Updated: Jan 6, 2020
If you're unsure about which type of box you would like, go back and read my post here which describes the options to help you decide what kind of box would best suit YOU!
So first up is dimensions. Hopefully you read the previous post and you already know what size box you want to build. The size will dictate how much wood you need, so this is an important first step.
If you have no obstacles and you are going for a simple straight, rectangular box I suggest keeping it at or below a maximum of 4' deep. This size is perfect for cutting 8'x's in half, and as long as you have access from both sides of your box, you should be able to reach the entire box to tend to your crops.
If you do NOT have access from both sides, you should definitely opt to go narrower.
I suggest 2' deep so you can reach across, and this way you can cut your wood into quarters without waste.
As I mentioned in my other post, I feel that 2' is a good height for most garden boxes. At this height you don't have to bend over far to tend to your plants which saves you from lower back strain. It's a comfortable height to perch on the edge of the box.
If you have young children who enjoy helping in the garden, then this is also the ideal height for them to help.
If you are tall, or don't have children, you may decide to make your box slightly taller.
This is YOUR box, so decide what will work best for your needs!
I like to keep things simple, and eliminate the need for cuts and waste as much as possible, so unless you have obstacles that make it necessary to make your box a specific size then I suggest making the length of your box one of the standard lengths that wood typically comes in. This is usually 8' long, 10' long, 12' long and so on.
Keep in mind that wood has a tendency to warp and bow at longer lengths, so if you want a really long box it may be best to opt for two or more individual boxes.
*Untreated 2"x 6" lumber
*Untreated 4"x 4" lumber
*Star Deck Screws 2.5" I really love these. They cost a little more, but they don't strip or rust and the aggravation they save me is well worth the added cost. I buy the 5lb box because it's more cost effective, but if you don't think you will have the need for more, you can opt for a smaller size box.
*CLEAN dirt ( I recommend filling the bottom of your box with raked leaves and compost which will break down over time, thereby enriching your soil, and reducing the amount of dirt you need to purchase.
*Quikrete concrete mix
Start with a level area of your yard.
If it isn't level you're going to want to rake it out and tamp it down as thoroughly as you possibly can. If you are planning on building in an area that slopes severely that's okay too, if just requires a little more work. We actually ran into this issue when building one of our boxes over our wine cellar, and it CAN be done, so don't fret!
Dig a hole for your first corner post 12" deep.
Measure your first 4"x 4" post so that it is at least 9" longer than the total height of your box.
For example: if you are planning on making the height of your box 24", then the length of your post should be 33" (24"+9"=33").
The hole you dug in step two is 12" deep.
You will be screwing your cross pieces (the sides of your box) into this post, so the post needs to be tall enough that you can screw all your wood into it.
By the same token, it makes for a much nicer, neater looking box if these corner posts are buried beneath the soil line, which is why we don't make them the same height as your box.
Place your post into the hole you dug and pour Quikrete around it.
Make sure that your post is level and square. It's helpful to have a second pair of hands for this, but it can be done solo with a little patience!
Repeat steps 1-4 for each of your four main corner posts.
If you are building an L shaped box or another shape, you may have more than 4 corners.
As you do this you will want to check that your measurements from the outside of one post to the outside of the opposite post are correct.
If you want the length of your finished box to be 48" make sure to subtract the width of your exterior pieces of lumber from the total to ensure that your corner posts are spread out correctly.
For example: 2"x 4" pieces of wood are 1.5" wide.
If you have one of these attached to either side of your corner posts to create the walls of your box than you will want to subtract this total from your total finished width. (1.5"x 2=3")
48" finished length - 3" total width of wall pieces = 45" apart from exterior corner to exterior corner of your posts.
Make sure you do this for each of your sides.
Consider how you will water your plants!
If this area gets attention from sprinklers, than you are good to go, but if it isn't- this is the time when you should consider adding a drip irrigation system to your box.
If you have a spigot close by install a splitter with shutoff valves on it. I like this one.
Then install a water timer so that you can decide when and how often you would like your crops to be watered. Doing this will save you both time and money! I like this one.
Next attach a drip irrigation hose to it and bury it to the entrance of your box, leaving a LONG length rolled up.
You can cut it after you run it up and down your rows of crops.
I've used this kind for the last several years and been super happy with the results.
Start attaching your side walls!
To do this I like to start on my MOST level side of the box.
Lay your first 2"x 6" piece across.
Make sure that you cut it 3" longer than the total length of your posts. If your posts are 45" apart, then this should be 48" and so on.
I find that the best way to ensure level when doing this part is to start by marking and measuring 1.5" from the end of your cross piece.
Line this mark up with the outside edge of your corner post and screw it in, making sure that your 2"x 6" is flat at ground level.
Only use one screw at the moment so that you may make necessary adjustments to the level of your 2"x 6".
At this point I lay my level across the top of my 2"x 6" and check the level.
It may be necessary to dig out a little dirt along the way, or raise the other corner slightly to make sure that it's correct.
Once you have the correct level, screw 2 screws from your 2"x 6" into the corner post and go back and screw a second screw into the first post. Double check that your 2"x 6" extends 1.5" past the outside edge of your corner post on each side.
Move around your box attaching the first row of your side pieces to your corner posts.
You will want to alternate the length of your side pieces so that they overlap.
For example on the second side your length should be the exact length as your posts edge to edge.
Your third side will be 3" longer (like the first) and so on.
Make sure that the top of each 2"x 6" lines up with the height of the 2"x 6" on the side that you just completed.
You may notice that you have small (or large) gaps between the bottom of your first row of 2"x 6"'s and your ground level.
If you have a steep slope on one side particularly. We will address how to fill in these gaps further down.
You should have now completed the first row of your garden box all the way around.
You are now going to complete the rest!
Beginning on the first side where you began, start by cutting your second row 3" shorter than your first row.
This row should line up exactly with the width of the outside corners of your posts (on both sides). The third side you will cut the same way. Your second and fourth sides will be 3" longer to cover the gaps.
Repeat the first row for your third, and your second row for your second until you've completed the full height of your box.
This is where you will address any issues with bowing due to length, or gaps underneath due to slopes in your ground level.
If you have any issues with bowing you should now add additional support posts along your longest sides.
Do this by determining how many additional supports you need.
Typically one in the middle of each of your longest sides is sufficient, but if your box is extra long you may opt to do 2 extra posts spread out at 1/3 and 2/3's of the way down.
Dig these posts in like your corner posts and make it extra strong by reinforcing with Quikrete.
Now about those slopes!
If your gaps are very narrow you may not feel it's necessary to fill these in, particularly if you'll be adding a path around your box.
Typically the height of your path will be enough to fill in any 1-2" gaps.
If you've got larger gaps or a steep slope than these definitely require additional pieces of wood.
Begin by digging down where the ground begins to slope UP towards your ground level side rail.
You will want to dig down enough to fit in an additional full 2"x 6" piece.
This piece doesn't have to run the entire full length of that side.
For example: Let's say your side is 10' long, and 3' from the corner post your ground begins to slope downward creating a gap that gets gradually larger.
You should dig down at the 3' mark so that you can fit an entire 2"x 6" from here to the opposite corner.
Cut your 2"x 6" to the appropriate length (in this case 7') and screw it into your outside post so that the top of this additional piece is flush with the bottom of your existing ground level piece.
Now take a scrap piece of wood (any 2"x piece will do) and screw it perpendicular to your cross pieces on the INSIDE of your box where your new piece will end.
In this case it should be 7' in from the end where you began.
You will then screw the opposite end of your filler piece into this piece of wood.
One piece of wood may be enough, or you may need to repeat this with additional pieces to fill in all the gaps.
Making it pretty!
Now you get to paint (if you wish) or stain and seal the exterior of your box!
You don't have to do this of course, and once you paint you will have to keep up with it, as it will eventually begin to peel, but painting it WILL extend the life of your box.
Plus, I love adding a little splash of color to my garden!
If possible you will want to do this PRIOR to filling in your box. It's a lot easier to paint the edges if you're not getting dirt all over your paint brush!
Now, fill in your box, plant it in, and create your path around it (if you wish!!)