I'm trying to get seed oil and having poor luck getting walnut saplings. I've got silver limes and hill cherries planted right up against each other next to my apiaries, but I'm still just getting lime and cherry saplings. I understand the drop rate for walnut saplings is low, but does the lineage of the trees matter...do purebred silver limes and hill cherries yield better results than hybrids, or vice versa?
Best thing to do with this, is to set up only those two trees, with no other breeds close by, and play the waiting game. I recommend Industrious bees for fast pollination. Eventually, you'll get the sapling you want.
Depending on any Forestry add-ons you've installed, you may get others. I know you also get Lemon Saplings from this combo, and the "Extra Trees" pack adds loads to this combo.
I have found breeding trees to be an incredibly slow process. Yiazzy's advice about the bees sounds good. I have only one thing to add, I have read somewhere that purebred are better for breeding than hybrids in regards to bees and mutations. Since trees and bees seem to share similar mechanics I would assume that this hold true for them as well. I haven't gotten too much into the mutations part of forestry breeding though as I have been mainly concerned with getting traits like 4xoffspring onto bees and getting rid of poison on tropical ones.
5 Apiaries. Two of those have Industrious Bees in, the other three have Imperial, Heroic, and Soul respectively. Those are just the five I chose to use, and Soul takes a while to get, plus you need to adapt them to the climate as well. They're all pure breeds though.
I have all the apiaries automated so that when the queen dies, the drones and princess loop straight back into the apiary in a neverending cycle. It works so well with the trees, that I managed to get 4 Common Walnut Saplings in 10 minutes today. I've just planted Mundane Larch and Bull Pine to get Coast Sequoia saplings, which I now have 3 of.
But the point is that yes, Pure Breeds work incredibly well for cross-breeding trees.
One thing I forgot to tip you on last post. Say for example you've planted 3 Silver Limes, and 3 Hill Cherries. Make sure they alternate, so that the pollen is more likely to land on a different tree breed.
Edit: Well, after another 2 hours of playtime, I managed to get my 9 Coast Sequoia saplings, and Grafting just one grown Sequoia got me another 192.