What's inside
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A wooden window sill planter can keep parsley, chives, and thyme within reach through winter without taking up counter space. The key is managing water: a planter that leaks onto the sill can damage paint or wood, and herbs sitting in soggy soil often fail. This simple box includes a waterproof liner and drainage layer, so excess water has somewhere to go without dripping indoors.
Contents
Plan the size and check the window
Measure the usable sill depth and width before buying materials. Leave at least ½ inch of clearance at each end and make the planter no deeper than the sill unless you can support its overhang. A practical size for three small herb pots is about 18 inches long, 5 inches wide, and 5 inches high. For a deeper box planted directly, allow around 6 inches of soil depth.
Check that the window gets enough light. Most culinary herbs need roughly 6 hours of bright light a day; a north-facing or shaded window may need a grow light. Also check for cold drafts and hot radiator air, both of which can stress plants. This project is best for herbs in nursery pots placed inside the box, rather than soil planted directly into the wood.
Choose materials and weigh the trade-offs
Use untreated, straight-grained lumber. Pine is inexpensive and easy to cut, though it dents more readily than hardwood. Cedar resists moisture better but costs more and can still rot if water is trapped against it. Avoid pressure-treated lumber for an indoor planter, and avoid food-contact claims for finishes unless the product label specifically supports that use.
| Build choice | Best for | Trade-off |
|---|---|---|
| ¾-inch pine boards | Low-cost indoor box | Needs careful sealing; softer wood dents easily |
| ¾-inch cedar boards | Better moisture tolerance | Costs more and can split near board ends |
| Nursery pots inside a lined box | Easy watering and plant replacement | Uses some of the box’s interior space |
| Soil planted directly in the box | A continuous planter appearance | More difficult to drain, clean, and protect from leaks |
For a first build, nursery pots inside a lined box are the safer choice. A roll of waterproof planter liner or a fitted plastic tray is useful; compare planter box liners by checking the dimensions and whether the material can be cut without tearing.
Gather materials and tools
For an 18-by-5-by-5-inch planter, buy one 1-by-6-inch board at least 6 feet long. Actual 1-by-6 lumber is usually about ¾ inch thick and 5½ inches wide. Cut the front and back to 18 inches, and the two ends to 3½ inches so they fit between the long sides. Add a ¾-inch plywood bottom, cut to about 16½ by 4 inches, fitted inside the walls.
You will also need exterior-rated wood glue, 1¼-inch screws, sandpaper, a drill with a pilot bit, a saw, and a waterproof liner or plastic tray. A waterproof liner is more reliable than relying on finish alone. If you want to protect the outside, use a low-odor finish labeled for interior use and let it cure fully before placing plants in the box. Do not coat the inside if the liner already protects it.
Build the planter box
1. Cut the boards square and dry-fit the four walls around the bottom. Check the box for square by measuring diagonally from corner to corner; the two measurements should be close.
2. Drill two pilot holes through each long side into each end piece. Pilot holes reduce splitting, especially in cedar. Apply a thin bead of glue to each joint, then drive the screws. Wipe away squeezed-out glue before it hardens.
3. Fit the plywood bottom inside the walls, leaving a snug fit rather than forcing it. Drill several small drainage holes in the bottom only if you plan to use a tray underneath and can contain runoff. For a box sitting directly on a wooden sill, skip those holes and use an intact liner or removable tray instead.
4. Sand the edges and corners with 120-grit paper, then smooth with 180-grit. Check for splinters and protruding screw tips. Add small rubber or cork pads underneath to let air circulate and reduce scratches on the sill.
Line it and test for leaks
Place the liner inside the box with enough material to cover the bottom and rise at least 2 inches up the walls. Fold corners neatly; do not puncture the liner with staples or screws below the soil line. If using a tray, confirm it sits flat and extends under every pot. A few minutes of testing can prevent a costly water stain.
Set the empty box on a protected surface, add a small amount of water to the liner or tray, and wait 10 minutes. Check the outside and underside for damp spots. If water escapes, replace or refit the liner rather than trying to solve the problem with extra paint. Never leave standing water in the tray after watering; empty it once the pots have drained.
Plant herbs and maintain the box
Keep herbs in individual pots with drainage holes, using a quality potting mix rather than dense garden soil. Parsley and chives handle cooler indoor conditions well; thyme prefers strong light and soil that dries slightly between waterings. Avoid crowding: three 4-inch pots fit comfortably in an 18-inch box, while larger plants need more space.
Water each pot until a little drains through, then let it finish draining before returning it to the planter. Check moisture with a finger rather than watering on a fixed schedule; indoor pots may need water every few days or only once a week, depending on light and room temperature. Rotate the box a quarter-turn weekly so plants grow evenly, and trim herbs regularly. If natural light is weak, a basic full-spectrum LED grow light for herbs can help, but a sunny sill is fine when plants remain compact and healthy.
Inspect the liner and sill monthly, especially during the first few weeks. A musty smell, soft wood, or swelling at a joint signals trapped moisture. Remove the pots, dry the box, and repair or replace the liner before using it again. That simple check matters more than choosing a costly wood species or finish.