Ask a Question forum: Keeping roots out of raised beds

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Manitoulin Island, Ontario
chapais
Oct 18, 2016 2:57 PM CST
I have several raised beds, each about 12' x 5' x 12'' deep. Sides are 2" x 6" cedar. Some beds have a tight plank bottom and some have a poly tarp bottom with drainage holes and anchored to the wooden sides. The beds sit on clayey rubble with cracked limestone 6" to 10" below. Big roots find their way around the outside perimeter of the beds not far below the surface. Smaller roots then find their way in by forcing themselves through the plank joints. They are not big but grow extensively in the soil. I am thinking that if I dig a small trench 4" x 4" around the outside of the cedars sides, fill it with salt pellets and cover these with soil then the roots will not attempt to pass though the salt saturated ground down to the limestone. Do people think this will work and will the salt find it's way up inside the beds and kill all growth?
Name: Barbalee
Amarillo, TX (Zone 7a)
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Barbalee
Oct 18, 2016 3:00 PM CST
Welcome! chapais! I don't have an answer, but that's a great question! Somebody who knows more than I do will undoubtedly come along soon!
Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
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RickCorey
Oct 18, 2016 3:56 PM CST
Hi chapais! Welcome to NGA.

I'm very, very "down" on adding salt, even unintentionally via "salty" manure. So see if others speak out against it before taking my advice to heart.

I would never intentionally add salt anywhere near a garden, or even a lawn, or anywhere upstream from anywhere that anyone would ever want to grow plants.

Salt is very soluble, so first it would wash out of your trench, then kill the surrounding lawn, perhaps even damage the tree, then damage whatever soil is downstream (based on the flow direction of ground water.)

It would eventually flush out of your neighborhood and go "salt up" someone else's yard.

There are entire regions where agriculture is more difficult or no longer practical because of salinized soil. Maybe it is only a BIG problem where there isn't enough rainfall to flush all the salts OUT and into a river or ocean.

1.
I would suggest some thick plastic, like 4 mil plastic, to line the trench, but I think I read that bamboo roots can pierce that. Maybe trees can, too.

2.
>> Big roots find their way around the outside perimeter of the beds not far below the surface. Smaller roots then find their way in by forcing themselves through the plank joints.

Maybe buying a tube of construction adhesive and using it like grout between the planks would keep them out?

3.
It is possible (but I'm not sure this works with trees) to make a trench unattractive to roots by filling it with coarse gravel. Water drains out instantly, leaving no water behind for roots to follow. In containers, this would be like "air-pruning plant roots". But does that work with tree roots in gravel? I don't know.

4.
I've read that some roots avoid copper, so painting the outsides of the walls with some paint supplemented with copper might work. Or copper foil glued over the joints.

5.
The "guaranteed" solution is one proposed by some bamboo fancier who had the gall to call this "easy": "just" dig a moat and fill the moat with concrete. In your case, I guess the "moat" would have to extend under the entire bed or else go so deep that tree roots would never turn up after diving under it.

Since unused fertilizer and organics will be leaching down out of your bed, I would expect tree roots to do somersaults in order to follow them back to their source and steal all of them!

6.
There ought to be an intermediate solution, like deepening your trench a little and then lining the side of the trench that is up against the bed with something solid enough that tree roots can't penetrate it, but affordable. ("Affordable" might mean "see what's available at a Habitat for Humanity Restore", or scrounge-able elsewhere.) Very heavy plastic? Corrugated fiberglas? Phoney wood?

7.
If you don't care about the tree at all, you MIGHT try relying on an annual "haircut" on the tree roots. Keep your existing trench filled with something loose like gravel, but once per year, dig down deeper around the moat and CUT any tree roots that have crossed your Line Of Death.

Or cut a second trench between the bed and the tree, farther away from the bed, so that you might only need to cut the tree roots deeply every other year.

That will annoy the tree every year and might be bad or very bad for the tree, but how many feet can a tree root grow in just a few months?

(I wonder whether it would work better cutting roots in spring, or in fall?


I'm just guessing at solutions, so don't hesitate to say "THAT won't work!"

I tried Googling "tree root barrier" and it was suggesting multiple alternatives even before I got "barrier" fully typed. There are so many different products available that my guess is that NONE of them are both effective and affordable. If any one of them were both cheap and effective, there wouldn't be so many options about "depth" and "cost" and "systems". And there would be no "patented mechanical guides" or "A Review of Root Barrier Research". And there wouldn't be a Google suggested search phrase "do root barriers work?"

But looking at the products offered may give you ideas suited to your situation, or reasons why some approaches are seldom used.

I hope someone suggests more attractive solutions!

Eventually someone is bound to suggest the method that DOES work: "move the bed away from the tree at all costs", but I figure that, if that were an attractive option, you would already have done it to get away from both roots and shade.

When tree roots invaded my compost heap, I moved the heap. Or, rather, I abandoned the old heap since it would have been easier to get a bone away from a pit bull than to recover any soil from those tree roots.

Name: Sandy B.
Ford River, Michigan UP (Zone 4b)
(Zone 4b-maybe 5a)
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Weedwhacker
Oct 18, 2016 4:17 PM CST
Welcome to NGA, @Chapais !

I'm afraid I don't have a solution to offer either, but I agree with Rick about not using the salt. I think the only sure way to have raised beds with no tree roots invading them is to locate the beds far away from any trees. Or have raised beds that are literally raised up above the ground, like big planters.
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Manitoulin Island, Ontario
chapais
Oct 24, 2016 2:47 PM CST
Many thanks for the replies that you have given me. I have to agree about the problems salt will cause. I had only done this for a short distance and have now removed it. I do not think there is an easy way to prevent the roots. I am sure that any barrier will not work too well as the roots just go underneath. My solution is to dig my trench around the perimeter about 4" x 4" and cut out every root I find and rip it back as far as I can. Then fill the trench with good soil. The roots will no doubt grow back in time but the good soil will be very easy to just hoe it out, cut the roots again and replace the soil. The effort is all in the initial work. The follow up will be easy!
Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
I helped beta test the first seed swap Plant and/or Seed Trader Seed Starter Region: Pacific Northwest Photo Contest Winner: 2014 Vegetable Grower
Avid Green Pages Reviewer Garden Ideas: Master Level Garden Sages I was one of the first 300 contributors to the plant database! I helped plan and beta test the plant database. Charter ATP Member
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RickCorey
Oct 24, 2016 4:02 PM CST
That sounds like a plan. Good luck!
Name: Rick Moses
Derwood, MD (Zone 7b)
Region: Mid-Atlantic Region: Maryland Hostas Ferns
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RickM
Oct 24, 2016 4:43 PM CST
Make it 3 for no salt.

Trees are very persistent critters. We have a lot, ok, mostly clay and rock. Anytime I put in a new bed, I just resign myself to the fact that the trees are going to find it. I've had roots come to the top of a 18" deep bed because it was rich soil.

That said, if you have a way of sealing the bottom of your beds, go ahead and do that. Once that is done, you will need to address the drainage issue. Drill some strategically placed holes about an inch above the grass line. You can run a piece of PVC pipe through it, or even a piece of old hose. Either will work to keep the hole open. Don't make the hole too big, or you'll have 4-legged critters setting up housekeeping.

The next time you're out and about, if you see a brick or block retaining wall, look along the bottom. You will most likely see a number of these drainage or 'weep' holes. For retaining walls, they're put in to relieve the pressure from excess water build up. In your case, you want any excess water to be able to drain out.

BTW, for 'running' varieties of bamboo, as opposed to clumping, I believe you have to put a non-permeable barrier down at least 6 feet to keep the bamboo in check. Sorry, but that's just too much work. Then again, tearing out our bamboo patch was no easy feat either. It took me a good 10 years one summer to get it all out!
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Name: Greene
Savannah, GA (Sunset 28) (Zone 8b)
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greene
Oct 24, 2016 5:26 PM CST
I read through this thread and do not see anywhere that the offensive plant has been identified. Would it help if we knew what kind of plant or tree is producing these roots? Are these roots from a tree? if so, what kind? Are they from shrubs/weeds/perennials? Maybe knowing this we could offer suggestions to contain the problem closer to the source - before the roots reach the garden bed.
Sunset Zone 28, AHS Heat Zone 9, USDA zone 8b~~"Leaf of Faith"
Name: Kat
Magnolia, Tx (Zone 8b)
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kittriana
Oct 24, 2016 7:20 PM CST
Raised beds for any plants is work. The trees will always be attracted to the nutrients and water. Many trees like oak shed tannins that restrict younger growths thereby protecting the parent tree. Can't say if that would work for raised beds, but I find I need to renew soils and nutrients every so many years in my raised beds anyway. SOME trees have compartmentalized root systems- if you chop this root, these branches die. There are trees like salix that can find water and send a root for miles to access that water.
If your raised beds are perhaps 3' plus high this may be enough to keep roots from finding your beds as the water won't seep all the way to the ground. Drawback to this raised bed is drainage in high rainfall areas I would think. Barriers at any depth can be a time limited endeavor.
No salt. Salt would be a Trojan Horse. Whistling
kitt

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