The forecast promised showers. Your beds got two millimetres and the water butt stayed empty. Regional forecasts are a reasonable starting point, but they cannot tell you what happened on your patch of ground: how cold the soil fell on Tuesday night, how much rain actually landed in the gauge, or the morning your first ground frost arrived. That gap is what a garden weather diary fills. Keep one for a season and you stop guessing and start planning.

Why a diary beats a forecast

Forecasts are regional; gardens are local. A sheltered, south-facing plot against a brick wall can run a couple of degrees warmer than an open allotment half a mile away, and cold air draining down a slope will settle in the dip at the bottom of the garden. Those differences decide whether your courgettes survive a late May night.

After two or three seasons you have something no app can give you: your own averages. Your last frost date, your wettest month, the weeks your soil warms quickest.

What to record, and why each entry matters

Keep it simple. Three lines a day is enough to be useful; a page of notes you abandon by June is not.

• Rainfall in millimetres, read at the same time every morning. Monthly totals tell you how much watering your plot really needs.
• Maximum and minimum temperature in degrees Celsius. The overnight minimum governs when to harden off, close the greenhouse or throw fleece over seedlings.
• Ground or grass minimum — the temperature at soil level, where frost bites first.
• Soil temperature at sowing depth — the figure that decides whether bean and sweetcorn seeds germinate or rot.
• Frost yes or no, and whether it was an air frost or a ground frost. Note what it damaged.
• Wind direction and strength — useful for shelter, staking, and knowing which side of the garden dries out first.
• A one-line note on the day: "hail, shredded the lettuce", "dry for a fortnight", "warm, everything bolted".

Kit and siting: the simple setup

You do not need a weather station. A basic rain gauge, a maximum-minimum thermometer and a notebook will do more for your growing than any gadget with a screen.

• A rain gauge with millimetre markings, or a proper five-inch gauge if you want to compare with official records.
• A max-min thermometer that records the highest and lowest reading since you last reset it.
• A second minimum thermometer for ground frost, laid on bare soil or short grass with the bulb exposed.
• A soil thermometer, pushed in to roughly 10 cm.
• A notebook or spreadsheet — on paper, keep it in a plastic bag.

Where to put things

Site the rain gauge in open ground, away from walls, fences, sheds and overhanging branches. As a rule, keep it at least as far from an obstacle as that obstacle is tall. Set the rim around 30 cm above the soil so splash cannot get in, and check it is level.

Mount the thermometers in a shaded, north-facing spot about 1.2 m above short grass — the same standard used for official air temperature readings. Direct sunshine will push the numbers up and give you a false record.

Reading rainfall without fooling yourself

Consistency matters far more than precision. Same gauge, same spot, same time each day. Empty it every morning rather than every few days; in a hot spell, evaporation can quietly rob you of the very rain you are trying to measure.

Snow is the classic mistake. Do not log the depth of snow as rainfall. Bring the gauge inside, let the contents melt, then measure the water — that is the figure that matters for soil moisture.

It also helps to count dry days, not just totals. A fortnight without measurable rain tells you more about when to water than a monthly figure ever will.

Frost dates: the record that pays for itself

This is the heart of a UK weather diary. An air frost is recorded when the thermometer in its shaded screen falls below zero. A ground frost happens at soil level, and it can strike on a clear, still night when the air a metre above is still a degree or two warmer. Ground frost is what blackens potato shoots and tender seedlings, so it deserves its own column.

Each year, note two dates: the last spring frost and the first autumn frost. The stretch between them is your growing season, measured on your plot rather than in a book. Once you have three or four years of records, plant by the latest spring frost you have seen, not the average. It is the awkward year that costs you the crop.

Note where frost hits hardest, too. Cold air drains downhill and pools behind hedges and fences, so the bottom of the garden can be frostier than the top by a surprising margin. Move the tender crops, not the thermometer.

From records to planting decisions

A diary is only worth keeping if it changes what you do. Some of the ways it earns its keep:

• Delay sowing tender crops until your soil thermometer has held steady for a week, rather than trusting a date on the calendar.
• Use your latest recorded frost date to decide when tomatoes, courgettes and beans go outside, and keep fleece to hand for a fortnight afterwards.
• Match watering to the gauge. A wet month means the mains tap can stay off; a dry fortnight means water thoroughly and less often.
• Record what you sowed and when, beside the weather that followed. Next year you will know whether the failure was the seed or the cold snap.

Be honest about the bad entries. The page recording a late frost that killed your runner beans is more valuable than a summer of pleasant averages.

A five-minute routine that

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