Fighting off tree roots

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When establishing our raised garden beds, I had thought this would proof us against issues in the soil, including the impact of neighbouring trees. How wrong we were. The camphor laurel has proven to be very vigorous, and when I turned over the soil in the bottom two beds, I found feeder roots coming up through the bottom of the bed. No wonder the plants weren’t growing properly!

No choice but to take drastic action (short of cutting down the noxious tree itself).

Digging out the soil from the garden bed (1.5 tonnes in total)

The starting point was to fully dig out the garden bed, which was no small task, as there’s 1.5 tonnes of soil by a previous calculation.

A layer of bark mulch to create a low nitrogen zone

Having dug the bed down the level of the original gravel, I laid down a layer of bark mulch. This is something that I saw on Gardening Australia, where the nitrogen draw-down of the mulch rotting creates a low nitrogen layer. This should help to discourage or deflect the tree roots. (This is the experimental bit, I hope it works!)

Multiple layers of weed mat goes in

Multiple overlapping sheets of weed mat should then form an impervious barrier to the roots, at least to some degree.

Old carpet to protect the weed mat

With the gardening in mind, I’d scavenged some old carpet from the side of the road. This was laid in to protect the weed mat against garden tools, and to provide an additional layer of proofing against roots. (And yes, I’m aware of the debate about whether carpet should be used or not for these purposes; in the end I decided that more was better than less when it came to roots.)

The finished bed, ready for planting

Getting 1.5 tonnes of soil back into the bed was my final workout. Having read that roots breaking down release toxins, I carefully removed the camphor laurel roots when refilling the bed, which slowed the whole process down.

Hopefully that’s all fixed now, and I can get back to growing vegetables!


14 thoughts on “Fighting off tree roots

    Carolyn Graham said:
    June 27, 2011 at 11:02 pm

    Hi James, I have exactly the same problem. I had to remove two raised bed veggie gardens, each one 10 metres long by 1.5 metres wide, two sleepers high, filled with the best soil I could make over three years. Everything started off great, then the cocos palms next door found them, and took over. I’ve eventually had the whole thing removed which required a bobcat to come in and remove all the soil choked with roots. It’s been a painful and expensive exercise, but I’ve now bought two large beds, like the ones in your pictures and am pondering whether or not weed mat will work if I use several layers like you have done? How are yours going so far? I am reluctant to go ahead and fill them up without being sure of what I’m doing in case I have the same problem all over again. What happens when the bark mulch breaks down eventually? Presumably the low nitrogen situation is then lost and the tree roots are no longer deterred? Carpet does seem like a good idea though, as well as weed mat. I would appreciate any comments you have.

      James responded:
      June 28, 2011 at 1:54 pm

      The obvious problem is that the garden beds are high in nitrogen by design: it’s great for the veges! So far so good, but how long it lasts, hard to tell.

      For my beds, which contain about 1.5 tonnes of soil, it took about 2 hours per bed to completely fix them up. If I have to do that again in 2-3 years, then I’ll be sad, but not heartbroken.

      But between you and me … I’ve got plans to cut down the offending camphor laurel before then (it’s a listed weed) 😉

    Carolyn Graham said:
    June 28, 2011 at 3:34 pm

    Well yes, if you can get away with cutting down the camphor laurel, and won’t upset anyone too much in the process, then go for it! Unfortunately, I can’t cut down the three cocos palms that are the cause of my problem, as they are not on my property. They are also a weed, but not listed as noxious, so I have little chance of convincing the owner of the next door property (who lives in Melbourne and is apparently not interested in doing anything to the property for the tenants anyway) to cut them down. I’ve thought of all sorts of possibiities, even poisoning them (!) but at the end of the day I can’t do that for ethical reasons. I will just have to go with the pine bark, carpet and weedmat suggestion of yours which, incidently, is the best solution I have found yet. So thank you.

      James responded:
      June 28, 2011 at 4:23 pm

      Well the other approach, if you’re not afraid to get in a mini-digger …

      You could dig in a metre deep worth of root barrier along the side of your property. That should seriously slow up the palms…

    Carolyn Graham said:
    June 28, 2011 at 5:59 pm

    I already tried digging a trench about a metre deep 18 months ago, and then filled it with crusher dust. But the roots went deeper! I got hold of some really cheap pavers today, 400 x 400. They were seconds. I’m going to put down weed mat first, two layers, then crusher dust, then the pavers, then two more layers of weed mat, then the pine bark mulch, then more weed mat, then hay, then soil. I think that should do the trick. If not, then I’ll just have to re-do it in a couple of years, as you said yourself….. unless someone else comes up with a better idea in the meantime! But if they still come through that, then I’m obviously not meant to grow veggies in my back garden, and there will be another solution somehow. There’s always a way, just got to get creative in this situation. Thanks for sharing your insights.

    Winnie Chai said:
    October 12, 2012 at 8:20 pm

    I spent the past week digging a trench about 1 foot deep next to our back fence. Our neighbour has a native garden with half a dozen huge gum trees. We have been here for 20 years and experience the same problem of the tree roots getting into the vegetable beds. They suck up all the moisture and the soil does not seem to get wet at all even after copious watering. As a result, our back yard is like a clay desert. In the last two days, I have dug up about 10 roots all around 10-15cm diameter and was quite happy with myself. However, I am sadden to read what Carolyn wrote, ‘1 metre deep trench; and the neighbouring roots gets deeper. I am quite discouraged. I have been growing lovely beetroots, sugarsnap and rockets in 3 new beds we built in April and I wanted get a few more beds made for me. Oh dear!

    Carolyn said:
    October 14, 2012 at 9:34 pm

    The update on my new beds, that look exactly like James’ beds in the photos above, is….. I laid geotech fabric down (two layers), then 4 layers of weed mat, then more geotech fabric, then 4 more layers of weed matt, then filled the beds with soil. I spent 6 months building the soil (all over again), and finally they were rich with worms and everything was growing a treat. I thought that the cocos palm roots were now deterred. How wrong I was! I dug down, just out of curiosity, and there they were again!! I was devastated. I had to dismantle the beds, with the help of a very strong able-bodied young man to help me, and remove all the soil and I ended up storing it all in a huge silver tarp, which we wrapped up and put bricks on top of for the time being, so as to save the soil and the worms. Since then I have slowly accumulated 25 large terracotta pots and filled them all with the saved soil, and mushroom compost and started again!!! This all happened a few months ago. Now I have the very best greens and herbs growing I have ever seen. The pots are standing up on brick pavers, to raise them slightly off the ground. They are well mulched. Only thing is I have to water them daily as they dry out in the warm weather fairly quickly. But the results are amazing. I have finally triumphed over the cocos palm roots, as vast expense (!!), but it’s worked and at this point, I now have 30 pots, growing rainbow chard, kale, spinach, rocket, parsley, borage, sugar snap peas, bush beans, land cress, herb robert, vietnamese mint, mint, jalapeno chillis, and some self-seeded cherry tomatoes from my compost! It’s all going gangbusters and I’m happy again. But what a journey. The messsage in all this is that trenches don’t work, unless they are very very deep (at least 1 metre, and even then I wouldn’t trust it), geotech fabric and weed matt don’t work, the only thing that works is getting the beds up off the ground. It’s been a very frustrating journey, but now a rewarding one. Hope this helps you a bit, Winnie. It does also depend on what trees you are dealing with. I understand cocos palms are extremely persistent and hungry feeders.

      James responded:
      November 9, 2012 at 10:53 am

      Yes, I fear my efforts have also failed!

      I have yet to fully dig over the garden beds, but some light work in the top layer of one bed revealed a tree root just mm below the surface 😦

      So I think the plan now has to be:

      • pull out the current raised garden beds
      • purchase beds the same size, but with a solid corrugated iron base
      • establish new garden beds as “wicking no-dig beds”
      • put back in all the soil, and refresh with a heap of compost

      No small job! I’m going to wait until the end of summer, and do the change-over in preparation for the Autumn/Winter crop.

      So the lesson learned: never underestimate the power of tree roots…

    Carolyn said:
    November 9, 2012 at 11:05 am

    Yes, James, it’s certainly sad to make this discovery, just when you thought everything was going so well. You may need to just get the beds off the ground somehow. Goodluck! Even if you have a corrugated iron bottom to the beds, you still need holes for drainage and then you have tree roots again. I thought about this option too, and finally came up with the pots. A young guy around here who builds permaculture gardens for people locally, did offer to build me some timber/hardwood beds up off the ground. They only need to be about 6 inches off the ground, but it all sounded too expensive at the time. But, in fact, I have probably spent as much on pots and setting all them up as I would have done on a genuine raised bed (off the ground). However, the end result is that it’s now working well. I hope you can find a solution that suits you James.

      James responded:
      November 9, 2012 at 11:12 am

      Actually, in a “wicking bed”, there’s no need to have drainage holes in the bottom. Instead, there’s an overflow pipe some way up the side of the bed. So that should leave no holes for those pesky roots!

      I’ll report back once I’ve done some more planning and investigation…

        Carolyn said:
        November 9, 2012 at 11:15 am

        Sounds like a great idea. I’m interested in finding out how you get on. …. will await updates!

    Gina said:
    November 17, 2012 at 5:23 pm

    So great to read this dialogue. We have two tall (approx 1m high) raised beds, one round (diameter 1200mm) the other larger clover shape, set up in the vicinity of two Cape Lilacs and a Mulberry tree. Gardens went well for first couple of use and now, you guessed it – they are full of roots. I have the circle bed dug out and have been researching what to do.

    I have looked into all the barrier options and decided that all they buy you is a bit of time, but eventually the roots will prevail – that is after all their job! We love our trees in the hot summer and the leaf fall, not to mention the magnificent mulberries (particularly over the last few years!!) I have been thinking ways of solving two problems at once by installing an underground rain water tank under the raised bed to be a root barrier and provide water storage. However this is looking too costly and disruptive for our little urban garden.

    My thought of the day(after reading the above) is why not turn the round raised bed into a BIG pot plant, using a round ready – made concrete tank lid as the base and sit it up on a layer of bricks or bessa blocks.

    Alternatively we could just make a cement base to sit it on. However I am guessing the roots would eventually get through the concrete. My preference is to not have to dig out the raised beds again!

    I am also experimenting with wicking beds and may consider going that route also.

    […] April 2011, I wrote about our efforts to fight off tree roots that were invading our garden beds. We took the job pretty seriously I think, digging up the beds, […]

    missylulu said:
    July 26, 2014 at 7:13 pm

    I had a similar problem with palm tree roots invading my raised garden bed from below and matting so thickly in the soil that nothing else would grow.

    So, I removed the raised garden bed and the dirt to about 1″ or 2.5cm below ground level. Levelled the area off with large wood chips then placed a heat treated (not pesticide treated) pallet on top. Staple gunned shade cloth (or you could use any other porous ag material you want not weed mat as it doesn’t seem to drain very well) tightly over the top of the pallet then placed the raised garden bed on top and refilled with soil, compost, manure etc. It is now a very productive vege garden and has no roots growing up from below. The pallet was free and the shade cloth a few dollars.

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