August 10, 2026

Got a Rainwater Tank? Here's How to Know Which Beds Actually Need It

A rainwater tank feels like free water, so it's easy to treat it as free-to-waste — fill the can, walk the beds, tip it out, repeat every couple of days. But the tank isn't the hard part. The hard part is knowing which day the garden is actually dry, and which bed needs the stored water while the one next to it is still wet from Tuesday's shower. That's the question the tank-and-pump listicles never answer.

The tank is the easy half

Every top result for a rainwater tank sells you the tank, the pump, the first-flush diverter. All useful. None of it tells you the thing that decides whether the tank pays off: when to spend the water, and where. A tank you empty on a habit isn't saving water — it's just moving mains-versus-stored around. The savings show up when you skip the beds that don't need it.

And beds don't dry evenly. A raised bed in full sun runs dry days before a shaded border. A slope sheds water the flat plot holds. One garden-wide schedule pulled from an article treats all of that as if it were the same patch of dirt, and it isn't.

Don't spend the tank right before it rains

Siltwell gives the whole garden one plain-language call — deliberately garden-level, a single verdict for the plot rather than per-bed:

• Hold. Real rain — about 5 mm or more — is in the forecast for roughly the next day or two. Leave the tank alone; the sky is about to fill your beds for you.
• Covered. About 5 mm or more already fell in the last three days. The ground is still carrying that water, so wait before you draw down the tank.
• Water. No meaningful rain has landed and none is coming. Now is when your stored water earns its keep.

The 5 mm line is the honest cutoff: a dusting that barely darkens the soil doesn't count, and Siltwell won't pretend it does. If you've connected a weather station, this reads that station's own rain gauge — the millimetres that actually fell on your yard — alongside the Open-Meteo forecast for what's coming. Without a station it still works, but it falls back to the forecast alone, which is a coarser signal for something as patchy as rain. Given that rain is exactly what fills your tank, letting the same gauge decide when to empty it is a nice closed loop.

Which bed, not just which day

The hold/covered/water call keeps you from wasting the tank the day before a downpour. But it won't tell you the sunny corner is parched while the shaded border is fine. That's a per-bed question, and it needs a per-bed sensor.

Map a buried capacitive soil probe to a bed and Siltwell sends a "this bed is low" alert when the bed's two most recent readings are both below its threshold — the default is 25%, which you tune per bed once you've watched how that bed actually dries out. Two readings, not one, so a single odd blip doesn't trigger a false alarm. Instead of walking the plot and guessing, you know which bed to carry the can to and which to skip.

That's the plain-language version of a dry alert: water this bed, skip that one. It's also the difference between a tank that stretches through a dry spell and one you drain in a week.

What Siltwell does — and doesn't — do here

Here's the honest boundary. Siltwell tells you which bed is dry and when to hold off for rain. It does not open a valve, run a pump, or control your irrigation — there's no automation on the watering itself. It alerts; you water, whether that's from the tank or the tap. It replaces the guessing, not the watering can. If you want the fuller picture on timing, the guide to when to water covers the root-zone side of the same question.

Your yard runs its own weather

The reason "read your own yard" beats a schedule isn't only that rain is patchy — it's that your garden runs its own microclimate. A sheltered bed holds moisture; an exposed one dries fast. A low corner sits cooler on a still night. Averages for your postcode smooth all of that away.

Curious before you buy hardware? The public microclimate checker compares the forecast low to what nearby home stations recorded over recent nights and tells you how much cooler or warmer your area tends to run — an area indicator, not a guaranteed reading for your exact beds.