FORMULA SHELF
13 of 370
The arithmetic behind the work: what to measure, what to type in, and what the answer comes out as. Every formula says what the symbols mean and gives a worked example, so you can check it against a job you have already done.
SHELVES
Growing
13 formulas
Bed planning. Work out how many plants you want, divide the bed area by the number, and that is the space each one gets.
Brassicas at 60 cm: 1 / 0.6 x 0.6 = 2.8 plants a square metre, so about 0.4 m2 each.
For a watering can shift or a hose timer. A 10 L can covers about 2 m2 a day, and a hose sprinkler covers about 20 m2.
A 6 m x 2 m polytunnel bed at 5 L/m2 = 60 L a day, or six watering cans.
Work out the total area in square metres, then divide by the area each plant needs (row spacing x in-row spacing). This tells you how many plants to order for a plot or field.
20 x 10 m plot, rows 0.75 m apart, plants 0.3 m apart: 20000 / 0.225 = 88,888 plants.
How much compost to barrow across the plot. One wheelbarrow is about 60 litres, so a 10 m2 bed at 5 cm needs about 5 barrows.
A 2 m x 1.5 m bed at 5 cm: 3 m2 x 0.05 x 1,000 = 150 L, or about 2.5 barrows.
Ten minutes and it tells you whether a packet is worth sowing. Old or badly stored seed is the most common reason a row does not come up.
18 of 20 seeds sprout = 90% germination, so sow 10% more than the packet says.
Every bed plan starts with this. A 10 m bed with rows 30 cm apart has 33 rows, and at 25 cm in the row that is about 1,300 plants.
A 10 m bed, 30 cm rows, 25 cm plants: 10 / 0.25 = 40 plants a row.
Only worth it if you grow one variety at a time, or if the neighbours are not growing the same crop. Otherwise you will save a hybrid.
Saving brassica seed on a smallholding usually needs an agreement with the neighbours.
A plant that goes out too early loses a fortnight. Harden it off properly and it will lose a week of growth instead of a month of plants.
Last frost 15 May: plant out the tomatoes from 27 May.
The quickest way to plan a bed or a border. Onions at 10 cm square give 100 a square metre; potatoes at 30 cm give 11.
Carrots at 5 cm in 15 cm rows: 1 / (0.15 x 0.05) = 133 a square metre.
For beans, beetroot, carrots and onions sown in a drill in a bed. Sowing three seeds in a station and taking the best one is often more reliable.
Onions at 3 cm: 33 seeds a metre, and at 0.05 g a seed that is 1.7 g a metre of row.
The halving rule: seed loses half its life each year, so freshness doubles the years it lasts. Buy fresh seed for the crops that go over quickly.
Two-year-old onion seed is about half as good as new.
A cover crop is worth nothing if it is too thin to shade the soil and hold the rain. See Cover Crops.
A vetch and rye mix at 40 + 50 kg/ha gives a canopy in eight weeks.
How to keep a small bed productive all season. Sow little and often, and swap a fast crop for a slow one when the space frees up.
A 3 m2 bed of radishes at 5 cm squares gives 120 plants, or 40 a sowing for three sowings.
SOURCES & PROVENANCE
1 listed
This page is an AI-organised index. The label means the grouping and cross-links were compiled with AI; the information itself comes from the public-domain books, reports and manuals listed below. Open any source to read the local copy or follow the link to the original publication.