Elevated planter box on a balcony: how deep, how high, how heavy

Size an elevated planter box for a balcony by crop depth first, then working height, then filled weight, in that order. Per University of Maryland Extension, 8 inches of mix is enough for lettuce, beans and cucumbers, and tomatoes and peppers want 12 to 24 inches. Legs set how high you reach, not how deep roots go, and every inch of depth you add makes the box heavier on a floor that has a limit. Start with the balcony weight limit if you have not yet checked what your balcony is rated for.

Pick the depth from the crop

A box standing on legs has nothing beneath it. Roots cannot reach down into the ground, so the box has to hold everything the plant needs. University of Maryland Extension says raised beds on hard surfaces are fine at least 8 inches deep for leafy greens, beans and cucumbers, and 12 to 24 inches deep for peppers, tomatoes and squash. Its caution comes with the figure: such beds dry out more quickly because roots cannot reach water in soil below the box.

University of Kentucky Cooperative Extension gives a slightly different split for containers. It calls 6 to 8 inches adequate for beets, broccoli, cucumbers, kale, lettuce, onions, radishes, spinach, Swiss chard and turnips, and puts green beans, cabbage, carrots, collards and peas at 10 inches or more. For its own patio herb planter it uses a box 12 inches wide, 48 inches long and 8 inches deep, planted with chives, garden thyme, basil, marjoram and summer savory.

Put those together and the choice is mostly binary:

• Salad, herbs, radishes, cucumbers: 6 to 8 inches by Kentucky’s figure, 8 inches by Maryland’s. This is the case where a legged planter earns its place, because it is light and easy to reach.
• Green beans, carrots, cabbage, peas: 10 inches or more, per Kentucky.
• Tomatoes, peppers, squash: Maryland’s range is 12 to 24 inches, and the plant may do better at the deeper end. Per-plant volume is a separate question from box depth; tomato pot size covers how much mix one plant wants, and one box will not serve several of them unless it is long as well as deep.

If you want only herbs and a few salad crops, a long, shallow box is a better fit than a deep one, and window box herb gardens cover the same 8 inch format on a rail.

What the legs actually change

Legs raise the working height. K-State Research and Extension describes raised beds ranging from 18 to 30 inches in height and up to four feet wide, as long as the gardener can bend and reach the middle from both sides. On a balcony, one long side is often against a wall or railing, so you reach from one side only. That argues for a narrower box than the four-foot ceiling, not a wider one.

K-State also points out that a table-height planter leaves clearance for the knees, which lets a seated gardener sit closer to the plants. K-State Research and Extension’s own legged planter has the cut edge of its barrel 29 inches off the ground for that reason. If kneeling or bending is what you want to avoid, build or buy at the height where your hands rest comfortably on the rim.

Two things the legs do not do. They do not add rooting depth, and they do not make drainage better by themselves. A box with a solid bottom and no holes holds water just as a pot would.

Weight is the real limit

The volume of a box is length times width times depth, and the weight follows the volume. A 4 by 1.5 foot box filled 12 inches deep holds 6 cubic feet of mix, about 45 US gallons. Water alone at that volume would weigh about 375 pounds, at the 8.34 pounds per gallon the balcony weight page uses. That is a ceiling for the water component, since the mix’s own particles take up part of the volume, so your real figure comes from weighing a gallon of your own soaked mix. The balcony weight calculator walks through exactly that, and it is the number to compare with your building’s documentation.

Two consequences follow. By simple arithmetic, going from 8 to 18 inches deep more than doubles the volume on the same footprint, so a box sized for tomatoes carries more than twice the load of a salad box of the same length. And the weight reaches the floor through the feet, not evenly across the whole box. A heavy, full box on four legs puts its load on four small contact points. Ask your engineer or building manager how concentrated loads are treated, and place the box near the wall rather than the outer edge, as the balcony weight page advises for heavy containers on a cantilevered balcony.

For scale, K-State’s legged barrel planter holds about 3 cubic feet of soilless mix. A box of 6 or 12 cubic feet is two to four times that.

Fill it with mix, not with soil

Soil that suits a ground-level bed does not suit a box with no ground beneath it. University of Kentucky Cooperative Extension says plainly that using only garden soil in containers is not recommended because it will not allow proper drainage. University of Maryland Extension fills beds on hard surfaces with compost and soilless growing mix in a 1:1 ratio, and adds topsoil only up to 20 percent by volume in beds at least 16 inches deep. A shallow balcony box falls below that line. Potting mix versus raised bed soil explains the structure difference and which bag to buy.

Do not put gravel or shards in the bottom to help drainage. Washington State University Extension states that planting containers must have drainage holes, and that drainage material added to containers will only hinder water movement. The mix goes all the way to the floor of the box. A gravel layer also takes volume you would otherwise fill with mix. Holes need to be generous in a long box: K-State’s legged planter has ten holes of half an inch along its lowest line. Drainage holes covers how to add them to wood or plastic safely.

Watering and heat on a hard surface

A box on legs dries out faster than the same mix in the ground. University of Maryland Extension notes that beds on hard surfaces are exposed to extra stress from absorbed and reflected sunlight and heat from surrounding surfaces, and advises checking the soil regularly and watering to keep the root zone moist. University of Kentucky Cooperative Extension says container soils can dry out very quickly on a concrete patio in full sun, and suggests moving plants or shading them in the hottest part of the day if reflection from pavement or a building is severe.

Metal and dark-coloured boxes add to this. University of Illinois Extension warns that intense summer sun can make a container hot, that the heat transfers to the soil, and that roots are damaged if it gets too warm. A pale or wooden box is the easier choice on a south-facing balcony. If the daily watering is what worries you, a DIY self-watering planter and a balcony watering system deal with that problem directly.

Check the wood before you buy

If the box is wood, the preservative matters when you grow food in it. University of Kentucky Cooperative Extension says wood used around vegetable plants should not be treated with creosote or pentachlorophenol (Penta). University of Maryland Extension adds that CCA-treated wood, which contained arsenic, has not been available for residential use since 2004, and advises avoiding CCA, creosote and Penta lumber. Oregon State University Extension adds that reusing or salvaging lumber over 20 years old may risk introducing CCA-treated lumber into your garden, so a free pallet or an old fence board is not a safe default.

Current pressure-treated lumber uses micronized copper azole. University of Maryland Extension describes it as low in toxicity, and cites Oregon State research finding that it raised soil copper only within 1 inch of the bed edge, with no increase in copper in the plants grown. University of Illinois Extension is more cautious: it advises caution with treated lumber when growing food, and suggests wood that resists decay naturally, such as cedar or redwood. If you want to avoid the question, Maryland lists a heavy plastic liner between the wood and the mix, with drainage holes cut in its base, painting the wood or choosing a non-wood material.

On concrete blocks, University of Maryland Extension says only that research data is limited and leaves the choice to you.

A short decision path

1. Pick the crop and read its depth from the table above.
2. Choose the shallowest box that meets it.
3. Choose the leg height from how you will work, 18 to 30 inches being K-State’s range.
4. Work out the filled weight and compare it with your building’s limit before buying anything.
5. Buy a soilless mix, drill generous holes, skip the gravel.
6. Choose food-safe wood or line it.

If the weight is the sticking point, a box that is too heavy can often be replaced by a hanging or wall-mounted format; vertical pocket planters cover that route, and compact vegetable varieties lets you grow more in a shallow box.

Sources

• University of Maryland Extension: Growing Vegetables in Raised Beds
• University of Maryland Extension: The Safety of Materials Used for Building Raised Beds
• Oregon State University Extension: Pressure-treated wood for raised bed construction in Oregon
• K-State Research and Extension, Sedgwick County: Wheelchair Accessible Gardening
• University of Kentucky Cooperative Extension, Daviess County: Vegetables Grown in Containers Improves Accessibility
• University of Illinois Extension: Container material choices
• Washington State University Extension: The Myth of Drainage Material in Container Plantings