Extra long planters for bamboo

twopointfour1980

Good morning 

I'd like to build a long planter to house bamboo as a screen (as per image below) . The total length I'd like is around 7.2m but when drawing up plans for construction I'm unsure as to how to go about end to end joining lengths of timber together whilst keeping the outside looking as clean as possible and still giving strength to the box. The overall dimensions would be about 7200mm wide, by 500mm deep by 500mm high. 
I was planning on using pressure treated 47mmx150mm planks or 22mmx150mm so three planks high by 3 x 2.4 metre lengths. I was worried that perhaps the 22mm thick wood would be too flimsy but would like to preserve space if i can by using a more slender option. This would be a long term fixture. 

Apologies the quality isn't better on the image below. There are no external supports so I'm guessing the corners have posts inside to help support and I can see (on a better quality image that lengths of wood are joined together along the main external face but can't see any screw holes externally. There are dividers in the box every metre or so but don't appear to join up to where the planks meet. 

I was thinking of using internal cuts of 6x2 to brace the wood internally and joining the ends by screwing from the outside. I'm not against external screws if I have to but would prefer not. 

Other thoughts would be to use a dowel jig for dowels and then also wood glue to join the wood and pocket holes but unsure of strength over the long term. 
I'm also thinking of how to secure the underside (which I would like to sit on gravel) to keep screws obscure. Thinking pocket holes again from inside to out 

Any suggestions or advice would be much appreciated. Thank you

  

mi-key

You could either use dowels or biscuits to join the planks end to end with glue. Glueing end grain isn't very secure though as the glue doesn't penetrate into the wood that well. 

I would also get some metal brackets ( something like an 8" long piece of 5mm thick metal with 2 or 3 screw holes on each side of the joint ) to screw on the back to hold them securely ( these will be hidden on the inside anyway and will add a lot of extra strength and resist the wood opening up at the joints from moisture or warping ). Use two or three on each joint, and it should hold them securely

mi-key

As for the base, if you aren't planning on having to move them, dont fit a base. The soil will be fine on the gravel, and it will also provide good drainage. The problem with a base is that it will rot anyway from the retained water, even with drainage holes in it

twopointfour1980

Thanks mi-key - appreciate the response.

Do you think a tabled lap joint or even half lap would add to the strength? Appreciate it might be more work (I don't have a table saw so would be a circular saw then chisel or router) but could add to longevity? I have a circular saw and router but not a dowel or biscuit tool so would be good to save where i can

As I'm planting bamboo I was worried about it spreading under the planters but guess i could membrane down at the bottom with drainage or just take a chance! We have it already and it does seem to clump more than spread.

If no base, how would you sit the sides on the gravel? Thinking i could fix some plastic on the underside and then insides to keep the damp off if i bury them by a couple of cm in to the gravel to not leave a gap

twopenny

There are types of bamboo that don't spread

Go for a clump forming one.

FreeBear

twopenny said:

There are types of bamboo that don't spread

Go for a clump forming one.

Even the clump forming ones spread. But the stuff is shallow rooted and the rhizomes don't generally grow downwards to escape a container.

With a planter ~7.2m long, you are going to need stakes at roughly 1m intervals to stop the sides from bowing out. I'd suggest using a recycled plastic fence post rather than timber for the stakes. Maybe consider using stainless steel coach bolts to hold everything together.

TELLIT01

22mm thick timber will severely restrict what can be used as fastenings both between sections of timber and to support the height.  I would suggest a minimum of 50mm so any screws have plenty of purchase into the timber.  Plates similar to these https://www.builderdepot.co.uk/building-roofing/builders-metalwork/timber-connectors/br_metalwork_type--jointing-plates can be used to join the timbers both vertically and horizontally but you will still need something like angle irons every metre or so to keep the walls vertical. 

twopointfour1980

Thanks TELLIT01. 
Not sure how i see the angle irons working to support the walls? Are you saying these should be fixed to the stakes?

Also, would you recommend any particular wood for this? I've been looking at pressure treated 6x2 marked up as C24. I'd like it to look nice, similar to the one in the picture colour wise so should I be going down a redwood route or just sand and stain any old 6x2? 

TELLIT01

Sorry I didn't explain clearly.  The angle irons would be hammered into the ground inside the planters and screwed to the timber sides of the planter.  Any pressure treated timber should do the job, just make sure you line it thoroughly with heavy duty plastic sheeting before filling with soil.  It won't stop the timber rotting completely, but will certainly slow it down.

mi-key

twopointfour1980 said:

Thanks mi-key - appreciate the response.

Do you think a tabled lap joint or even half lap would add to the strength? Appreciate it might be more work (I don't have a table saw so would be a circular saw then chisel or router) but could add to longevity? I have a circular saw and router but not a dowel or biscuit tool so would be good to save where i can

As I'm planting bamboo I was worried about it spreading under the planters but guess i could membrane down at the bottom with drainage or just take a chance! We have it already and it does seem to clump more than spread.

If no base, how would you sit the sides on the gravel? Thinking i could fix some plastic on the underside and then insides to keep the damp off if i bury them by a couple of cm in to the gravel to not leave a gap

A lap joint probably wouldn't help much. Any strain on the joint is going to be coming from the soil, so from the inside out. Removing wood for a lap joint would make it weaker in that area. You are better off with as much thickness of wood as you can get, then bracket fixings on the back.

Membrane would be ok , but I would also make holes in it for extra drainage. Plastic sheet would work well, you can staple it to the bottom and wrap it around the inside for a few inches and then staple there as well

twopointfour1980

Thanks everyone. So I've decided to make two 3.6m metre frames from 6x2 treated wood with ends on each and then join them together so there is a bit more strength. I've also ordered some more wood the same size which I could cut to the same size as the ends and bridge and fix across at intervals which i thought would add some strength? 
Is this a false economy - i.e does it put stress on the overall frame and would i still be better to put in angle irons to then fix as well as or instead?