--- title: "Best Plants for Window Boxes: What Actually Grows Together" description: A window box holds about 5 gallons of soil. Root depth by crop, combinations that share it well, what to plant by window aspect, and the soil mistake to skip. image: https://growperma.com/hubfs/blog/images/gp-0437-hero-20260810-063820.webp --- [Skip to content](https://growperma.com/blog/best-plants-for-window-boxes#main-content) [![GrowPerma Logo](https://growperma.com/hs-fs/hubfs/icon-512.png?width=40&height=40&name=icon-512.png)](https://growperma.com/) - [Blog](https://growperma.com/blog) - [About](https://growperma.com/about) [Join the Weekly Dig](https://growperma.com/newsletter) - [Blog](https://growperma.com/blog) - [About](https://growperma.com/about) [Join the Weekly Dig](https://growperma.com/newsletter) ![Wooden window box below a sunny apartment window planted with trailing thyme, basil, parsley and a compact cherry tomato](https://growperma.com/hs-fs/hubfs/blog/images/gp-0437-hero-20260810-063820.webp?width=1200&height=520&name=gp-0437-hero-20260810-063820.webp) In This Article Table of Contents ![Peter Vogel](https://growperma.com/hubfs/Peter%20Vogel%20Portait.png) Peter Vogel Peter Vogel is the founder of GrowPerma, bringing together evidence-based gardening advice with permaculture principles. When he's not writing about companion ... [Companion Planting](https://growperma.com/blog/tag/companion-planting) August 10, 2026 # Best Plants for Window Boxes: What Actually Grows Together 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. ## How Much Soil Does a Window Box Actually Hold? This number surprises people. According to the [Swansons Nursery soil calculator](https://www.swansonsnursery.com/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). 5 Gallons in a 3-Ft. Box 6 in. wide, 6 in. deep 6 in. Workable Root Depth Greens, herbs, strawberries 20 in. Pot Width One Tomato Wants Penn State Extension 1x Waterings Per Day Summer, Texas A&M AgriLife Put that next to what extension services recommend and the constraint becomes obvious. [UMass Extension](https://www.umass.edu/agriculture-food-environment/home-lawn-garden/fact-sheets/container-gardening-with-vegetables) 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: - Root depth requirements crop by crop, so you stop planting things that cannot fit - Which combinations genuinely work in a shared 6-inch root zone - What to plant by window aspect, north through west - Why garden soil ruins containers and what to fill the box with instead 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. ## What Root Depth Does Each Crop Need? ![Cutaway view of a window box soil profile showing shallow lettuce and thyme roots above deeper basil, parsley and tomato roots](https://growperma.com/hubfs/blog/images/gp-0437-body1-20260810-063824.webp) [Penn State Extension](https://extension.psu.edu/container-vegetable-gardening-four-keys-to-success/) sets the benchmark that matters most: a single tomato plant needs at least a 20-inch-wide pot, peppers and [eggplants](https://growperma.com/blog/what-to-plant-with-eggplant-in-your-summer-garden) 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](https://aggie-horticulture.tamu.edu/wp-content/uploads/sites/12/2011/03/E-545_vegetable_gardening_containers.pdf) 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](https://extension.psu.edu/container-vegetable-gardening-four-keys-to-success/), [Texas A&M AgriLife Extension](https://aggie-horticulture.tamu.edu/wp-content/uploads/sites/12/2011/03/E-545_vegetable_gardening_containers.pdf), [University of Maryland Extension](https://extension.umd.edu/resource/growing-vegetables-containers) 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. ## Which Plants Actually Grow Well Together in One Box? ![Hand pinching the top growth from a bushy basil plant growing in a window box beside parsley](https://growperma.com/hubfs/blog/images/gp-0437-float-left-20260810-063837.webp) 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](https://growperma.com/blog/companion-planting-mistakes-12-combinations-to-avoid) covers the pairings that genuinely conflict. Why This Works: Grouping by Need, Not by Lore [NC State Extension](https://content.ces.ncsu.edu/how-to-create-a-container-garden-for-edibles-in-the-north-carolina-piedmont) reduces container companion planting to one rule: group plants that have similar water and light requirements. That is the same logic behind [permaculture guilds](https://growperma.com/blog/permaculture-guilds-for-every-climate-zone), 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. ## What Should You Plant Based on Which Way Your Window Faces? ![Infographic showing recommended window box plants for north, east, south and west facing windows by hours of sunlight](https://growperma.com/hubfs/blog/images/gp-0437-infographic-20260810-063829.webp) 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](https://www.umass.edu/agriculture-food-environment/home-lawn-garden/fact-sheets/container-gardening-with-vegetables), [University of Maryland Extension](https://extension.umd.edu/resource/growing-vegetables-containers) 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. ## What Should You Fill the Box With? ![Hands filling a long window box with light potting mix on a balcony, with herb seedlings and a watering can nearby](https://growperma.com/hubfs/blog/images/gp-0437-body2-20260810-063825.webp) Not garden soil. [Illinois Extension](https://extension.illinois.edu/container-gardens/soil) 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](https://gardenprofessors.com/container-planting-intuition-vs-reality/) 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](https://extension.psu.edu/potting-media-and-plant-propagation/) 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](https://growperma.com/blog/container-soil-mix-diy-potting-soil-for-every-plant) breaks down the ratios. ## How Do You Keep It Alive Through Summer? ![Trailing creeping thyme and orange nasturtium spilling over the front edge of a weathered wooden window box](https://growperma.com/hubfs/blog/images/gp-0437-float-right-20260810-063844.webp) 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](https://extension.illinois.edu/blogs/good-growing/2022-04-08-helpful-tips-creating-successful-container-garden) 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. 1 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. 2 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. 3 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. 4 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. ## Frequently Asked Questions ### What are the best plants for window boxes in full sun? 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. ### What should you put in the bottom of a window box? 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. ### Can I use regular garden soil in a window box? 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. ### How often do window boxes need watering? 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. ### Can herbs and vegetables share the same window box? 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](https://growperma.com/blog/companion-planting-for-small-spaces-and-containers) goes deeper on this. Get the Weekly Dig One practical permaculture guide a week, written for gardeners who do this on weekends: permaculture design for real backyards. Free, no spam, unsubscribe any time. [Join the Weekly Dig](https://growperma.com/newsletter) [Browse All Guides →](https://growperma.com/blog) A 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](https://growperma.com/blog/food-forest-in-containers-balcony-scale-ecosystems) is the next rung up, and the [complete companion planting chart](https://growperma.com/blog/companion-planting-chart-the-complete-guide-for-every-vegetable) covers pairings once you have real space. ## Resources - [Penn State Extension — Container Vegetable Gardening: Four Keys to Success](https://extension.psu.edu/container-vegetable-gardening-four-keys-to-success/) - [UMass Extension — Container Gardening with Vegetables](https://www.umass.edu/agriculture-food-environment/home-lawn-garden/fact-sheets/container-gardening-with-vegetables) - [Texas A&M AgriLife Extension — Vegetable Gardening in Containers (PDF)](https://aggie-horticulture.tamu.edu/wp-content/uploads/sites/12/2011/03/E-545_vegetable_gardening_containers.pdf) - [University of Maryland Extension — Growing Vegetables in Containers](https://extension.umd.edu/resource/growing-vegetables-containers) - [NC State Extension — How to Create a Container Garden for Edibles](https://content.ces.ncsu.edu/how-to-create-a-container-garden-for-edibles-in-the-north-carolina-piedmont) - [Illinois Extension — Soil for Container Gardens](https://extension.illinois.edu/container-gardens/soil) - [Illinois Extension — Helpful Tips for Creating a Successful Container Garden](https://extension.illinois.edu/blogs/good-growing/2022-04-08-helpful-tips-creating-successful-container-garden) - [Penn State Extension — Potting Media and Plant Propagation](https://extension.psu.edu/potting-media-and-plant-propagation/) - [The Garden Professors — Container Planting: Intuition vs. Reality](https://gardenprofessors.com/container-planting-intuition-vs-reality/) - [Cornell Cooperative Extension — Container Gardening](https://cceniagaracounty.org/gardening/food-gardening/container-gardening) **Tags:** [Companion Planting](https://growperma.com/blog/tag/companion-planting) ## Related Articles ![Late autumn raised bed of frost-touched collard greens interplanted with sweet alyssum and a row of dill going to seed behind](https://growperma.com/hs-fs/hubfs/blog/images/gp-0548-hero-collard-bed-20260908-064751.webp?width=400&height=220&name=gp-0548-hero-collard-bed-20260908-064751.webp) 8 Sep [Companion Planting](https://growperma.com/blog/tag/companion-planting) #### Collard Companion Plants: What the Evidence Supports Peter Vogel ![Red seven-spotted ladybugs feeding on flowering dill and yarrow umbels in a late summer vegetable garden with cosmos and calendula behind](https://growperma.com/hs-fs/hubfs/blog/images/gp-0544-hero-ladybugs-on-dill-20260907-143702.webp?width=400&height=220&name=gp-0544-hero-ladybugs-on-dill-20260907-143702.webp) 7 Sep [Companion Planting](https://growperma.com/blog/tag/companion-planting) #### Plants That Attract Ladybugs (and What They Eat) Peter Vogel ![Autumn raised bed with a dense band of young arugula growing between rows of kale and carrot tops in golden morning light](https://growperma.com/hs-fs/hubfs/blog/images/gp-0525-hero-20260902-064850.webp?width=400&height=220&name=gp-0525-hero-20260902-064850.webp) 2 Sep [Companion Planting](https://growperma.com/blog/tag/companion-planting) #### Companion Planting Arugula: What to Grow Beside a Fast Fall Green Peter Vogel ![GrowPerma](https://growperma.com/hubfs/icon-512.png) Gardens That Work Like Forests — the smart-friend guide to permaculture for normal backyards. #### Explore - [Blog](https://growperma.com/blog) - [Companion Planting](https://growperma.com/blog/tag/companion-planting) - [Soil & Composting](https://growperma.com/blog/tag/soil-composting) - [Food Forests](https://growperma.com/blog/tag/food-forest) - [Permaculture](https://growperma.com/blog/tag/permaculture-foundations) #### Resources - [About](https://growperma.com/about) - [All Articles](https://growperma.com/blog) #### Legal - [Imprint](https://growperma.com/imprint) - [Privacy Policy](https://growperma.com/privacy-policy) - [Affiliate Disclosure](https://growperma.com/affiliate-disclosure) - Cookie Settings As an Amazon Associate I earn from qualifying purchases. 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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." }, "name" : "Can I use regular garden soil in a window box?" }, { "@type" : "Question", "acceptedAnswer" : { "@type" : "Answer", "text" : "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. 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