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Every year I remove and clean out the raised garden beds and add to the soil. This is the second year using the raised beds. I've noticed that when I mix in new soil and hit the soil, mostly clay, that the beds are sitting on there are tons off roots coming from the clay and spreading out within the raised bed soil. Is there a fabric of some kind that I can lay between the clay and the soil to prevent the invasive roots from getting into the bed? Also is there an alternative way such as placing stone, or some other type of organic base to prevent the roots?

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If tree roots are getting in your bed every year then you could do one or both of these:

  • every spring trench around the bed and cut any roots that have grown towards it. I suggest that either trees are close or water is in short supply if roots are that aggressive
  • food grade pond liner is environmentally inert, lasts for 25 years and roots cannot penetrate it. Although not cheap it is a one time purchase. You must ensure that water can drain out from the raised bed. Something like:

    • taking the bed down to clay
    • laying landscape fabric
    • lay crushed 5/8" gravel which is higher in the middle
    • pond liner on top.
    • top with soil
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If your raised beds are made with lumber or concrete and if you install a bottom you have created a POT.

Normally roots do not grow up from the ground unless the tree isn't getting enough water. How high are your beds? What is supporting these beds? Wood, concrete? Using the garden soil can work for awhile but as you've noticed the clay stays and compacts with little organic matter. The clay is inhibiting drainage and kind of becomes a structural bottom making the raised bed literally a pot.

The critical zone for all plant roots is the top 4 to 6" of topsoil. That is where the roots are able to get air and water and sometimes chemistry (not nutrients) if that has been added by a human. Something like 95% of all plant roots exist in the top 4 to 6" of the soil.

Is this bed beneath the canopy of this tree? Feeder roots normally are a mirror image of the tree's canopy.

It is easier and wiser to move the raised bed than it is to replace a mature tree. Do you have any other space for your raised bed than under the tree? This must be on the south side of the tree to get enough light, right?

Kevinsky, you correctly hit on the fact roots don't normally grow upwards unless that tree is desperate to find water. Watering the tree would make sense. I've been able to control Quaking Aspen roots just by watering the Aspens religiously.

Planting anything under trees is not a good idea except for a few hostas or pachysandra?

Kevinsky, roots are able to go through concrete and asphalt. Pond liner surely? Grins! I've uninstalled pond liner because roots completely riddled that stuff. I never use just one layer of pond fabric always 2. I don't even think two layers would have stopped the roots of the willow above the pond.

A raised bed above the roots will kill the roots beneath; no air no water unless there is water the tree can get at by growing up into the raised bed to get at water and air. One raised bed should not be a big deal, right?

Yes, landscape fabric would work in this situation. I take back my first statement. With one caveat: ONLY if you use potting soil not garden soil above the fabric.

That fabric slows the drainage and causes a barrier between the flora and fauna of the larger body of the garden. With that fabric installed, especially pond liner that raised bed becomes a pot. Literally a pot. And the hard and fast rule unless you 'make' your own soil and STERILIZE it in the oven, good old potting soil is the only medium to use in pots or 'planters'.

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    Landscape fabric or geotextile is also used for large retaining walls and as an underlay for ponds. Also consider that this is in a raised bed. Some of your suggestions may not be appropriate for a raised bed. – kevinsky Jun 30 '16 at 16:31
  • Kevin, I never put plants in anything other than raised beds. That is a duh howdy no matter what type of soil. You are correct, that stuff is great for even perforated pipe so it can be used without getting clogged up. For ponds certainly beneath the pond fabric liner (s...never ever use just one)...but NEVER to be used for weeds. Now roots getting into the garden is just not a big deal enough to blanket the soils with fabric. Roots just are not that big of a deal! My raised beds (always ending up a foot off the ground with sloped sides I use for plants that appreciate slopes...) – stormy Jun 30 '16 at 22:30
  • ...are from double digging and always a trench at the base of the bed to direct excess water. My raised beds are never done with boards, or rock, or CMU...and I've done this with blue clay as well as sand and pumice. One thing the OP could do is water his trees so they won't be so needy to go searching for watered raised beds? Or septic, water lines. Yes? – stormy Jun 30 '16 at 22:34
  • Be aware that if you cut the root of a large tree that is leaning away from the root, you are damaging part of the system that is holding the tree upright. My neighbor's got a 60-70 foot silver maple leaning towards his house. I don't mess with its roots much, besides, this winter it started blowing off bark up to the crotch of a monstrous limb leaning over his house. Looks like a crack is forming. When that limb falls, I want no part in any lawsuit, even if his roots on my land are sometimes annoying. The tree should have been removed 15 years ago. But people round here do not know trees. – Wayfaring Stranger May 2 at 3:41

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