How to Grow Herbs in a Window Box

Spend a sunny day tracking which windows get the most direct light. Most culinary herbs need at least 4-6 hours of direct sun, and Mediterranean herbs like rosemary, thyme, and oregano want 6-8 hours. South-facing windows are ideal in the Northern Hemisphere; southwest and west-facing also work. East-facing windows (morning sun only) are fine for partial-shade herbs — parsley, chives, mint, and cilantro. Skip north-facing windows for outdoor mounting unless you supplement with a grow light.

Pick a box at least 9 inches deep — anything shallower limits root growth and dries out hourly in summer. Length should be slightly less than your window width so it fits within the window frame. Cedar lasts 8-10 years untreated and resists rot naturally; fiberglass and self-watering plastic boxes are lighter and longer-lived but cost more. The box must have at least 4 drainage holes — if it doesn't, drill 3/8-inch holes spaced every 6 square inches across the bottom before planting.

A loaded window box can weigh 30-40 pounds, so anchoring matters. Use a stud finder to locate framing beneath the siding under the window. Hold the brackets in position, mark screw holes with a pencil, then pre-drill pilot holes through the siding into the studs. Drive 3-inch deck screws or 1/4-inch lag screws through the brackets into solid framing — never just into siding, stucco, or drywall alone. Set the box on the brackets and check for level before screwing the box itself to the brackets through the bottom.

A 30-inch box comfortably holds 5-6 herbs. Choose plants with similar water and sun needs so one watering schedule keeps them all happy. Moisture-loving group: basil, flat-leaf parsley, chives, cilantro. Drought-tolerant Mediterranean group: thyme, oregano, sage, rosemary. Don't mix the two groups in the same box — Mediterranean herbs rot if watered as often as basil needs. Keep mint in a separate pot entirely, since its roots will choke out everything else within one season.

Use a lightweight container/potting mix — never garden soil, which compacts and suffocates roots. Bags labeled "Potting Mix" or "Container Mix" (Miracle-Gro, FoxFarm Ocean Forest, Espoma Organic) are formulated with peat or coco coir, perlite, and vermiculite for drainage. Fill the box to within 1.5 inches of the rim, then mix in 1 tablespoon of slow-release granular fertilizer per gallon of soil — this feeds plants for 3-4 months. A 30-inch box typically needs about 1 cubic foot of mix.

Arrange your starter pots on top of the soil before planting to finalize the layout — tall plants (rosemary, basil) toward the back, mid-height (parsley, chives, oregano) in the middle, trailing or low-growing (creeping thyme) at the front edge to spill over. Space plants 6 inches apart center-to-center; herbs in 4-inch starter pots can go that close because the box restricts root spread. Water each plant in its starter pot, then dig a hole the same depth as the root ball, slip the plant out of its pot, place it in the hole at the same soil level, and firm soil around it.

Use a watering can with a rose-head (sprinkler) attachment to soak the entire box slowly until water runs out the drainage holes. This deep first watering settles the soil around the roots and removes air pockets. Add more potting mix to top off any low spots that appear. Avoid using a hose without a sprinkler attachment — the high-pressure stream washes soil away and exposes roots. Plan to check soil moisture every morning for the first two weeks while plants establish.