Most window box failures happen at the garden center. You buy a tomato, a basil, a trailing something, and a bag of whatever soil was on sale. Six weeks later the tomato is stunted, the basil has bolted, and the box dries out by noon.
The problem is volume. A standard 3-foot (0.9 m) window box holds about 5 gallons of mix, and that is the entire world your plants get. Everything in it shares the same water, the same nutrients and the same root space. Choosing plants for a window box is less about which plants you like and more about which ones can coexist in five gallons.
This number surprises people. According to the Swansons Nursery soil calculator, a 3-foot box that is 6 inches wide and 6 inches deep takes about 20 dry quarts of potting mix. That is 0.67 cubic feet, or roughly 5 gallons (19 L). A 4-foot box takes a full cubic foot, about 7.5 gallons (28 L).
Put that next to what extension services recommend and the constraint becomes obvious. UMass Extension puts most vegetable crops at 3 to 5 gallons each. Your entire window box is one crop's worth of soil. Plan accordingly.
Here is what this guide covers:
Key Takeaway
A window box is not a small garden bed, it is one container's worth of soil shared by everything in it. Match plants by root depth and water demand first, then worry about which ones look good together.
Penn State Extension sets the benchmark that matters most: a single tomato plant needs at least a 20-inch-wide pot, peppers and eggplants manage in a 14-inch pot, and all pots should be at least as tall as they are wide. A 6-inch-deep window box does not come close for either crop. Texas A&M AgriLife Extension puts tomatoes, peppers and eggplant in the 5-gallon class, with 6 to 10 inch pots being adequate for green onions, parsley and most herbs.
| Crop | Minimum Depth | Verdict for a 6-In. Box |
| Lettuce and salad greens | 6 in. (15 cm) | Excellent |
| Chives, thyme, oregano, cilantro | 6 in. (15 cm) | Excellent |
| Radish, round types | 4-6 in. (10-15 cm) | Good |
| Strawberries | 6 in. (15 cm) | Good, ideal spiller |
| Basil, parsley | 8 in. (20 cm) | Workable, better at 8 in. |
| Bush beans | 8-10 in. (20-25 cm) | Needs a deeper box |
| Peppers | 12-14 in. (30-36 cm) | Not in a window box |
| Standard tomatoes | 14-20 in. (36-51 cm) | Not in a window box |
Sources: Penn State Extension, Texas A&M AgriLife Extension, University of Maryland Extension
Common Mistake to Avoid
Do not put a full-size tomato in a window box. Extension guidance is unambiguous: it needs four times the depth a window box provides. If you must grow tomatoes at a window, use a true micro-dwarf variety in an 8-inch-deep box and accept a small harvest, or give it a dedicated 5-gallon bucket beside the box.
Traditional companion planting assumes each plant has its own root run. In a window box they do not, so the useful question changes from "do these help each other" to "can these share five gallons without one strangling the rest."
Three combinations hold up. A salad box of loose-leaf lettuce, arugula, radish and chives works because everything is shallow, fast and thirsty on the same schedule. A Mediterranean box of thyme, oregano, sage and trailing rosemary works because everything wants the same lean, dry conditions. A kitchen box of basil, parsley, chives and nasturtium works because they share water needs and the nasturtium spills over the front.
What does not work: mint in a mixed box. It runs, and within one season it owns the entire root zone. Grow it alone. Fennel is the other one to keep separate, since it is widely documented as inhibiting neighbours. Our guide to companion planting mistakes covers the pairings that genuinely conflict.
Why This Works: Grouping by Need, Not by Lore
NC State Extension reduces container companion planting to one rule: group plants that have similar water and light requirements. That is the same logic behind permaculture guilds, just compressed into a box. Plants that want the same conditions form a stable community; plants that want different ones make you choose a loser every time you water.
Aspect decides more than anything else you control. Fruiting crops need at least 6 hours of direct sun. Leafy greens and most herbs will produce on 3 to 4 hours, and in a hot climate they actually prefer the break.
| Window Aspect | Direct Sun | Plant This |
| North | 0-2 hours | Mint (alone), parsley, chives, lettuce, violas |
| East | 3-5 hours, morning | Salad greens, cilantro, chervil, strawberries |
| South | 6+ hours | Thyme, oregano, rosemary, sage, micro-dwarf tomato |
| West | 4-6 hours, hot afternoon | Basil, nasturtium, peppers in a deeper box, marigold |
Sources: UMass Extension, University of Maryland Extension
West-facing boxes are the trap. They read as sunny but deliver their light as afternoon heat, which cooks shallow soil. NC State Extension specifically advises avoiding small or dark-coloured containers for this reason. A pale wooden or light terracotta box on a west wall will run several degrees cooler than a black plastic one.
Get Our Free Companion Planting Chart
Join 10,000+ gardeners getting weekly tips on what to plant together, soil health, and permaculture techniques.
Send Me the ChartNot garden soil. Illinois Extension is blunt about it: never use garden soil by itself in a container, no matter how well things grow in it outdoors. In a pot it compacts, excludes air, and holds water where roots need oxygen.
Skip the gravel layer too. The Garden Professors explain the physics: water stops moving at the interface between two different materials, creating a saturated "perched water table" above it. Adding gravel at the bottom does not drain the pot, it just lifts the soggy zone higher and steals root space. In a 6-inch box you cannot afford to lose an inch.
Penn State Extension gives a simple mix that works: one part compost, one part vermiculite, one part peat moss, screened through quarter-inch mesh. Illinois Extension's soil-based version is one part garden soil, one part peat moss, one part perlite or coarse builder's sand, never fine play sand. Mix it uniformly. Do not layer it. If you would rather buy than blend, our container soil mix guide breaks down the ratios.
Water and food, more often than you expect. Texas A&M AgriLife Extension puts container vegetables at roughly once per day in summer, and recommends flushing the box with plain water once a week to leach out accumulated fertilizer salts. Illinois Extension gives the practical test: water when the top inch of mix feels dry, and keep going until it runs from the drainage holes.
The nutrient side follows from that. All that water washes nutrients straight out. Illinois Extension notes that slow-release fertilizers typically supply nutrients for 3 to 4 months, while liquid feeds are usually applied every two weeks or diluted and used at every watering.
Check the drainage holes first
NC State Extension is firm that containers must have drainage holes. If your box has none, drill them. Holes along the lower sides work better than the base alone when the box sits flat on a sill.
Fill to one inch below the rim
Illinois Extension recommends leaving about an inch (2.5 cm) of headspace so water does not sheet over the edge. Moisten the mix before planting, since dry peat repels water.
Plant thriller, filler, spiller
One upright anchor at one end, two or three bushy fillers through the middle, and a trailing plant at the front edge. For a 3-foot box, five to seven plants total is the realistic ceiling.
Harvest hard and often
Pinch basil tops weekly, cut lettuce as leaves rather than heads, and shear herbs before they flower. Constant harvesting keeps plants compact, which is what a shared root zone can support.
Key Takeaway
The three failure points, in order, are too little depth, wrong soil, and inconsistent water. Get those right and plant choice becomes forgiving. Get them wrong and no combination of plants will save the box.
Mediterranean herbs are the standout: thyme, oregano, sage, marjoram and trailing rosemary all evolved in thin, hot, dry soil and handle a south-facing box better than anything else. Add nasturtium and marigold for colour and pollinators, and strawberries along the front edge as a spiller. Basil does well on a west aspect if you can water daily. Avoid lettuce and cilantro in full sun, since both bolt within weeks and turn bitter once the soil heats up.
Potting mix, and nothing else. The common advice to add gravel, broken pottery or packing peanuts for drainage is backwards. Because water stalls at the boundary between two materials, a drainage layer actually raises the saturated zone and cuts your usable root depth. In a box only 6 inches deep, that is a significant loss. Use a uniform, well-structured mix from top to bottom and make sure the drainage holes are open.
Not on its own. Illinois Extension advises never using garden soil by itself in a container because it compacts, drains poorly and starves roots of oxygen. If you want to stretch expensive potting mix, Illinois Extension suggests blending about 25 percent garden soil with 75 percent soilless media, or a one-to-one-to-one mix of garden soil, peat moss and perlite or coarse builder's sand. Whatever you use, blend it uniformly rather than layering it.
In summer, plan on daily. Texas A&M AgriLife Extension puts container vegetables at about once per day, and a shallow box on a hot south or west wall can need a second watering during a heatwave. The reliable test from Illinois Extension is to water when the top inch of mix feels dry, then water until it runs out of the drainage holes. Unglazed terracotta dries faster than plastic or glazed pots, so factor in your box material.
Yes, provided they want the same water. Salad greens with chives and radish work, and basil with parsley and nasturtium works. What fails is mixing a drought-tolerant Mediterranean herb like rosemary with something thirsty like lettuce, because whichever you water for, the other suffers. Keep mint in its own container regardless, since it will colonise a shared root zone in a single season. Our guide to companion planting for small spaces and containers goes deeper on this.
Ready to Grow Smarter?
Get our free 20-page beginner's guide to backyard food forests. Two printable worksheets, a year-by-month food-forest calendar, and a curated reading path.
Read the Free GuideA window box will not feed you, but it will change how you cook, and it is the cheapest way to find out whether growing food suits you. When you outgrow it, our balcony-scale food forest guide is the next rung up, and the complete companion planting chart covers pairings once you have real space.