AusBonsai

Wondering what are the best species to grow in the ground for a few years to fatten them up rather then just in pots

Trident Maple & JBP.
Nothing worth having grows faster in the ground.

Almost any tree species you have in the ground will thicken quicker than one in a pot, whatever you choose.
If you want to maximise thickness, planting in the ground and letting it run for a couple of years (or more) is the fastest way to get the girth.
If you want to develop a root system, taper and scaled internode length etc that is good for bonsai, planting in the ground and letting it run for a couple of years (or more) is probably the worst thing you can do. Some species might work out better than others, but my point is ground growing is no silver bullet.
I really only ground grow pines (JBP, Austrian Black and Scots predominantly), although if I had room, I'd like to have a go with a few mels too.
Before my trees go in the ground they are at least 1 year (normally 2) from seed and have had their roots worked two or three times so that I have a developing nebari/ramified root system. I have also looked to develop potential sacrifice branch options at various points along the stock (not just the main leader, but others waiting to kick in once that is removed) and ensure I have the start of some lower branching so I can nurture this as the future bonsai while the tree is doing it's thing in the ground. I would say whatever you wish to ground grow for bonsai, try and keep these things in mind with the trees you plant.
I lift my trees annually to check/work the roots. Obviously this has an impact on the development as they subsequently allocate energy into re-growing a root system rather than just thickening up, but it also means that when it comes time to dig them out, I am pretty confident I can transition to a pot with a good set of viable roots close to the trunk/nebari.
Remember, a tree wants to be a tree, preferably a big one. It has no interest in your plans for it as a bonsai and will grow in a way that gets it as big as it can as quickly as it can. Without strategy, that means you get a tree that is tall, with a massive root system and branching way up high on a taperless trunk because all those things fit in fine with the tree and its own goals.
So, I would think there is no reason not to have a go at ground growing whatever you want provided you go into it with your eyes open and have a plan to get a bonsai out of the experience, not just a thick tree. I would think something like Chinese Elm would be an easy one to get experience, or Trident Maple as suggested. Both of these are easy to layer/ cut back hard (both above and below the soil) if you find it gets away from you.
There are also other techniques (like grow bags etc) that are a kind of 'hybrid' of ground/pot development ... but that's another kettle of fish!

Almost any species will develop quicker in the ground. I've ground grown Melaleucas, Banksias, Japanese black pines, Junipers, crab apples, Japanese maples, Chinese elms, Plums, hawthorn, Buxus, Ginkgo, Cotoneaster, Euonymus and trident maple.
Trident has been the star performer in my area but may not be quite as good in Taree. Japanese maple is much slower and more difficult to get great roots. Crab apples grow and thicken fast but trunks often end up unattractive. A couple of trials with Banksia integrifolia gave some good trunks.
Juniper chinensis has been slow, even when grown in the ground. My experience is minimum 5 years to get any size on the trunks. Shimpaku growth is long and straight so it appears to be better to shape the trunk for a few years before thickening it.

I reiterate what Ryceman said.

• Ground growing is NOT a magic bullet.

• Big is not necessarily best for bonsai. We also want trunk taper, attractive bends, good nebari and don't want huge scars.

• Definitely do root work to set up a good, lateral root system before committing to the grow bed. Planting small tangled roots will yield thick tangled roots that will be difficult and time consuming to correct after.

• Lifting trees each year gives much better control over trunk shape, roots and overall development. That may slow initial development slightly, though I'm still not absolutely convinced that growth is very much slower, if at all, BUT the well developed root system will make transplant very much safer.

• Pruning regularly usually gives much better trunk shape and much better taper. Again, this may slow growth a little but the following phase where you grow the new leader and branches is very much quicker if you've done regular pruning so overall I find it quicker to do regular chops for most species.

Don't bother trying to develop branches while trees are growing in the ground. You don't have enough control over shape or growth rates. Internodes tend to be long which means later ramification is sparse and branches too thick.

shibui wrote: ↑August 17th, 2025, 3:11 pm Trident has been the star performer in my area but may not be quite as good in Taree.

I reckon they will be fine in Taree.
I'm in Northern Rivers NSW and they are also the star performer for me.
They thrive on the sunshine and heat, as do JBP.