Creating an outdoor stratification box is rather straightforward. While you might be able to find stratification boxes through gardening stores or restoration suppliers, it helps to be able to build your own in a pinch.

Whether you are measuring in metric or imperial, you may have size differences as lumber sold in the EU is usually rounded to the nearest millimeter whole. These plans are for a rather large outdoor box with a final size of 1’ (300m) squared area and 1’ deep (300m) which gives you an area of about 6’ (1.82m). Most small-scale stratifications can be done at a fraction of this size.

Build Your Own Cold Stratification Box

This resource will guide you through the creation of a cold stratification box, and for smaller applications, a cold stratification pouch. These are intended to be used outdoors.

Seeds that require stratification either require a cold or warm stratification period to break dormancy. Before starting, ensure your seeds require a cold stratification that simulates winter.

Supplies Needed

• 12 pieces of lumber (1’x1’, or 300m x 300m)
• 16 wood screws (#8 or 4.0mm)
• 20 staples
• 6 fine wire mesh (1.5’ or 0.45 meter); see note about fineness.
• Drill with a boring and pilot hole bit and screw bit to match the wood screws
• Wire cutters
• Stapler
• Sand (can be gathered from site.)

A Note on Fine Mesh

Fine wire mesh should be small or fine enough that your seeds cannot fall through it easily, but it still allows airflow. This will vary greatly based on the seeds that you are using, so you can always measure the typical seed size and round down. You will also need your habitat sand to stay in the mesh, which should be achievable by adjusting down to the seed size; though you may need to be careful when using particularly large seeds and substitute sand from outside of the habitat.

You also need to be mindful of water; if your seeds are moisture-tolerant, the mesh should allow water in. If they’re not moisture-tolerant and you are stratifying in a zone that receives a lot of rain and snowmelt, you may benefit from an even finer mesh.

First, a Note on Habitat and Soils

To build your stratification box, first make sure your site is suitable for the habitat of your seeds, and digging a hole would not disrupt any sensitive species or habitat area.

Ensure the area is safe to dig in and free of any pipes or lines, and you should also research local soil surveys, or consult a soil scientist to determine the type, depth, and safety of the soil. 

You should also inspect the local water table during dry and wet periods to ensure the area you intend to dig will not flood.

You will need to dig the hole either before or after building your stratification box; if the project will take more than a day, or there is inclement weather, it’s best to wait on digging the hole so it does not fill with sediment.

You may also need to adjust the sizing of the box to work around the natural layering of the soil; for example, in some regions and habitats, the bedrock may be too high and would make inserting the box difficult to impossible.

If any of these conditions are unsuitable, you can adjust by using a smaller box, finding a different suitable site, or using a pouch instead (instructions below.)

When you dig your hole, preserve the topsoil (soil at the root layer of any plants) as best as possible, as you will be laying this back over the box.

Assembling the Stratification Box for Outdoor Use

To build your box, you are essentially constructing a cube. You should first lay your wood in pieces, in the shape of two squares. These are the top and bottoms of the box (or cube) and you will have four pieces of lumber leftover.

Next, drill your pilot holes into the adjoining edges of the box, so that you can attach the pieces together. If the lumber is too thick to allow your screws to reach the adjoining piece of lumber, you can use a boring bit to get closer before you drill your pilot hole in the two pieces.

Next, attach the remaining four lumber pieces vertically by drilling pilot holes and attaching them with screws. It should look like some semblance of a table that is missing its top.

Then, place the other wooden cube on top of the legs, and repeat the process of pilot holes and screws to attach. Optionally, you can use wood glue rated for outdoor use to further secure the box in addition to your screws.

Once any glue is dried, cut your mesh to size. You want the mesh to overlap with the wood so that there are no gaps, but it should not go beyond the edges of the wood.

Staple the mesh in place across all sides of the box except the top. The top mesh will be done last, once the box is filled.

Fill the box. The layering instructions are under a different heading on this page. Based on the dimensions, type of sand and seed, and habitat, it may be easier to place the box in the hole and then fill it.

Once the box is filled, staple the stratification mesh on top.

If you haven’t already, place the box in the hole and cover it entirely with the preserved topsoil. The topsoil covering the box should be level with the rest of the topsoil in the area.

Summary of Steps

1. Lay the first 8 pieces of lumber in the shape of two squares.
2. Drill pilot holes to attach these pieces with screws.
3. Repeat pilot holes and screws to attach the “legs” of the cube to one square.
4. Repeat pilot holes and screws to attach the remaining square to the free side of the legs.
5. Cut your mesh to size, to overlap with wood on all sides.
6. Staple meshes to all sides of the box except the top.
7. Fill the stratification box. (Layering instructions below.)
8. Staple the top mesh on.
9. Place into the habitat hole and replace the topsoil on top.

Changing the Box Dimensions? Read This First

If you change the dimensions of the box, please note that the diameter of the materials should also adjust to suit your needs. For example, wood lumber that is 1’ (300m) are likely too large if you are downsizing the box to ¼ of these plans; you can size down the materials to be proportional to your new size with a calculator and pre-build planning.

Regarding the amount of mesh to purchase, you should especially note the amount of mesh when changing the size of the box. Know that you will need 1.5’ or 0.45 meters of mesh for every foot (300mm) of wood. You want the mesh to overlap the wood for every Too much material will always be better than not enough, and you can never have enough leftover mesh in seed science.

Building a Stratification Pouch for Smaller Applications

If you only need small-scale stratification, it’s easier and more economical to create a stratification pouch. This is essentially a mesh that is shaped into a small bag with an opening at the top.

Supplies Needed

• 1 Fine Mesh, 1’ x 1’ (3m x 3m)
• Wire Cutters
• Staples and stapler

Assembling Your Stratification Pouch

First, cut the mesh into a 1’ by 1’ (3m x 3m) square.

Next, roll the next in your hands so that it has a wide opening on one side, and you are gripping the rolled edge.

Staple one end of the open mesh together.

Fold the stapled end over itself again to create a hem, then staple that side again to secure the fold.

Fill the pouch with your stratification material and seeds.

Staple the remaining open end, fold the edge over to create a hem, and staple again to fully close the pouch.

To place the pouch, instead of digging a large hole, you only need to create a small opening under the top soil for the pouch to lay. Alternatively, if you have a refrigerator or freezer dedicated to stratification, you could place this pouch there. Just be sure to cover the pouch with about an inch of soil so no seeds are exposed to freezing temperatures directly.

Summary of Steps

1. Cut mesh into a square.
2. Hold the mesh by the center and staple one side of the mesh together.
3. Fold that seam over to create a hem, and staple again.
4. Fill the stratification pouch.
5. Staple the remaining open end of the pouch and fold over to create a hem.
6. Staple the hem closed and place your pouch under topsoil.

Layering a Stratification Box or Pouch

Seeds that require stratification to sprout after winter, but they require that cold period to germinate. Layering material may be different for different types of seeds and their habitat, so research the native habitat of the seeds you are working with. You should also note the amount of moisture required for these seeds; some seeds require moist environments during stratification, while others do better when dry.

If you are layering before the first frost, you shouldn’t need to add a lot of moisture to your sand mixture because natural rainfall during the first frosts and melt cycles will sink down into the sand.

From the bottom up, layering a stratification box should start with a layer of sand about an inch (25mm) thick.

Then, layer seed and sand in the same thicknesses until you reach the top of the pouch or box; you should be sure that the top layer is also sand.

If you are using this layering method for an indoor stratification method, you will want to add a small amount of moisture and keep it moist on a routine basis. If you are struggling with stratification, it can help to add an additional topsoil layer over the top.