How to Construct a Raised Bed with Low Tunnel

whoose · February 2, 2017

How to Construct a Raised Bed with Low Tunnel

This post is intended to be an experiential based discussion of how to make a raised bed. There are many consideration that I will try to cover and some controversies. I will have Tomatoville post for reference from many discussion that already have been had controversial topics.

My credentials: Psychology Professor by trade (retired), I have been a home owner level carpenter all my life, I built a log house, two garages and a greenhouse. I have done maintenance on these building for the last 40 years. I have tools, not that modern, but sufficient for the task. Last year I built seven raised beds, five with low tunnels. I plan one more raised bed for this Spring.
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Please feel free to add to the discussion, we all are here to learn from each other.

Why build a raised bed?

Many of us are happy with our container gardens or regular garden spaces. Some of us want to expand to the next level of the raised bed.

Some of the advantages of raised bed are:

Maximizing the amount of soil for growing plants.
Allowing for easy applications of compost and fertilizer.
Easier control of weeds.
The ability build soil is enhanced.
Raised beds just look nice.

Where to build a raised bed?

Ideally a south facing exposure on level ground is the most desirable. Many of us are not lucky enough to have this ideal.

Try to avoid these situation.

Trees, brush, tall grass, large roots.
Rock outcrops that are not level.
Long distances for your water source.
Shade.

What to use to build a raised bed?

Almost anything you can stack, form or pile will work:

Clay Bricks.
Concrete walls.
Cinder blocks.
Retaining wall blocks.
Hay bales.
The buckets from your container garden.
Wood.

Most of use think of wood as the primary material used for construction of raised beds. There are many types of wood available depending on your location and how much money you are willing to spend.

Types of wood include:

Redwood.
Western Red Cedar.
Eastern Cedar.
Cyprus.
Pine.
Fir.
Hardwoods.

Just about anything that is natural wood will work.

No plywood please.
No railroad ties either.

The amount of time it takes wood to rot in soil depends on the wood type. The most expensive woods (Redwood, and Cedars) last longer than Pine and Fir. So depending on how long you expect the raised bed to last and your budget you can pick the type of wood.

There has been a lot of discussion on using treated wood on this forum.

Pressure treated wood is treated with an arsenic-like compound in most cases.

The some questions to consider:

How much leeches into the ground?
How much leeches into your plants?
How much longer does treated wood last than regular lumber?
How long does it take for the chemical to break down in the environment?

For question that you will have to consider, see the following:

Pressure treated lumber and raised beds http://www.tomatoville.com/showthread.php?t=43231&highlight=treated+wood

Treated lumber?http://www.tomatoville.com/showthread.php?t=4771&highlight=treated+wood

Paint for raised beds?
http://www.tomatoville.com/showthread.php?t=38776&highlight=treated+wood

My view is why take the chance???

For me it is hard to say Organic and treated lumber in the same sentence. So if you are organic probably do not used treated wood. I use 2X Western Red Cedar, very expensive but why take a risk? The cedar beds will out last me for sure. I have two grandkids and prefer not to take any more chances than necessary with their health. If you sell your produce you must consider the implications of using treat wood.

So Lets Build Some Raised beds

You now have decided on the site and type of material. I will discuss using dimensional lumber for raised beds.

Buy your lengths of lumber carefully, this stuff can be really expensive. Use even 2 foot dimensions for the long sides and try to have to make only one cut for the short sides. For example 8'x4' is a no waste bed with only one cut from 3, 8 foot boards. Many of us do not have power carpenter tools like precision saws. A good way to solve this problems is to have the big box or local lumber yard make the cuts for you, only a dollar of two, and very precise cuts, which you will need.

How to join the sides, front and back of the bed?

I use 4X4 posts in each corner and sink large bolts through the 2X into the 4X4, this will not fail for a very long time. The 4X4 method also allows for easy addition of the next course of boards. I leave the 4X4 about 1 inch below the finished soil for a nice look. You can use metal corner braces, get as big as you can, they will also last a long time. Use only treated non-rusting screws, nails loosen and work their way out. I put a 2X4 as a brace between the front and back in the middle to keep the boards from trying to bow out from the weight of the soil. This brace is also a good place to sit.
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How high should my bed be?

I start with 2X12 boards for the bottom course and then can add 12, 10, 8, or 6 inch boards for the next course. I find 20 inches is about right for me, try out different heights and find the one you like. Remember to leave your corner 4X4 the correct height for the new course of lumber. The bed makes a great seat, the older you get the more you appreciate a good resting area.
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Raised bed height
http://www.tomatoville.com/showthread.php?t=43033&highlight=treated+wood

Building on a sloping area?

You will need to drive large 2X4 stakes in the corners and middle of the bed to keep it from moving down hill. I also place stakes on the uphill side to help slow the movement. Worst case is the bed migrates down hill until it breaks the wood or pulls the screws.

Where to Build the raised beds?

I like to comfort of my garage for the construction of the raised beds. I then put the bed in my truck and take it to the site. I almost always have projects during what we call the "mud season" or Spring here in the West, so nice dry, warm work place is nice. You can build in place but use saw horses to elevate you work.
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Screening the bottom?

I use hardware cloth, small metal mesh, on the bottom of my beds. I have Voles, Moles, Gophers, and other burying rodents that I do not want in my beds. The hardware cloth stops them dead. If you are a double digger leave the screen out. See, below.

Birds? Mice? Rats? Squirrels?
http://www.tomatoville.com/showthread.php?t=42604&highlight=mice

Landscape fabric

Also consider using landscape fabric in the bottom of the beds to help control any weeds trying to grow from the bottom of the raised bed.

The soil?

First decision, organic or not. Your call, see.

Organic/growing compendium and resource listing
http://www.tomatoville.com/showthread.php?t=4636

How to buy soil?

Bags are easy but expensive. Landscape contractors are cheaper and have good access to very good soil. They will also will deliver large amounts to you site relatively cheaply. Try to make your own compost going forward. See.

Compost bin and building compost question
http://www.tomatoville.com/showthread.php?t=36787&highlight=treated+wood

Chicken poo and sawdust
http://www.tomatoville.com/showthread.php?t=40453&highlight=treated+wood

Additives

Another consideration with commercial soil is the uses of bio solids, sewer bi-products (sludge) in soil mixes. The manufacturer must list bio solids in the list of ingredients. Very different views by different people, but it is you choice. See:

Sewage (bio solids) in your compost?
http://www.tomatoville.com/showthread.php?t=42694&highlight=whoose

Low Tunnel

While you are at it add some low tunnels to your raised beds. Very easy, quick and relatively inexpensive. The advantages of low tunnels are about 1 month early planting in the Spring and 1 month longer before killer frost in the Fall. That is a big deal at 6000 feet in the Northern Rockies.

You also get what I consider to be a very important edge, higher soil and air temperatures. These might be the missing ingredients in a successful garden, especially at higher altitudes with short seasons. Where I live the temperature rarely gets over 80f and 50f at night, so hot soil and air really area must.
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Lets build a low tunnel

I use schedule 40 white 1/2" PVC plastic pipe for the hoops in a low tunnel. For the end supports of the hoops I use 1' schedule 40 white PVC plastic pipe, attached with galvanized brackets. Very easy to install and last for a long time. Be sure to try and bend the pipe for the hoops after it has been in the sun for a while, or use a heat gun. Schedule 40 is tough stuff but you can break cold pipe. Stay away from the cheaper thin wall pipe, it is bound to fail. I disassemble my hoops for the winter and store out of the weather.
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Covering for a low tunnel

I use 6 mil clear poly plastic. Lumber yards sell very large roles of the stuff, it is not cheap, but worth it. Do not try to save money with 2 or 4 mil it will fail far too soon. The 6 mil will last one season and then it is off to recycling. Some specialty green house stores have higher quality plastic but I find the cost too high.

You live in a HOT climate and do not need the heat provided by a low tunnel or the extended season. But you can still use the low tunnel, you will want to stop lost of uninvited guests. See:

Birds? Mice? Rats? Squirrels?
http://www.tomatoville.com/showthread.php?t=42604&highlight=mice

You can use netting/screening to stop most mammals, insects, birds for destroying your plants.

Too hot in the summer is a problem many of us would like to have but if you do have too high temperatures you need to mitigate the problem. You can put shade cloth over the low tunnel and provide shade and lower temperature. See:

Thoughts on shade cloth
http://www.tomatoville.com/showthread.php?t=43248&highlight=whoose

Ending Thoughts

So let me know your experiences and how to improve my basic raised beds and low tunnels.