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The decking between my property and my neighbours is divided by a very large concrete planter box. It is very roughly 4x1x1 metres (13x3x3 feet) internally. Over the top of the planter, down the middle is a wooden fence dividing our properties. So each of our gardens shares the same soil.

Due to waterproofing issues, the existing soil is currently being removed to repair the concrete, giving me an opportunity to improve the set up.

A knowledgeable friend has warned me that sharing the soil in the planter with the neighbours is a recipe for trouble - if we have different plants with different needs for water, fertiliser, etc., it will be difficult to maintain. There may also be difficulties preventing weeds and enthusiastic plantings from taking over the other side's garden.

She recommends I put some sort of divider between the two halves so we can maintain our own soils separately.

My question is in two parts:

  • Is this good advice?
  • What sort of product (thin plastic lining? thicker plastic? wooden sheets? weed mats?) should I be looking for to put in the planter box before it is filled with new soil?

I tried asking at nursery, but the assistant looked confused about the request and had no suggestions.

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  • What kind of plants do you actually want to put in the planter? What kind of plants does your neighbor want to put in? Do you have any problems with whatever is in the planter right now (besides the waterproofing)? How much time/energy are you and your neighbor planning to spend on tasks like weeding and fertilizing? – user3067860 May 12 at 17:22
  • @user3067860 (It is actually two sets of neighbours, but I simplified.) One loves succulents. The other I approached to find out what plants they had because they seemed to be thriving, and they said they hated them, couldn't wait to get rid of them, and I don't know. My long-term plans have been discarded, but included doryanthes, bamboo that I was assured would not spread, probably some Australian native shrubs, and if I am feeling very ambitious an espalier citrus in a couple of years. – Oddthinking May 13 at 3:26
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I think this is good advice, and your friend has given you good reasons as to why sharing the soil without some sort of barrier could be a problem. One thing to consider, though is drainage: would a barrier restrict any drainage holes in the bottom of the planter?

I use aluminum flashing (used to prevent water from infiltrating around chimneys and occasionally to prevent leaks in valleys on roofs) between my yard and all four of my neighbors' yards. Why? Because lawn grass is a very difficult weed to control in a border here, and a couple of neighbors like to see incredibly invasive weeds in their yards (they kill the grass, which means less lawn to mow. Yes, they are that lazy). This is helped by my own fences between our properties, as this defines the borders and allows for the flashing to be place directly up against the lower half-inch/cm of the fence.

This setup could work for you if you are able to attach the aluminum directly to your side of the fence, so - do you own the fence that's between the properties? If you do own the fence, then get flashing large enough to reach the bottom of the planter. Note that flashing has sharp edges, so wear gloves when working with it. Once in place, the top edge will be against the fence and the bottom against the planter, so there's no danger of cuts.

If you don't own the fence and therefore cannot attach the flashing to it, I'd suggest a "floating" barrier such as cedar planks or, if available, "lumber" made of recycled plastic.

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  • Thank you; I'll look into aluminum flashing. Your question of who owns the fence is an interesting one, but the complexity of Australian strata law isn't on topic here. In any case, I am on good terms with my neighbours and any action I take here will only be with their full consent and cooperation. – Oddthinking May 12 at 13:37
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It's not terrible advice, but also maybe a bit of over-engineering something that's more of a social problem.

You are unlikely to have significantly different soil needs since the plants will all have similar temperature, humidity, light, etc. (One side may get more light than the other, but still.) Also, the types of low-maintenance plants that people tend to put in such planters generally do well on "all purpose" soil and watering. You would probably already know if you were planning to have plants with more exotic requirements. (And if your neighbor is planning to do that, then probably they should put the effort in for the barrier.)

A barrier might protect against minor plant incursions, but plants are very, very good at spreading (it's kind of their thing). Any plant that spreads by runners will grow right through the fence (bypassing your barrier). Any plant that spreads by seeds will spread on air or by animals (bypassing your barrier). Even rhizomes which are supposedly below ground will grow up and through the fence (bypassing your barrier). It's better to just have a talk with your neighbor about what type of plants you envision in your shared planter and agree to avoid invasive or destructive plants and to speak up if some plant starts turning invasive/destructive.

A barrier might be your only option if you can't agree with your neighbor or if you anticipate the house changing hands and will have unknown new neighbors, but it's a poor substitute for picking out good plants and spending the time maintaining them.

(Unless both parties agree that you want a really low maintenance planter, then you could pick one hardy type of creeping/climbing plant and just cover the whole planter with it and call it a day.)

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  • I've been successful in using a barrier to keep such invasives as Chinese Lanterns (Physalis), Bishop's Weed (Aegopodium), Creeping Charlie, all lawn grass, and Vinca minor out of my property - you over-estimate the ability of plants to "climb" through a fence. – Jurp May 12 at 22:53
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I have used EPDM pool liner with some success. It can be bought in whatever quantity off the roll at pond supply stores. It is UV stable, flexible and tough.

Used in the ground it stopped creeping bellflower that has shallow roots but did not stop horsetails (equisetum) that seem to send roots out a deeper level.

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