Ideas for large bed under pin oaks
I have 3 pin oaks in my south-facing, full sun backyard. They've been there about ten years and they're big enough now that I removed their mulch rings (not volcanoes, just normal rings) and am creating a large continuous bed beneath them, which will run to the corner of the lot and then flank the side of the backyard against the property line, essentially in a large L shape. The shorter, straight side abuts the neighbor's property and the longer, deeper section encloses the pin oaks and is straight at the back along the property line and has sweeping curves on the side facing the house.
I measured and laid out the area with twine, then spraypainted the borders and sprayed the turf inside it with Round Up over the weekend. The grass is starting to die nicely, and once it's good and fried then I will be laying a two inch layer of mulch in a mix of shredded oak leaves, pine needles and pine bark. I will be planting an informal hedge of crimson pygmy barberry along the short (approx 32 feet) straight leg of the L to delineate the property and visually disown the leggy, scraggly assortment of unidentifiable, half dead shrubs scattered on the neighbor's rocky slope facing my yard.
I won't be planting anything in the other leg of the bed until next spring, but I want to work out a full plan for any shrubs or perennials this year so that it's well thought out long before anything actually goes in the ground. I was originally going to transplant some low junipers that I have in a large bed on a front western corner of the house, but I've since changed my mind. They're not thriving where they are now and they're pretty thin looking, so I'll be taking them out and disposing of them. My soil is on the sandy side and well drained except for one wet spot in the corner of the L, where I'm considering putting a dwarf blue arctic willow because I understand they like it wet.
I'd love some ideas for what I might do with this bed. Deer are not an issue at all for me, but rabbits can sometimes be a problem (mostly in the winter and I suppose I could cover or cage things then if I have to). The leg of the bed beneath the pin oaks is about 50 feet long and 5 feet deep at the shallowest points inside the curves. I don't want to fill that entire leg of the bed and would prefer short things, no taller than 4 feet at the absolute max. I already have the trees for height so I'd like small shrubs and perennials, just enough to keep the bed from looking spartan. The yard is very visible from the elevated deck and from large picture and bay windows along the back of the house, but I don't actually go down there much other than when I'm doing yardwork.
The trees have been pruned by Bartlett annually and they're quite tall and have an elevated canopy, so light is not a problem. The shade they throw mostly hits the house and foundation bed, and won't affect the bed beneath the trees much except on the far western end by the last pin oak, where I probably won't plant anything because it gets dappled shade for much of the day. I don't have a problem with annual or even bi-annual pruning to keep shrubs tidy and full, but I don't want anything that will outgrow its intended space or end up looking leggy as it ages. I'm not familiar with too many things that prefer acidic soil, so I would really love some help. I know rhododendrons and azaleas like acid, but they're a bit ugly when not in bloom so I'd prefer to skip them. I'm considering putting in 2 or 3 blueberries because they have nice foliage, lovely fall color and the dwarf varieties remain small. I have no idea what to do with the rest of the space.
Sorry this is so long, thanks for taking the time to read through it and thanks in advance for any help!