Raised Garden Bed longevity

[AlenG]

Hi everyone,

I’m planning to build a raised garden bed on legs for next spring. Dimensions of the bed are 4’ by 8’ and either 6” or 12” depth. Have not yet decided on depth but the plan is to do SFG (square foot gardening).

My question is does anyone know or have tried a method of using untreated pine for the bed and spraying it with borax/water mixture then sealing with exterior paint to increase longevity instead of using pressure treated wood. Any advice would be appreciated, untreated will obviously be far less expensive but I don’t know how effective the borax/water mixture will be vs PT.

 

[Marck]

So this raised bed will be more like a shallow table top sized container on legs. That may work well for some plants though others might prefer more root room. In any case, it will dry out faster than a full raised bed connected to the ground. What types of plants are planning to grow? Do you have a photo or drawing of what you are intending? Are you building high to avoid bending over?

I doubt borax and water will add that much protection to otherwise untreated pine.

Some woods are much more rot-resistant than pine. Redwood, for instance, but I don't know how available that is in your area.

Also consider that pressure-treated wood does release toxic compounds, so you might not want to grow edibles in such material.

You might be able to construct something out of galvanized sheet metal or plastic. If the planting area is only going to be 6 to 12 inches deep, maybe you could use something like a metal patio table and build or find a large container to set on top of it. things like plastic strage boxes ans livestock feed tanks could possibly work.

 

[Mick Emery]

I am just now replacing my original untreated (1" thick bought from local sawmill) hemlock beds that were on the ground.
They worked for 9 years.
I'm replacing with 2" pine from the same sawmill that I treat sho shugi ban style.
I've also used a product from Canada called Tall Earth.

 

[Meadowlark]

Not all Pines are equal and certainly not all climates are the same...but Pine in my climate used like that would be good for one season, two at the most with or without the Borax.

 

[AlenG]

I am just now replacing my original untreated (1" thick bought from local sawmill) hemlock beds that were on the ground.
They worked for 9 years.
I'm replacing with 2" pine from the same sawmill that I treat sho shugi ban style.
I've also used a product from Canada called Tall Earth

Thank you Mick, this is great information. I looked into both Sho Shugi Ban and Tall Earth - both look like interesting options. Any experience with applying either technique/product to untreated wood outdoors and see how long it lasted?

 

[AlenG]

So this raised bed will be more like a shallow table top sized container on legs. That may work well for some plants though others might prefer more root room. In any case, it will dry out faster than a full raised bed connected to the ground. What types of plants are planning to grow? Do you have a photo or drawing of what you are intending? Are you building high to avoid bending over?

I doubt borax and water will add that much protection to otherwise untreated pine.

Some woods are much more rot-resistant than pine. Redwood, for instance, but I don't know how available that is in your area.

Also consider that pressure-treated wood does release toxic compounds, so you might not want to grow edibles in such material.

You might be able to construct something out of galvanized sheet metal or plastic. If the planting area is only going to be 6 to 12 inches deep, maybe you could use something like a metal patio table and build or find a large container to set on top of it. things like plastic strage boxes ans livestock feed tanks could possibly work.

I will be working on detailed drawings soon - I am preparing the area the garden beds will go. I will be putting down landscaping fabric and riverrock first on the ground, then building the planter boxes to sit ontop.

Currently I only have the dimensions of the bed and thinking of having it 3-4' off the ground on legs. Partly to avoid bending over but mainly to deter pests (i.e. rats/rabbits) which are plentiful in my area. I can either jimmy rig a mesh deterrent on the legs or attach a smooth surface to the legs to deter rats from climbing up and into my garden box and don't expect rabbits will be able to jump high enough into the box.

Great suggestions with storage box/plastic/steel but not sure how the aesthetics of those materials would be. I love the look and feel of wood, the aesthetics in this case is just as important to me as the function. Mick talked about Shugi Ban style and after researching it, it has an absolutely beautiful finish to it and looks like it does a great job to increase longevity - certainly an option at this point.

Great point about PT, I have read that PT after a particular year (2004?) is not as hazardous as they banned a particular chemical but I still don't like the idea of any chemicals leaching into a garden box which I anticipate will feed my family alot of produce in the growing season.

 

[Mick Emery]

Thank you Mick, this is great information. I looked into both Sho Shugi Ban and Tall Earth - both look like interesting options. Any experience with applying either technique/product to untreated wood outdoors and see how long it lasted?

Started sho shugi 4 years ago. No sign of deterioration. Same with Tall Earth started 7 years ago.

 

[DirtMechanic]

I thought that name looked familiar. It also explained something to me. I was identifying some trees out front and found out we call them Japanese red-cedar, although correctly they are called something else. Sugi is the national tree of Japan.

Cryptomeria is a monotypic genus of conifer in the cypress family Cupressaceae, formerly belonging to the family Taxodiaceae. It includes only one species, Cryptomeria japonica. It used to be considered by some to be endemic to Japan, where it is known as Sugi. Wikipedia.

The term “Shou-Sugi-Ban” is Japanese (焼杉板) and literally translates to “burnt cedar board”. The term is commonly used to describe the centuries old Japanese technique of charring “Sugi” (cedar) planks used for residential siding, fencing, and decking projects.

There is also YakiSugi, where "Yaki" means heat.

A very modern way to treat a lot of wood is to use plastic sheet to form a very large sealed bag which can be filled with wood and preservative liquid and use a wet dry vac to pull a vacuum within it. The vacuum pressure will ease as the liquid finds its way into the wood but may be increased several times as it sits. A deeper penetration can be achieved this way for boric acid or other chemicals.

 

[Mr_Yan]

I think most people way over estimate the speed where standard kiln dried stud grade dimensional lumber rots when used as a garden bed. Yes, some areas will be faster than others. Here in Michigan a 2x8 untreated garden bed will last a solid 6 years before it needs replacement.

I built a raised bed out of T111 exterior plywood with 2x3 frame to make the panels and it lasted 5 years.

A 4x8 bed up on legs with 6 inches of soil will have a LOT of weight on it and is not a trivial design and build. My guess is look to no less than 3/4 inch ply for the base with a 2x6 joists 16" on center. If you're worried about the dirt contact to the ply you can place a sheet of clear PVC fabric (link to US craft store) over the ply. I'd fold the edges of the plastic down so you're not making a pool but just a waterproof sheet. Blue tarp is a woven plastic and will not hold up as waterproof more than a few days - don't try to use that.

Or shift from 4x8 to 3x6 and use a concrete board for the base. These are sold as the backer board for tile install. I've worked with a lot of raised beds and greatly prefer the 30 to 36" wide beds to the full 48" wide that most people say to build.

 

[Mick Emery]

Thank you Mick, this is great information. I looked into both Sho Shugi Ban and Tall Earth - both look like interesting options. Any experience with applying either technique/product to untreated wood outdoors and see how long it lasted?

All my experience, with both, is outdoors. I've used in on my beds AND my home & porch. BTW>>> the 9 yr old beds I'm replacing are simple to do. After treating, I assemble the new bed, dig a trench, with a garden trowel, around the inside of the old bed's perimeter (putting the soil in the center), remove the old frame & set the new one down. DONE!

 

[Meadowlark]

I think most people way over estimate the speed where standard kiln dried stud grade dimensional lumber rots when used as a garden bed. Yes, some areas will be faster than others. Here in Michigan a 2x8 untreated garden bed will last a solid 6 years before it needs replacement....

My average rainfall is over 53 inches a year. I see where Western Michigan runs about 32 inches annually.

Nope, I don't way overestimate the speed that untreated pine lumber rots. Why would I or anyone do that?

I guarantee you won't get 6 years and will be very lucky to get anything useful more than two years here.

At today's price of lumber, let alone the cost of labor, using untreated pine in the ground is pretty much foolish here.

 

[Marck]

I’m planning to build a raised garden bed on legs for next spring. Dimensions of the bed are 4’ by 8’ and either 6” or 12” depth. Have not yet decided on depth but the plan is to do SFG (square foot gardening).

For the legs, consider cinder blocks, either, unmortared, mortared, or braced with rebar. You could put the blocks sideways for cubbyholes. I truly do find cinder blocks attactive, and they take paint and stain well for further design options.

 

[DirtMechanic]

My average rainfall is over 53 inches a year. I see where Western Michigan runs about 32 inches annually.

Nope, I don't way overestimate the speed that untreated pine lumber rots. Why would I or anyone do that?

I guarantee you won't get 6 years and will be very lucky to get anything useful more than two years here.

At today's price of lumber, let alone the cost of labor, using untreated pine in the ground is pretty much foolish here.

Here too. I finally got old enough to recognize that internet advice has no context, and that is the most important detail, even though it is very cool to share ideas. Perhaps this could be a goal for future Ai, a red screen that says WHOA that dude is in ICELAND!

@Mr_Yan your size advice is good stuff. I am a fan of the metal siding as well.