How To Make A Grow Box

Learning how to make a grow box can put fresh produce on your porch or balcony-even in your closet! The trick is to use durable materials to create a sub-irrigated system where water is stored and delivered to your plants right under the soil.

How To Make A Grow Box From Simple Materials

A sub-irrigated grow box conserves water and space. Sub-irrigated grow boxes store water at the bottom of the container, where it is wicked up into the soil as plants require it. By adding a bit of liquid fertilizer to the water, plants can absorb nutrients at a rate that will keep them growing healthy all season. Follow these steps to get started:

  • Materials. You'll need a medium heavy plastic storage container that is about 16" wide by 24" long by 18" deep-an 18 gallon container should be fine; about two dozen 16 ounce disposable plastic cups; a 2' x 3' section of window screen netting; a 24" long section of 1" pipe (PVC or similar); potting mix (NOT potting soil); a few heavy duty, black garbage bags; strong tape (duct or packing tape).
  • Plan ahead. Your grow box will be heavy once it's full of soil and water. Placing the box in its final position before you fill it will save you some heavy lifting. If the storage container came with a lid, you can place it underneath the grow box to catch any spilled water or potting mix.
  • Place the cups. Line the bottom of the storage container with the plastic cups-the cups should loosely cover the bottom of the container. Although most of the cups should sit with their open sides down, about one in four should face upward. Take the upward facing cups and drill several small holes in the bottom of each cup. Fill the drilled cups all the way to the top with potting mix and return them to their original position. The filled cups will act as a wick to carry water up to the plants above.
  • Position the fill pipe. Attach the plastic pipe in one corner of the container so that one end touches the bottom of the container. Duct tape or plastic zip-ties work well for fastening the pipe. For best results, the bottom edge of the pipe should be cut at an angle.
  • Drill the fill indicator hole. Using a cup as a guide, drill a small hole in the side of the container near the bottom, just below where the top of the cup would be if it were sitting in the container. This weep hole will tell you when the grow box has been filled with enough water.
  • Add the screen. Lay the section of window screen over the layer of cups, making sure the screen drapes up the sides of the container.
  • Add potting mix. Carefully fill the container with potting mix, stopping about 3" from the top of the container. Try not to spill any of the potting mix under the screen. Adjust fertilization.
  • Add plants. Plant your plants in the potting mix. Be sure to space plants so that they will not become over crowded when they reach full size-two to four plants is usually plenty.
  • Fill the water reservoir. Using the fill pipe, add water to the reservoir until it just begins to pour out of the weep hole. Depending on where you've sited your grow box, you'll need to add more water every two to four days.
  • Cover with plastic. Once the plants are about 3" to 6" above the top of the container, cover your grow box with black plastic, taping it in place with duct or packing tape. Allow the plants to poke out of the plastic. The black plastic will act as mulch, retaining moisture and retarding the growth of weeds.
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