For most urban planter boxes, the best mix is a lightweight, moisture-retentive potting mix rather than garden soil. A reliable starting recipe is 55–60% high-quality peat-free potting mix, 20% mature compost, 10–15% coconut coir, and 10% perlite or pumice. If your box is very exposed to sun and wind, add a little more coir or compost, but keep perlite for drainage. Avoid garden soil alone, as it compacts in containers and can become either waterlogged or bone-dry. A small amount of biochar or water-retaining granules can help, but do not rely on them; good structure and mulch matter more. After planting, apply 5–7 cm (2–3 in) of organic mulch, such as bark chips or straw, leaving a gap around stems. Water deeply until it runs from drainage holes, then check the mix 2–3 cm below the surface before watering again. Self-watering inserts or drip irrigation are useful in hot urban sites. The goal is a mix that holds moisture like a wrung-out sponge but still drains freely, so roots get both air and water.
What soil mix works best in an urban planter box to keep plants from drying out?
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