Raised beds without killing trees.

Couldn't find a raised bed forum so I'm coming to the tree and shrub experts. I have a fairly shady area under tree canopy where I would like to plant some hostas and other shade plants. I'm thinking of raised beds for several reasons but have read all the warnings abt how bad it is to cover the root zone with beds or in some cases, even just a couple of inches of soil.

I am wondering if there is a percentage of the root zone that can be covered without harming the tree. Suppose I covered less than 1/4 of the root zone. I could do that by placing the bed in between 2 trees/shrubs. I would place the bed so it is no closer than 4 - 5 ft from the trunks. Specifically the 2 I am talking abt are a large red tip (photinia) and a tall tea olive (over 9 ft.).

So can any percentage of the root zone be covered safely?

Hi firstyard. I did something like this a few years ago under a pine tree. Here is a link to my thread about it. The tree is fine, I think the lightness of the fake soil mix allowed enough air to get to the tree roots. I had read in a book about shade gardening that this is usually OK. I just spent a few minutes searching DG- I recall a thread that discussed this a bit, one of our resident experts, ViburnumValley, did warn about smothering roots. Now I can't find the thread. Anyway, there are some trees that really do not want the roots to be covered. I don't know about yours, except I had a Photinia hedge that had a lot of soil heaped over a lot of the roots during a landscaping project, it was not bothered at all, grew like a weed that year.
I suspect covering just 1/4 of the root zone should be fine. It sure was easy to plant my shady bed that way.

I've tried similar thing a few times but have had trouble w/ the vigorous tree roots quickly invading the raised bed, creating the same root competition I was trying to avoid.

Pistil, yes, I remember ViburnumValley from my first stint on DG abt 10 yrs. ago! I looked for the thread you mentioned but also could not find it. Glad to hear your raised bed project worked out.

Weerobin, you bring up the other problem I have read abt and worry abt. I don't want to go to the extra work only to find my boxes invaded from below!

I think the only answer (pending other replies) is to try a small one and see what happens. If no roots invade the 1st yr., then add on the 2nd yr. and repeat.

Thanks, y'all!

A simple Dmail will get my attention - no need to bandy about references to epistles past.

Welcome back, by the way. Where've you been all this time?

Raising soil levels around established trees is usually poor practice. There are some plants that will tolerate it, and some that will make you sorry you ever tried (think Silver Maple - Acer saccharinum).

That said, it doesn't mean you can't take a crack at it in a more deliberate and thoughtful manner such as firstyard suggests in the last paragraph above - try a small one and see how things fare.

To avoid the intrusion of tree roots from around the yard, one should place a water-permeable landscape fabric on the site first, and the best method is to tack it to the sides of the raised bed walls so that roots can't slip through a seam. This is the method used for raised beds for community gardens that are built on sites where the soils are not amenable to gardening.

Now you've got my attention, I'm going to see if a little archaeology won't turn up past rants...

Hello, VV! Yes, I've been away quite a while. I have a couple of chronic illnesses that get the better of me now and then. But I'm back now, working like a dog to undo as much damage as possible from 7 yrs. of neglect. At least I know which of my plants really are tough!

I guess I'm in btw a rock and a hard place with my raised bed. It will not be "raised" very far. To keep it affordable I plan to use 2 x 8's. I will first cultivate the top 6" or so of the existing soil (which is rocky) and then lay the frame on top. So, I'll have an 8" deep frame which I will fill with 6 " of soil. The plants, of course, will need more than 6" of soil so I want them to be able to grow down into the original soil. Therefore I can't use the landscape fabric.

Maybe this is futile. Truth to tell, as far as soil, I couldn't have picked a worse place in my yard for a garden. But it's in a sight line that I really want to have landscaped.

So, I'm just going to go for it and see what happens. If it's a total failure, I can always relocate the frame and the soil in it. Then I could find flowering plants that will grow in the existing inhospitable soil. Hey, that's not a bad idea! I had my heart set on certain types of plants but I have another area they can be planted in. I will give that some thought. I may have just talked myself out of a problem!

I think you could grow many perennials quite successfully in an 8" deep raised bed in shade. You might need to irrigate more often than in-ground plants, but prepared soil should provide plenty of nutrients and growing zone.

BUT - do as you wish. There's no harm or shame in the attempt; there'll be learning earned nonetheless...

I agree with VV (imagine that), lol. I grow many plants at the base of my Acer saccharum and sometimes the depth is only 1". I imagine the plant roots root out some space, no pun intended. I also don't really have to water much more giving the fact that it's mostly shade. I suppose it all depends on the kind of plants you'd like to grow and their individual abilities for survival.

Some plants actually thrive crowded amongst tree roots. Cyclamen comes to mind. It hates being wet during summer - those thirsty tree roots keep them dry.

I constantly am adding soil mixes around my trees for perennial and shrub plantings.
The great majority of my trees are oaks.
They love it and respond with subsequent new growth.
I think the concern comes when developers pile up huge amounts of soil, usually clay, which suffocates the roots.
Seems like what I'm doing is mulching.

firstyard- are you there? How did it work out. My garden under the pine is doing fine, and the tree is fine too.

Pistil, I am still here but have not tried the raised bed yet. Things have changed now- the red tip is diseased and I am cutting it down.

So that area is in limbo right now until I can decide what to replace the photinia with. My neighbor's hardwood tree (don't know what kind) has grown so far over onto my property that I have got to select a shrub that won't get higher than 6 ft. (Don't want to get into pruning her tree back. Already have to do that with one of her other trees.)

I still want to try the bed though. I even have the wood. Hoping to get my other neighbor to build a couple of beds for me to try next spring on that side of the yard but they will be in sun and not close to a tree.

Glad to hear your garden and tree are doing fine!

Oh I had to get rid of a Photinia, it had some icky disease with red spots on the leaves, also it was clearly intending to turn into a huge forest tree! I had to prune it yearly to keep it a hedge (would cut off 6-8 feet of growth, then it looked bizarre- only the new growth was red so it looked really stupid, pretty much all the time.
Cutting it down was a chore, that wood was like iron. However, it made good firewood, slow burning like oak.
Good luck with your decisions, keep us posted next year.

quercusnut, I have 2 oak trees on the other side of my backyard. Under the first one, I have a little over a dozen pots of hostas and hydrangeas and 1 fern.

I also have a hosta plantaginea planted in the ground - it's probably been there 10 yrs. This Spring I took a division and potted it. What a surprise! For several months, the leaves have been at least twice as large as the ones in the ground. So I am going to dig up the one in the ground, put in a small (but not too small!) raised bed right where it was planted, and replant the dug up plantaginea in the raised bed. If things go well I'll try another one a little larger. Hope to have the success you have had!

Good luck to you. I've had lots of hits and misses with raised bed planting. Even now I am still trying to find the 'perfect' soil mix.

This message was edited Sep 15, 2016 10:24 PM

quercusnut, how tall are your beds and what do you include in your soil mix (even if it is imperfect).?

My 'beds' are very improvised and fluid. I'd say the average is 6 to 8 inches. No particular soil recipe-just depends on the plant. Loam, peat moss, soil conditoner, well-rotted hardwood mulch, etc. in all kinds of combinations and ratios. Still experimenting and probably always will be. I certainly haven't mastered it yet.