This is one of my favorite cheap ways to occupy beginners insatiable need to constantly get more trees and work on developing them.
I ideally start with a deciduous fast growing species that is simple to propagate. Fast growing species include: Maple, Elm, Tamarisk, Redwood, and I’m sure a few others I’m missing. Easily propagated species through cuttings or seeds includes: Elm, Maple, Euonymus, Ficus, Redwood, Pomegranate, Tamarisk, a surely more.
You can also start with an opportunistic species. I have a friend with a large Chinese pistache tree, and it drops seeds every year that sprout in the rocks below. They pull them out like weeds every year, so I went ahead and offered some free labor. Here is the blog post on a Chinese pistache forest creation on a slab. You can use suckers from fruit trees, pull up those pesky Siberian elm seedlings that are somehow everywhere, or take cuttings off of larger trees you already have (Ficus, Elm, Euonymus, Redwood, Pomegranate, Tamarisk) and using them for the beginning of your forest.
You’ll notice not all of these species are the ideal choice for a forest. You can definitely take these same steps with Beech, Birch, Hornbeam, Ginkgo, Sweet gum, Junipers, Cypress, Cedar, and others, but the results are slower (nothing wrong with that!).
Above you can see a full flat (1 of 3) of silver maple trees. I have a neighbor with a massive tree, and it is virtually impossible to fail growing them from seeds.
I went ahead and took a chunk out of the corner to a forest. I was able to have some smaller trees, some taller ones (in the middle) and a few medium height trees all within that 1/4 of the flat.
I then sat it on the bench to see what there was to work with. I was pretty surprised how well it worked without much help at all, so I went ahead with preparing the pot.
One of the more important parts of a forest creation that can make a HUGE difference between 12 sticks in a pot and 12 sticks that look like a forest is contour. Below you’ll see I put a mound in the center to help me accomplish this. It worked well because I have a flat root mass that I could easily contour to the soil below.
After laying the root mass down and pushing it down into place, I was left with this. Pretty simple.
After finishing the potting up I was left with a pretty good starter forest that has potential in the future. I’ll wait a growing season and then put wire on the trunks to give some more movement.
Another option is to just cut any section out and pot it up, then you can prune back certain trunks to differing heights to get the asymmetrical triangle look that a refined bonsai aspires to.