Dug out by a Trackhoe to make a new road or dug out by me and stick in a pot? It’s more like move 1 piece of shale and put in a pot. I guess we will see if it makes it.
Given you are collecting from construction sites, now is better than never. You did not answer the question about where you are growing these, whether it is Seattle or in Utah, at lower elevation. After collection from 6000 feet in Utah in January, these will need to be protected from hard freezing for the remainder of the winter. Easy to do in Seattle, much more difficult in Utah.
I found I need at least 25 t0 to 50 trees in the yard so that I can work on one, then leave it alone to recover, with 50 trees there is always a different tree that needs attention, thus making the patience part of bonsai easier to bear.
Collecting out of season is often possible, if your aftercare is better than average. So think about their after care.
first task after collection is ROOTS, you need to let the tree grow a new root system. Within reason, the more foliage a tree has, the quicker it will create a new root system. Do not prune off foliage if at all possible. Of course remove damaged and diseased branches, but keep initial pruning at a minimum. Pot the trees into a pumice based bonsai mix. Many collectors use 100% pumice. One possible substitute for pumice is Perlite or Sponge rock. Both are the same material, just different coarseness of particle and different distributor. Both are fired to expand feldspar or similar mined mineral. Pumice is preferable. Pumice and composted bark is one good enough mix. Like I mentioned, pumice alone is quite good for establishing new root systems on newly collected material. Conifers do well with 100% pumice. Deciduous trees do well with at least some composted fir bark in addition to the pumice. Maybe 20% to 30% bark.
Use large containers, collanders or purpose built grow out boxes. Bonsai pots are too small. You need to build a root system and a larger than the final tree size root system. In order to have branches to choose from, you need the possibility of growth. For growth you need a root system. I know I'm repeating myself, but catch the theme? Roots. Collected material can require more than one summer to grow a new root system. The tree is not ready for any work until you have vigorous bushy growth. One or two buds is not enough, even many buds, while a good sign, does not mean the tree is ready. You need vigorous growth, then the following season work can begin in earnest. With good vigor, the tree will recover from dramatic work. If you start working the tree too soon, you can send it into decline, which if you don't recognize the symptoms, decline can lead to death of the tree. I have a Ponderosa pine, and it has taken 3 years, and finally this 3rd summer it had a good flush of growth, so I put the very first piece of wire on it. I still have not pruned it at all. But it needed time to settle in. I have a colorado blue spruce, nursery material that was horribly pot bound. I removed 75% of its root system to get it into an Anderson flat, a 16 x 16 x 5 inch tray with a mesh bottom. It has been 3 summers since repotting, and as of yet to get sufficient growth to be comfortable to start on it. So I let it recover. Since it is only 2 feet tall but has a nice large diameter trunk, I can wait. Work will not begin until the
season after every branch has ''blue shiners'' fesh young growth. It could take another year or two. Six years to recover is long, but not unheard of. I was brutal with the roots. SO, back to the theme, roots first, don't rush until you have good roots.
The above more or less applies to all species.
Douglas fir is what I also believe you have collected. They have only recently become popular for bonsai, you won't find a lot written about them as bonsai. They are a good species. They have their own schedule for when it is ideal to do various tasks. I don't have any, so you will have to find the information yourself.
Each species of tree has its own habits, and growth pattern. This pattern determines the schedule for when it is optimal to apply different bonsai techniques. For this reason you need to become more skilled at identifying trees in the wild. You need to know the identity, at least to genus, the various trees you collect. This way you can optimize your care, and get better results with a lower mortality rate. Low mortality rates are good.
So collect when the opportunity presents itself, and improve your after care to give the trees a fighting chance of survival.