Hi Deanna,
Some species can cross. It depends on how closely related they are. Like I know some Echinacea species can cross since the species are actually a sub-species. So those plants when grown alone will come true from seed, but once grown in a group setting with the closely related species will cross.
Sometimes you will see a subspecies listed as such which makes it easier to know for seed saving like the Yellow MG relative I grew this year. Ipomoea hederifolia var. lutea. The VAR. means that it is a variant of the species. Ipomoea hederifolia is red. Var. lutea is yellow. Yellow is not the dominate color so once grown together. The red will take over in the offspring.
Oh, sometimes you will get people saying certain plants will cross that never cross because they are count laboratory crossing. Those are extremely difficult to make happen and only happen in the laboratory not in a garden.
Anyway, back to your original question, lol. I don't believe R. hirta and R. fulgida will cross. If your plants are isolated enough from neighbors, your chance of cross pollination are low. : )
Remy

Hi remy,
I have a question for you,
I live out in the country with neighbors a long way away, so I shouldn't have to worry about polinators,
If I take seed from say a hybrid petunia , and plant it the following year, will the seed that i save that following year be true or will i keep on getting variations . Was wandering if at some point in time they will become true ?

Hi Heavyoilguy,
Being a hybrid, you will still get variations. What you want to do is save the babies that look like what you want(if you get any, petunias can vary a lot from the parent) and remove the other plants. Then the following year do the same and keep on doing this in subsequent years. You need to do this for at least 5 years possibly up to seven.
Remy

Some native species are known to "hybridize" quite readily, so one can never be sure about the parentage of saved seeds. Two examples are asters and ironweeds. That's why saving seeds/trading seeds can be fun, but if you want the pure species, you are better buying from a reputable nursery or wild flower grower.