Planet zoo doesn't have brushes but if you group a bunch of trees you can duplicate them over and over again.

Ngl that brush idea is pretty good, would be really useful!

There are a ton of workshop items that are groups of scenery (rocks, planets, both combined, etc.) to allow quick placement of natural features.

The difficult part is finding them in the workshop because so many people use tags that are wrong so you get a ton of other crap in your searches.

Another useful thing is using "contro-d" to duplicate your selection, which is handy when trying to fill out the big flat maps with foliage.

Brushes would be so nice for this if they added it to the game though.

You can also make your own blue prints of areas you like your nature/gardening work. Press "i" for group select tool. Hold ctrl and click all the plants, mulch, decorations, etc you want to be in it. Depending on what you selected you can merge it all into one group by clicking the little tree in the menu at top right to "merge scenery items together". You can also create blueprint by clicking the little button to the left of that. I tend to do both so I can easily click and ctrl d to duplicate in that zoo and I have it for future zoos.