Two part question:

1. I have a number of large water oaks that are a pain to trim around. The exposed roots and crevices make for a difficult time when new weeds pop up. Adding a little mulch (not smothering) around the base of the trees should help alleviate this problem, but I wanted to double-check with the OB to make sure I'm not missing any potential issues.



2. I also have a number of younger trees that are supported with an irrigation system and wire cage to prevent critters getting to them. I like to fertilize the saplings with MicroLife a few times each year, but I'm concerned that if I add mulch to the bare ground to prevent weed growth, I'll be wasting money with fertilizer run off. Thoughts?

I probably would. It is not like there is lush grass growing around the base of the tree anyways.

Big trees: I would just mulch around it. You can make a nice bed around them or just put enough down to cover the hard dirt and keep it from turning to mud when it rains. If you make a bed, just don't smother the tree. However, it you use a hardwood mulch (not a fine mulch), I don't think you could smother the mature trees. Mulch is porous and will break down slowly to dirt.

Small trees, I would just cover with a light layer of mulch primarily to cover the dirt, restrict weeds, and keep moisture from evaporating so fast.

For the small trees, I use a 1-2 inch wide auger that connect to my drill to drill holes around the tree. Usually at the edge of the canopy (yours would be around the root ball b/c of not canopy). You can put some mulch or compost in the holes. This will allow water and fertilizer to reach down into the soil. It will keep the roots growing deeper, rather than at the surface competing with your grass.

Make sure to leave some of the flare exposed at the base of the trunk on the little one. Burying the root flare on a tree, especially a young one, is terrible for them.

Spray Roundup on the weeds

You won't have any issues on the bigger ones as long as you don't make volcanoes.

As others mentions, just don't go too deep on the small ones.

Bigger concern from my point of view is the included bark at the base of the trunks of that big water oak. Monitor for any separation and long-term structural integrity.

Specifically where the two trees connect?

Use Roundup for the weeds around the trees. You can mulch the heck out of them, just don't stack it right next to that root flair. Some nice beds will do wonders for your landscape.

Keep the glyphosate (and any other herbicides) off of the trunk of the young trees-or any other plants' trunks.
It can be absorbed into the phloem and although probably not lethal will cause damage. Such as bark cracking and "witches brooms". Damage may not show up for a couple of years.

You can wrap the trunks loosely with aluminum foil or just use a cardboard shield when spraying. Definitely keep it off the leaves.

A mix of mulch and compost would be great for the small trees.