I'm a hosta gardener here because of your link. Others are lots smarter than me but . . .

What kind of trees are they? If they're maples, you're out of luck. Do something artsy like pea gravel and a hammock. forget hosta.

Almost any tree with roots that close to the surface will cause real problems, hogging all the water, putting tiny feeder roots into your plantings.

And the existence of moss is troublesome; moss only grows when there is not enough sunlight for anything else to survive.

but you've already sprayed your Roundup so . . . Have you thought about spreading shredded bark, then putting plants in containers? Try planting a couple in the ground to see if I earlier comments about their survivability are right. But leave most of your plants in pots. One of the posts here a few years ago showed a ladder leaning against the tree with pots on every step. Can you get some really long ropes to hand baskets from the tree branches (long ropes so they're low enough for you to water them).

Best of luck -- Nancy

p.s. You may want to move this to the hosta forum where everyone is much more willing to help.

Nancy, Why "out of luck" if they're maple trees? I have 2 enormous maple trees with some visible roots in my back yard and plenty of hostas growing beautifully around them. Is this some sort of fluke?

You do not want to pile up soil around the base of the trees!! Changing the soil level around trees - both raising or lowering - is not a good thing. The bulk of a tree's feeder root system - what keeps it alive - are located just below the soil surface and adding soil, even as little as a couple of inches, can effectively smother the roots of oxygen and deprive the tree of nutrients, usually resulting in death. Not something you will necessarily notice overnight, but it is very well documented and happens with rather startling frequency - often the byproduct of construction activity but just as often, ill-advised landscaping projects.

The other issues with growing plants under large, established trees is the difficulty of digging adequate planting holes in very rooty soil, the far more aggressive tree roots stealing needed soil nutrients and any moisture and reduced rainfall and sunlight under the canopy. Ideally you need to start with small plants that tolerate or thrive in dry shade and even then provide considerable TLC the first couple of seasons to help get them established.

Here is a link that might be useful: cautions for planting under big trees

gardengal48 is right- you'll kill those old trees if you cover the roots with soil.

If it was me I would enlarge the beds so you aren't forced to cram plants up too closely against those trees.
You have the room and don't need to be wedging plants between tree roots and forcing them to grow in constricted areas- that soil is going to be very compacted and you can't be digging around amending without damaging the trees.
I would also make the bed an irregular shape that perhaps mirrors the shape of the lawn.
No matter what you do you will be doing supplemental watering so the trees don't suck up every drop.

Thank you all so much- tons of great advice!!!

Ok, I have a question about adding soil to the base of these trees.

These are VERY well established trees. They are about 50 feet tall, and 25 years old. The space between them is about 20 feet. They are very healthy. There is a dip between them from erosion, I'm sure. I want to add dirt to the area between, not a ton of dirt, just enough to give it some bulk to plant.

I don't plan on planting right up next to the trees or taking out any of the roots to do it.

Is this dangerous to my trees??

Thanks,

Gabby

All the previous advice sounds solid to me, and I would like to second the idea of pea gravel and pots, plus a couple of sturdy plant hangers secured in the ground or attached to the trees directly, some seating, an arbor close to the perimeter welcoming into the garden. Many shade plants do beautifully in pots: hosta, heuchera, impatiens. The trees are beautiful and graceful, so the difficult part of your garden is all ready to go. Some shrubbery planted directly into the ground might work, especially if you start with small plants. I am picturing something spreading or sprawling, and drought tolerant, evergreen, some kind of junipers. If you're not a fan of junipers, but want a more herbaceous look, some ferns do well in dry shade, check out the dryopteris, some of them are evergreen, too! and would probably thrive planted underneath these trees. (Note, the ferns need lots of water until they are established, THEN they will be fairly drought tolerant.) Google epimedium and see what you think of it. How about lamium? It's very pretty and will compete OK with tree roots. (Epimedium and lamium are ground covers) Pachysandra is an old standby. If you did a ground cover over all or part of the area, it would be instead of the pea gravel idea.

Good luck. This spot has so much potential.
Christine

I'd like to second the others; don't add soil under the trees. The surface roots are where the trees need them to be. Some trees have major surface roots, others (such as oaks) have roots that go deeper, but all have surface feeder roots that can be damaged or suffocated by adding soil or too-deep mulch.

I have a shade garden under a very challenging tree and years of (sometimes disappointing) experience have shown me some plants that can take the competition with tree roots. Believe me, trees with a tight network of feeder roots at the surface are amazing machines for sucking up water in the summer. For instance, for years I planted impatiens that never thrived or bloomed well--I thought the shade was too deep. One year I had the brainstorm of trying half of them in large pots instead of in the ground.
What a difference! They had that area of soil in which their roots didn't have to compete with the tree's roots, and they grew beautifully. So, yes, for the larger, showy stuff, think pots.

Whatever you plant in the ground will need frequent watering in the summer.

Asarum canadense grows well in the ground, as does the white cream violet, and sweet woodruff. Hostas do somewhat well, provided they get lots of watering, and the further from the tree trunk they are, the better they grow. Christmas fern and heuchera, also.

I have a mix of common blue violet, Confederate violet, and cream violet, with some patches of the Canada ginger and Virginia bluebells that form a very pretty carpet of flowers in the spring. These become green (the bluebells go dormant) for the rest of the season, and then I rely on pots with impatiens and ferns. Quite simple, but people admire it.

I have many hostas growing beautifully under 2 Huge maple trees. I also have euyonmous growing like crazy. I also have several ornamental trees growing, But they are A challenge. I potted one in a large planter and
it is doing much better.

Another possibility for an accent or focal point among a larger bed of low plants, whether you go with pachysandra or epimedium or my suggestion of violets, ginger, and mertensia, if you don't want to arrange some pots, is some kind of sculpture.

By the way, in reference to your comment about moss, the growth of moss should not worry you. It grows naturally where there is acid soil and moderate moisture. (It will not grow in wet soil.) It DOESN'T mean that nothing else will grow, contrary to what one person said. Moss likes the same conditions as many shade wildflowers. But where leaves collect and other, taller understory plants thrive, moss gets crowded out or smothered, so you will see it where other plants are thin enough to let it get some light. I encourage it in some raised places in my garden. Many kinds are very beautiful. (See Japanese moss gardens.)

The low area between the trees doesn't look eroded. Rather, the areas around the trees have raised as the trees' root systems have grown underground.

I have two different kinds of hostas under two maple trees that are doing fine also. I'm in Zone 4 and have a shade garden directly under a large maple and in the surrounding area. Some of the things that have done well are:

Columbine -- both Old Fashioned and hybrid in purple, lavender, and fuchsia.

Soloman's Seal

Lily of the Valley

ferns

Digging plants from a local timber or wooded area is a good way to add plants cheaply that should do well since they are native to your area.

My recommendation would be to let the dig up the area and rake out as much dead grass as you can. Or till it.

Plant your stuff, then lay heavy, heavy layers of newspapers down, wet it, and cover with mulch. If you plant to space your plants and keep them spaced, pea gravel is pretty and holds up. If you eventually want them to all fill in together, I'd use bark or pine mulch and replenish each year or two as needed until you don't need it anymore.

You have such a nice big area you could put a couple small shrubs in there too.

Don't know if this will help but see my article about gardening under shade trees at
http://www.examiner.com/article/gardening-under-shade-trees
Please let me know if it is helpful and if you would like to see another article on a different gardening subject (there are a couple hundred available for perusal).
I am growing a daylilly and some asian jasmine under our younger oak tree shade area.

As far as I'm concerned on the topic of not planting under large trees is pure Bunk!

I just lost two large trees in a recent storm and both the hosta and lilies over the years grew right up to, and around the tree trunks. The groping were so massive, I had to dig them out so I could get the chainsaw near the base of the tree to cut the trunk down!

Also Columbine and Corbels have done very well under the trees. Now I'm in the process of moving them back into another shaded area!!!

AL

Some woodland plants thrive in a shaded situation with root competition - cyclamen, sanguinaria, corydalis come to mind.