Mount a sturdy box under the sill with brackets or a cleat, anchored into studs or masonry, with drainage and a waterproof liner.
Want color right outside the glass? A window box adds charm and daily blooms without eating up patio space. This guide walks you through planning, fastening, weatherproofing, and planting so your box looks good and stays put. You’ll pick the right hardware for your wall, set a straight line, and give roots room to breathe.
Read the steps once, gather the parts, then work methodically. The job sits in the homeowner skill range with basic tools. The trick is matching fasteners to the wall material and giving water a clear exit. Do that, and the rest is straightforward.
Plan The Location, Load, And Style
Start with the window itself. Measure the width of the trim and sight any obstacles such as shutters, outlets, hose bibs, or light fixtures. A box usually spans the sash width or the full trim. Leave at least 1–2 inches of clearance from each side of the trim so the brackets or cleat don’t collide with casing details.
Weight matters. Wet potting mix is heavy, and wood or fiberglass boxes add their share. A filled 36-inch planter can weigh 40–80 pounds or more depending on material and saturation. Build for the heavy day after rain, not the light day after wind.
Pick a mounting style that suits the wall: screw-on brackets under the box, a French cleat, or a cage that holds a liner. Brackets give easy removal. A cleat makes the box slide off for winter. A cage looks airy and pairs well with liners.
Mounting Methods By Wall Type
| Wall Type | Mount Style | Notes |
|---|---|---|
| Wood Framing With Siding | Brackets Or Cleat Into Studs | Find studs, use exterior-rated lag screws. |
| Brick Or Stone | Brackets With Lag Shields | Drill into mortar or brick, set anchors, then bolts. |
| Concrete Or Stucco | Anchors For Masonry | Use masonry bit and corrosion-resistant hardware. |
| Vinyl Siding Over Sheathing | Through Siding Into Framing | Pre-drill, use spacers to keep siding from crushing. |
| Fiber Cement | Into Studs With Sealant | Seal penetrations; never hang from siding alone. |
Check Safety And Prep The Surface
On older homes, paint near windows may contain lead. If the house predates 1978, use Lead-Safe Renovations for any scraping or drilling and capture dust. Wear a respirator and lay plastic to collect debris.
Clean the wall area. Brush off dust and cobwebs so hardware seats flat. If you plan to paint or caulk trim, do that first and let it cure. A dry, clean surface helps sealant stick around screw holes.
Step-By-Step Window Box Installation
Tools And Materials
Box or cage, two or more steel brackets or a cleat set, exterior screws or lag bolts, anchors for masonry if needed, drill/driver, bits (wood and masonry), level, tape, pencil, silicone or polyurethane sealant, rust-resistant washers, spacers for vinyl, and safety gear. Add potting mix, liner, and plants for the finish.
1. Find Solid Backing
For framed walls, locate studs below the window. Typical spacing lands near 16 inches on center. Confirm with a stud finder and a small pilot hole at bracket height. For masonry, pick anchor spots in sound mortar or brick and avoid crumbly joints.
2. Mark A Level Line
Hold the empty box or a straight board under the sill and mark the top of the brackets. Use a level to draw a light line. A straight reference line makes the hardware layout painless and keeps the planter from tilting.
3. Pre-Drill
Drill pilot holes at your marks. On wood framing, size the bit just under the screw’s shank. On brick or concrete, switch to a hammer drill and the masonry bit that matches your anchor. Blow out dust so anchors grab cleanly.
4. Set Brackets Or Cleat
Fasten the first bracket snug, check level across to the second, then add any middle brackets. With a cleat, anchor the wall cleat first. Use washers under bolt heads so hardware doesn’t bite into brackets. Add sealant under each fastener head and into each hole to shed water.
5. Test Fit The Box
Place the planter on the brackets or hang it on the cleat. Nudge left or right so it centers on the window. Confirm clearance for shutters and the sill drip edge. Tighten the hardware once the line looks true from the curb.
6. Add Liner And Drainage
Line wood boxes to extend their life. A removable plastic insert or pond-liner offcut works well. Punch holes along the bottom so water exits. Many fiberglass and PVC models include built-in drains; keep those clear.
7. Fill And Plant
Use a high-quality soilless mix that drains well. Moisten it lightly, fill to two inches below the rim, and tuck plants with space to grow. Water until it flows from the drain points. Finish with a finger of mulch to slow evaporation.
Drainage, Mix, And Watering That Works
Roots need air as much as water. That’s why planters should drain freely and use a light, soilless blend. Look for ingredients such as peat or coir, bark, perlite, or vermiculite. Garden soil compacts and stays soggy in a box, so skip it except in very large troughs.
Holes are non-negotiable. If your container lacks them, drill several across the base. Add a slight outward tilt so water heads away from the wall. Skip gravel at the bottom; it can trap water above the layer. A strip of mesh over holes keeps mix from seeping out.
Wall-By-Wall Fastening Notes
Wood Framing With Siding
Screw through the siding into studs. Add nylon spacers behind brackets so siding sits free and can drain. A bead of sealant at the hole shields sheathing from water.
Masonry: Brick, Block, Or Stone
Use lag shields or sleeve anchors sized to your bolts. Drill with a masonry bit, vacuum dust, tap in the anchors flush, then drive the bolts. Seal the hole before setting the shield if you want extra moisture control. For a step-through on this process with photos, see the This Old House guide for brick fastening.
Fiber Cement Or Stucco Over Frame
Go through the cladding into framing with exterior screws. Pre-drill and use sealant. If no stud lands where you need it, add a cleat that spreads the load across two or more studs.
Right Size, Right Spacing
A good visual rule is a box length near the sash width or the full trim width. Depth in the 7–10 inch range suits annuals and herbs. Taller plants ask for 10–12 inches of soil. Keep at least 8–10 inches between the box rim and the sash so window movement stays clear.
For weight and airflow, use two brackets for short boxes and add one middle bracket every 16–24 inches on long runs. If wind whips your facade, add discreet safety screws through the back of the planter into the brackets.
Box Size, Mix Volume, And Watering
| Common Length | Typical Mix Volume | Watering Starter Range* |
|---|---|---|
| 24 Inches | 8–12 Quarts | Every 2–3 Days |
| 36 Inches | 16–24 Quarts | Every 1–3 Days |
| 48 Inches | 24–32 Quarts | Daily In Heat |
| 60–72 Inches | 32–48 Quarts | Daily In Heat |
*Start here, then adjust by weather, plant choice, and sun.
Hardware And Corrosion Choices
Pick hardware that stands up to rain. Stainless fasteners resist rust near salt air. Hot-dipped galvanized bolts work well inland. If your brackets sit close to water runoff from a roof edge, stainless pays for itself over time.
Use large washers under bolt heads to spread load on bracket slots. Where a bracket meets softwood trim, a dab of exterior sealant under the foot helps keep water out of the hole. Wipe squeeze-out right away for a clean line.
If the box is cedar or another tannin-rich species, match it with stainless to avoid black streaks. For pressure-treated lumber cleats, pick coated or stainless fasteners designed for chemical contact.
Cost, Time, And Skill Range
A ready-made 36-inch PVC box with brackets often lands in a midrange budget, while wood kits and cages can be lower. Custom boxes and copper liners sit higher. Add a drill bit set, anchors for masonry if needed, a tube of sealant, and potting mix to round out the list.
Plan a half day for layout and mounting, plus time for planting. Brick or stone takes longer due to drilling. A second set of hands helps hold the box while you level and mark. If the wall sits two stories up, rent safe access gear or place the box on the lower level.
Plant Choice And Seasonal Swaps
Pick plants for the light you have. Full sun welcomes petunias, calibrachoa, lantana, salvia, and herbs. Part shade likes begonias, impatiens, coleus, ferns, and trailing ivy. In deep shade, lean on foliage and texture: heuchera, golden creeping jenny, and ferns carry the show.
Mix spiller, filler, and thriller shapes for balance. Trailers soften the rim. Mid-height fillers knit gaps. One taller accent adds lift without blocking the view. Keep mature widths in mind so the box doesn’t crowd by midsummer.
Plan on refreshes. After a tough heat wave or a cold snap, swap tired annuals and trim leggy stems. A few fresh plugs make the whole box feel new.
Care, Cleaning, And Winter Steps
Feed with a slow-release fertilizer at planting, then supplement with liquid feed during peak growth. Water deeply until it drains, then let the mix lose a bit of moisture before the next round. In windy spots, morning and late-day checks help catch wilt early.
Clean drains midseason. Poke through any clogged holes and flush with a watering can. If the box sits under a roof edge, add a splash guard above to prevent a waterfall that can erode mix.
Before frost, lift the box off a cleat or unbolt the planter and store it dry. Empty wet soil from wooden boxes so boards can dry fully. Check hardware each spring and touch up paint or sealer.
Mistakes To Avoid
Skipping Studs Or Proper Anchors
Drywall or sheathing alone won’t hold a wet planter. Hit solid framing or use masonry anchors sized for the load.
No Drainage Path
A liner without holes or a flat bottom traps water. Add holes and tilt the box a touch outward.
Too-Shallow Soil
Shallow boxes dry out fast and stunt roots. Aim for 8–10 inches of depth for most plant mixes.
Crushed Vinyl Siding
Brackets pulled tight to vinyl can dent it. Add spacers so the face stays true and water can pass behind.
Quick Materials Checklist
- Planter box or metal cage sized to window width
- Two or more heavy brackets, or a matched cleat set
- Exterior-rated screws or lag bolts; masonry anchors if needed
- Washers, spacers for vinyl, silicone or polyurethane sealant
- Drill/driver, level, tape, pencil, wood and masonry bits
- Insert liner or pond-liner offcut; mesh for drain covers
- Soilless potting mix, plants, slow-release fertilizer, mulch
- Safety glasses, dust mask or respirator, drop cloth
Troubleshooting Common Snags
Box Looks Crooked
Recheck the level line. Loosen one bracket and shim behind the foot with a stainless washer until the front edge reads level.
Plants Wilt Constantly
Increase soil depth or shield from midday sun. Add more organic matter to the mix and mulch the surface. A drip line on a simple timer removes guesswork during hot weeks.
Water Stains On Siding
Confirm drain holes point outward. Add a small spacer between the back of the box and the wall so air can pass behind it.
No Stud At The Best Spot
Span with a French cleat that reaches two studs, then hang the planter where it looks balanced.
Method Notes And Sources
Anchor pick depends on wall type and load. Masonry often calls for lag shields with bolts, while framed walls favor lags into studs or heavy screws. For a step-through with photos, the This Old House guide shows a clean approach to brick fastening. If the house dates before 1978, the EPA’s lead-safe work rules outline setup, protection, and cleanup so drilling near painted trim stays safe for families and pets.