Building a Wooden Bonsai Grow Box

By Michael Greenstein

Valley Oak being transitioned

Grow boxes are very useful for transitioning collected trees from the ground to a bonsai pot and for rehabilitating weak trees. For example, this Valley Oak was dug up ("collected") but not yet ready for a bonsai pot. It will remain in this box long enough to build primary primary branches. The primary attributes of these boxes are a large (but still shallow) root volume, a high oxygen environment for the roots because of the breathable wood plus drainage in the bottom of the boxes.

This design uses seven 1 x 6 redwood boards (fencing) to build a box that is 18 x 18 x 6 inches in size. These boxes will last over 10 years with daily watering / rain in the SF Bay Area. No chemical treatment is used on the wood.

The boxes can also be designed with other shapes and sizes. 

When I make a grow box I usually make several at the same time.

Materials

Exterior deck screws

• 1 x 6” redwood approximately 10’ long

• 1 x 2” redwood for handles on the top and spacers on the bottom

• 2 x 16” strip of screen to cover gaps in bottom

• #8 x 1-1/4 inch (34) Exterior deck screws (stainless steel or powder coated)

Tools

• Tape measure and wooden pencil to mark lengths for cutting

• Power radial or hand saw to cut 1 x 6 and 1 x 2 boards to length

• Sanding block to break edges of sawed boards

• Power drill / driver to drill guide holes for the screws and to drive the screws

Procedure

  1. Measure and the 1 x 6 boards - five pieces 18-inches long and two pieces 16-inches long

  2. Measure and cut the 1x2 boards - four pieces 18-inches long

  3. Sand down cut edges to remove splinters

  4. The photo below shows the boards cut to size for 1 box

Wood boards cut to length

  1. Take two of the 1x6x18 boards. Place one flat and stack one vertically on top of the flat board. Adjust the placement so the so that the corners are at right angles. Now draw a line with a pencil along the horizontal board. This mark will indicate how wide edge to show the width of b - usually about 3/4" wide. Repeat on the other end of the board. Now swap boards and mark the second board.

  2. Mark holes on the side and bottom pieces 1x2x18 about 2" from end of the board.

  3. Drill pilot holes so that the screws go into the middle of the side boards. Pilot holes keep the boards from splitting when you insert the screws.

Pilot hole

  1. Start assembling the box by positioning a 16” board as shown below with blocking behind it so it will not slide away as the screws are driven. Position a predrilled 18” board perpendicular to the 16” board and drive (2) screws in. If your work surface isn't exactly even it can help to place another board underneath the boards being screwed together.

  2. Now attach the second 16’ board to the other end of this 18" board.

  3. Rotate this assembly of three boards around and attach the second 18" board to the two 16" boards.

  4. Although these first four boards may be a bit wobbly now it will be quite rigid after the next steps.

  1. Now position the remaining three 18" boards on top of the assembled sides. Leave a ½ “ gap between the boards for drainage and air circulation.

  2. If the assembled sides are't quite square, now is the time to wiggle it a bit so that it is. Drill pilot holes into the bottom boards and frame and then drive in screws.

  3. Flip the box onto it's side and position the side handles along the top of the sides.

  4. Drill thru the existing pilot holes in the side handles into the side boards and then drive screws to fasten the handles to the board. An alternate solution is to screw on galvanized handles.

  5. Repeat for opposite side.

  1. Flip the box so that the bottom is facing up again. Now screw on the (2) bottom spacers. They improve drainage and air circulation, as well as add rigidity to the box (cross bracing) and extend the life of the box. After about 5-7 years, these bottom spacers may need to be replaced if they rot out before the rest of the box.

  2. Use a sanding block to remove any sharp edges anywhere on the box.

Final assembly as seen from bottom

  1. After the box is completely assembled, staple the mesh screen into the bottom of the box to retain soil and exclude sow bugs, as shown below.

Screen stapled across drainage gap

Tie Downs

To tie your tree into the box drill holes in the bottom of the box and feed wire up to tie a tree into the pot - just like any other bonsai pot.

In addition I drive short screws into the sides or top of the box to act as additional tie down points. These are especially useful to pull down branches or provide extra anchoring of the trunk to the box.

Optional screws act as tie down points

Comments

• Estimated lifespan: ~ 10 years

• Alternate materials: I use kiln dried redwood because it is easy to work with and is straight and flat. Lower grade wood works as well and is cheaper .

• Chemical treatment: this would extend the life of the box, but I do not want these chemicals around my trees

• Different sizes: my standard is as shown, 18 x 18 x 6”. You could make a longer box for a raft project, or consider a 14 x 14 x 4” box.

• Stainless steel screws: if you use SS screws, you will be able to use them again in 10 years for the next box.