When to mow, water and cover the plants

The Home & Yard lens reads the forecast for the lawn and the garden. These are its rules, with the numbers.

What this lens tells you

Pick Home & Yard on the forecast page and the same weather gets read for yard work. The headline can say three things, in this order: whether a freeze is coming or a frost is possible tonight, whether to water, and when to mow. It says at most two of the three at once, so on a busy day mowing is the one that drops off.

Use it before you start the mower, set the sprinklers, or leave tender plants out overnight. When none of the three applies, it says "Nothing urgent in the yard. Pour something and look at it."

The rules below are ours. They read a forecast. They do not see your yard.

Open the forecast with the Home & Yard lens on.

Is it too wet to mow today?

We cannot see your lawn, so the page answers from the forecast. It looks for a mowing window: three hours in a row that pass every line of this table. It looks at today first, then tomorrow, and takes the first window it finds.

An hour counts as wet by the rule the whole site uses. When the forecast says rain, drizzle, snow, a wintry mix or storms, the hour is wet if the chance is at least 40 percent or the amount is at least 0.5 mm (about 0.02 inch). When the forecast says anything else, such as cloudy, it takes a chance of at least 60 percent and an amount of at least 0.3 mm (about 0.01 inch). In the United States there is one step before either rule: an hour the Weather Service gives a chance under 25 percent is read as cloud whatever word it uses, so it falls under the second rule.

When a window passes today and it opens within two hours, the headline says "Mow now, while it is dry." Early in the morning, that "now" can mean a window that opens at 10 AM. For a later window the headline says "Mow this afternoon after" or "Mow tomorrow after", with the hour the window opens. "This afternoon" is the page's word for any later window today, even one that opens in the morning. A window that falls today is also marked on the day chart as "Mow window", and the chart shows when it starts.

Late in the afternoon, when three full hours no longer fit before 6 PM, today is out and the page looks only at tomorrow.

When no window passes, and it is raining or a wet hour is in the next 24 hours, the headline says "Too wet to mow right now." It can say that on a dry afternoon, when today has run out of hours and rain is due tomorrow. When no window passes for some other reason, such as cold, heat or wind, the page says nothing about mowing.

Can I mow after rain?

By our rule, not right away. A mowing window cannot open until four hours have gone by with no wet hour, and then it needs three dry hours of its own. The thinking is simple: grass that was rained on this morning is still wet.

Four hours is our number. It is not a reading from your grass. Shade, long grass and heavy soil keep a lawn wet for longer, and the forecast sees none of them.

The page does not measure the rain that fell, either. For the hours already gone it reads a weather model's account of them, and a model can miss a shower over your street. A light shower that falls under the wet-hour lines above does not count at all.

Is morning dew too wet to mow?

The page does not know. This lens does not measure dew, and its mowing rule does not use the dew point or the humidity. All it has is a weather model's account of the rain that fell and a forecast of the rain to come.

What it does is open no mowing window before 10 AM, and say in the "Why we say that" box: "Grass is easiest to cut dry, once the dew has burned off." That sentence is the same every day. It is a rule of thumb, not a measurement. On a cool morning or a shaded lawn the grass can still be wet at 10 AM. Walk across it. Your shoes will tell you.

Is it going to frost tonight?

The page takes the lowest air temperature in the hourly forecast for the next 16 hours and holds it against two lines.

We put the frost line above freezing because the forecast is for the air, and grass and leaves can be colder than the air around them. The low the page used is printed in the "Why we say that" box as "Low tonight".

"Tonight" means the next 16 hours from whenever you look. Look in the morning and those 16 hours can run out before the coldest hour of the coming night. Look again in the evening.

This lens reads the air temperature and nothing else. Whether frost forms on your garden depends on things it does not read or cannot see: whether the sky clears and the wind drops, a low spot where cold air settles, what is under cover. So for frost the page says "possible", and it is no surer than that.

When the National Weather Service issues a frost advisory or a freeze warning, that is the official word, and it comes before anything this lens says. In the United States the page shows official alerts above the headline, as written. When it cannot check them, it says so. A freeze warning takes the puns off the forecast page. It does not change what this lens says, so read the alert first. Unless the Weather Service marks it as immediate, this lens still gives its answer under it. A warning marked as immediate, such as a tornado warning, takes over the headline in every lens.

Should I water the lawn today?

The page keeps two totals: the rain in the last 24 hours and the rain forecast for the next 24 hours.

• If either total is about 0.24 inch (6 mm) or more, it says "Skip the sprinklers."
• If both totals are under about 0.06 inch (1.5 mm), and the feels-like temperature reaches 85°F or more in the next 12 hours, it says "Water early. Dry and hot today."
• Anywhere in between, it says nothing about watering.

Both totals are printed in the "Why we say that" box as "Rain in the last day" and "Next day", so you can hold them against your own rain gauge. Both are a weather model's estimate for your area, from Open-Meteo. Neither is a measurement from your yard.

In the United States, if the page has no answer from Open-Meteo, it shows "Some data is late" and both totals read "a trace", whatever fell. Do not go by the watering line then.

That is all this lens knows about water. It does not know your soil, your grass, what the hose did yesterday, or whether your town has watering rules.

What to wear

The yard kit on the What to wear card is picked from the same forecast. The temperatures in this list are "feels like", averaged over the next six hours.

• On top. A T-shirt at 74°F or warmer. A flannel from 48°F up to that. A work coat below 48°F.
• On your head. A warm hat at 38°F or colder. Otherwise a straw hat on a sunny day, which to us means daylight, no rain or snow falling or due within eight hours, and a UV index of 3 or more in what is left of the day. Otherwise a cap.
• Always. Work gloves, jeans and boots.
• To carry. A water bottle at 80°F or warmer. A water bottle and sunscreen on a sunny day with a UV index of 6 or more. Weights for the bin lids when the wind in the next 12 hours reaches 20 mph, counting a gust at three quarters of its speed. If none of those applies, the list says "Something cold to drink after".

The kit calls the ground soft when about 0.16 inch (4 mm) or more of rain fell in the last 24 hours, or when rain or snow is falling or due within eight hours. That is a guess from rainfall. Nothing measured your ground.

The card prints a "Low tonight" of its own. That one is the lowest feels-like temperature in the next 12 hours, so it can differ from the low in the headline.

What the page does not know

• Dew. Nothing here measures it.
• Rain on your lawn. Every rain amount here is a weather model's estimate for an area. Nothing here is a gauge.
• Your soil and your grass. This lens does not know how wet the ground is, how it drains, or how long the grass is. The Ground & sky card on the forecast page can show a weather model's estimate of the soil. That is the model's account, not a probe in your yard, and this lens does not use it.
• Frost in your garden. The forecast is for the air over an area. It cannot see a low spot, a sheltering wall, or what you have covered.
• What your lawn needs. How much water, how short to cut and when to plant depend on the grass, the soil and the place. In the United States, a county extension office is the place to ask. Watering rules come from your water utility or your town.
• Warnings. Frost advisories, freeze warnings and every other official alert come from the National Weather Service. Follow weather.gov and your local officials before anything on this page. Official alerts are shown for the United States only. Elsewhere, check your own country's weather service.

See it for your place

Open the forecast with the Home & Yard lens on, search for your town, and these rules are run on its forecast.

An assistant can ask the same service about yard work, the best window for it, and frost. Its rules for yard work are kept in their own table and are close to these, not identical. How to connect one is on the page for agents.

The other lenses

• Run and ride. The best hour to get out, or a plain "treadmill day".
• Commute. Rain and ice at both ends of the working day.
• Hunt and fish. Pressure, light and storms.
• Build and finish. When a stain or a finish has the dry days it needs.
• Methodology. Where the data comes from, and where the page can be wrong.