Hi,

I would think you'd need to dig many extremely deep holes to even come close to damaging a 50 ft oak tree...so I'd say put in your perennials and don't worry about hurting the oaks :) You might, however, want to consider that oaks do provide a great deal of shade at that size and I'd think that some of your perennials might need a bit more sun in order to flower...so you might not want to place them directly under the oaks...

...yup, and the other factor to avoid is filling in with soil under and around these oaks. All trees resent having fill placed over their root zones, but oak and beech are the most sensitive to this practice.

If all you're doing is digging the small holes required to plant gallon, etc. stuff, shouldn't be a problem.

+oM

The trees are tall enough, so sun does come close to the foot of the trees.

The purpose is just to cover the ground currently occupied by the wild plants. Some are invasive, such as Jap honeysuckles, many wild raspberries.

When I tried to dig about 5 feet from the tree, I saw some large roots. This makes me uncomfortable. I do not want to cut the roots. If I leave the large roots, then I may still damage the smaller feeder roots. There may be more roots if I keep digging.

The other way is just to yank out the unwanted plants, spread some annual flower seeds and let it grow wild again. Or just mulch the ground....

I guess that many people still think that the roots of big trees are super deep and that's just not true. Many of the anchor roots are deep, but the critically important 'feeder' roots, the ones entirely responsible for the absorption of water and dissolved elements are actually very shallow. As a matter of fact, it is the shallow fibrous root system is also holds most of these trees up and in place!

When I mean shallow, I mean within the top 12 inches or so of soil. You'll see what I mean when you start digging. Those roots will always and forever be located where there is the greatest concentration of fresh water, oxygen, and essential micro flora like beneficial bacteria and fungi.

If you have the ability to drag a hose to the site where you want to plant, most trees can tolerate a bit of hole-by-hole planting with a sharp implement. I mean here and there, not in a solid, one plant per every 18 inches.

Established tree roots, those fibrous (feeder) roots will fight for all the water, nutrients, and elbow room underneath there, so be aware of that. You shouldn't compensate with too much extra everything in favor of the perennials, however, because that may disrupt the ecosystem of the trees.

It's the trees, after all, that are your most valuable things on your property, other than the house itself. I expect that those trees and the soil that they're growing in have already seen their share of challenges over the years.

Be conservative, careful, and thoughtful in your addition of plants around the roots of your trees. Plant a few at a time, over the years.

In addition to Rhizo's factual post, I'd like to assure you that trees can withstand having their smallish 'feeder roots' disturbed, to a degree. These mostly whitish and very tiny roots are not permanent structures, but rather, are formed and subsequently die and get reformed a number of times over the course of a single growing season. What's more, and again, Rhizo alludes to this, most of the actual work of nutrient absorption is performed by still tinier fungal hyphal structures, collectively known as mycorrhyza, that work synergistically with the big tree to do so. The tree in turn provides the mycorrhyzal fungus with carbohydrates to live on. The mycorrhyzal fungi have the ability-unlike plant roots-to degrade rock itself, thereby obtaining such critical elements as nitrogen and especially, phosphorus. Plants long ago "realized" it made more sense to form such a mutualistic association than to build all that structure themselves.

Pretty ingenious, eh? BTW, some trees, maples being preemminent, laugh at our efforts underneath them to get some herbaceous plants going, eagerly-some would say greedily-lapping up any and every input of water or nutrients we place in that root zone. You may have a little better luck with your oaks though-not quite as challenging.

+oM

"The purpose is just to cover the ground currently occupied by the wild plants. Some are invasive, such as Jap honeysuckles, many wild raspberries."

Do you have a plan of campaign to get rid of these plants first? Perennials will not be able to fight it out with the likes of Japanese honeysuckle so they need to be dealt with before anything else happens.

You might even find that once they are gone some nice natives get a chance to show up.