Advanced options
Weight check FIRST: a watered 80cm box runs 15-20kg: brackets into masonry, and boxes on sills need restraint (wind is patient). Drainage holes are non-negotiable: crocks over them, compost with added grit. Window boxes dry faster than any border: daily water in summer, feed weekly from June. Trailing at the front, upright behind: the hanging basket's rules, in a rectangle.
What The Window Box Calculator Works Out
- Plants per box: the box length divided by the planting distance, rounded down. Bedding plugs go 15cm apart for the packed summer display (half the border spacing, because crowding is the container aesthetic), 20cm for a standard planting and 10cm for an instant-effect blitz, so the common 80cm box takes five or six plants.
- Compost: length x width x depth in centimetres, divided by 1,000 for litres. A standard 20cm x 18cm profile needs about 29 litres per 80cm box, so a pair of boxes is one bag of compost and a tray of plugs.
- The recipe: upright plants behind, trailers in front, exactly as a hanging basket but in a rectangle. Geraniums shrug off drought, petunias and lobelia do volume, and ivy is the permanent scaffold.
- Box width and compost depth (advanced): override the standard profile for a deep trough or a slim sill box. Deeper boxes hold water longer and suit herbs; anything under 15cm deep dries out daily in July.
- Rows of plants (advanced): boxes 25cm or wider take two rows, uprights at the back and trailers at the front, which doubles the plant count and the cost.
- Bags to buy (advanced): total litres divided by the bag size, rounded up. Multipurpose compost is commonly sold in 40, 50, 60 and 70 litre bags; the spare goes to topping up as the compost settles.
- Cost per planting (advanced): plants x price per plant plus bags x price per bag. Plug plants run GBP 0.50 to GBP 1.50 each in trays, potted bedding GBP 2 to GBP 4, and a 40 litre bag of compost GBP 5 to GBP 8.
- Cost per year (advanced): compost bought once plus plants for each replanting. A summer display is one planting; adding a winter swap of pansies and violas, or spring bulbs underneath, keeps the box working year-round for the price of the extra plants.
- Watered weight per box (advanced): wet compost weighs roughly 1kg a litre, plus the empty box and the plants. A watered 80cm box runs 15kg to 20kg, which is a falling hazard on a sill and a load calculation for the brackets, so use masonry anchors and restraint fixings.