What's inside
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This planter box with a vertical trellis is one of the most useful builds for a small balcony. It gives you growing space without eating floor space, adds privacy, and handles vines, peas, or climbing flowers that would otherwise sprawl. The whole build can be done in a half-day with basic tools, and it breaks down into two parts: a watertight-ish box and a simple lattice back.
Contents
Planning Dimensions and Weight Limits
Before you cut anything, measure your balcony and check its weight limits. Wet soil is heavy – about 100-120 lbs per cubic foot. A planter that is 30″ wide x 12″ deep x 12″ deep of soil can easily hit 180 lbs after watering. Keep it under 36″ wide if you need to move it through a doorway, and keep soil depth to 10-12″ for most vegetables and annuals to save weight.
A good balcony size is 30″W x 12″D x 14″H for the box, with a trellis 48-54″ tall above the box rim. That total height of about 62-68″ gives climbing plants room without acting like a sail. If your balcony is windy above the 3rd floor, drop the trellis to 36″ or add a wider base. Tall, narrow trellises catch wind and will tip a light box.
Also decide how the trellis will sit. The strongest method is to extend the back legs of the planter box up to form the trellis uprights. A screwed-on trellis panel looks cleaner but will wobble unless you add diagonal braces or bolt it through the back wall.
Materials and Tools
Use rot-resistant wood. Cedar is the default for a reason, but pressure-treated pine and white oak also work. Avoid untreated pine or pallet wood for the box itself – it will rot in one to two seasons in constant contact with damp soil.
| Wood | Cost for This Build | Rot Resistance | Weight | Best For |
|---|---|---|---|---|
| Western Red Cedar | $45-70 | High, 5-7 years untreated | Light | Balconies where weight and longevity matter |
| Pressure-Treated Pine (ACQ) | $25-40 | High, but check label for ground-contact rated | Heavy when wet | Budget builds; line the interior if growing edibles |
| White Oak | $70-110 | Very high | Heavy | Permanent planters if you have the tools to work hardwood |
| Untreated Pine / Spruce | $15-25 | Low, 1-2 seasons | Light | Practice build or temporary use only |
For one 30″ planter you need: two 1×6 cedar boards at 8 feet for the sides and bottom, one 1×4 at 6 feet for trellis slats, and two 1×2 or 2x2s at 6 feet for the trellis uprights. You also need 1-1/4″ exterior wood screws, a small bottle of waterproof exterior wood glue, landscape fabric, and a 6-mil plastic liner or pond liner scraps.
Tools are simple: drill/driver, circular saw or miter saw, tape measure, square, and a 1/2″ drill bit for drainage holes. A pocket hole jig makes the box joints cleaner and faster, but it is not required. You can just screw through the face with countersunk screws and plug them if you care about looks. The cheaper option is fine here if you pre-drill – cedar splits easily near the ends.
Building the Planter Box
Cut the box parts: two sides at 30″ (front and back), two ends at 10-1/2″ (so they fit inside the front/back when using 3/4″ stock), and three to four bottom slats at 30″. If you are using 1x6s (actual 5-1/2″ tall), stack two boards to get about 11″ of side height. That leaves a 3″ soil gap at the top so water does not wash out.
Dry-fit the box on a flat surface. Check for square by measuring diagonals – they must match within 1/16″. Assemble the front, back, and ends with glue and two screws per joint. Pre-drill every hole. Failure mode to avoid: screwing too close to the end (under 3/4″) will split cedar even with pre-drilling. Stay 1″ in.
Attach the bottom slats with a 1/4″ to 3/8″ gap between each for drainage. Do not build a solid bottom. Without gaps, water pools and rots the bottom in a single winter. If your balcony floor must stay dry, you can still add gaps and set the box on 3/4″ feet (scrap cedar blocks) with a plastic tray underneath.
Drill three 1/2″ drainage holes in the front wall about 1″ up from the bottom. This is your overflow in a heavy rain. Without it, a lined box becomes a bathtub and drowns roots.
Building the Trellis Back
The simplest strong trellis is a ladder style: two vertical uprights with horizontal slats. Cut the uprights to 54″ long from 1×2 cedar. Cut six horizontal slats from 1×2 or 1×4 at 28″ long. Rip them down to 1-1/2″ wide if you want a lighter look.
Space the slats 6-8″ apart, starting 4″ above the top of the box. Closer than 6″ looks solid but blocks light and adds wind load. Wider than 8″ and small vines cannot grab. Attach with one screw per intersection from the back, with a dab of glue. If you use thin 1×2 slats, add a single vertical center strip to prevent sagging – otherwise a heavy clematis will bow them after a season.
For a diamond lattice, you need twice the slats and half-lap joints or just overlapping stapled joints. It looks nicer for privacy but is weaker and takes longer. Do not use store-bought thin lattice panels as the structural back on a windy balcony; the staples pull out. If you want that look, frame the lattice inside your 1×2 uprights.
Assembly, Lining, and Finishing
Screw the trellis uprights to the inside of the back wall of the box with at least three 1-1/4″ screws per side, plus glue. Better yet, run the uprights all the way to the bottom of the box so they act as the back legs. That ties the load directly to the base instead of hanging it on the back screws.
Line the inside with landscape fabric, not solid plastic. Staple fabric to cover the bottom gaps and sides, then poke holes over the drainage holes. If you must use plastic to protect a wood balcony, line only the sides and leave the bottom as fabric, or punch 6-8 drainage slits in the plastic. A fully sealed plastic liner is the number one cause of root rot and sour soil in balcony boxes.
Do not film-finish the inside. For the outside, you have two honest options: leave cedar bare and let it silver, or apply a penetrating exterior oil rated for planters. Film finishes like polyurethane or solid stain will peel where soil stays damp. If you use pressure-treated wood, let it dry 4-6 weeks, then seal the outside if you want.
Add feet. Screw four 3/4″ thick blocks under the corners to lift the box off the floor. This prevents trapped moisture, lets it drain, and keeps the balcony from staining. On a small balcony, add felt pads under the feet so you can slide it.
Planting and Keeping It Alive
Use potting mix, not garden soil. Garden soil compacts and weighs 30% more. Fill to 1″ below the rim. For a 30x12x11″ box that is about 1.8 cubic feet, or roughly two large bags of mix. Mix in a slow-release fertilizer at planting time – nutrients wash out fast with daily watering.
Water until it runs from the drainage holes, then wait until the top 1-2″ is dry. On a hot, windy balcony you may need to water daily. A box without drainage gaps will stay wet for days after rain; check with a finger before watering again.
Anchor the box if wind is an issue. A trellis with vines is a sail. Keep soil low and heavy, use a wide box over a tall narrow one, and if possible, strap the trellis uprights to the balcony railing with UV-resistant cable ties. Do not rely on the railing alone to hold it – the box should be stable free-standing.
Good plants for this setup: peas and beans (need 6″ spacing on the trellis), jasmine or clematis for scent and privacy, and strawberries or herbs in front where they get sun and do not need to climb. Avoid heavy perennials like small trees; the root mass will split the box in two seasons.