Rainwater for Cannabis Plants: Collecting and Using Rain Barrel Water

Rainwater works well for cannabis because it is soft and low in dissolved salts. Collect it in a screened, covered rain barrel under a clean roof, and divert the first flush off the roof. Keep the barrel clean, and apply the water to the soil at the base of the plant rather than over the leaves or buds.

Is Rainwater Good for Cannabis Plants?

For most outdoor and greenhouse growers, yes. Rain falls with very little dissolved mineral content, so it doesn’t leave the white crust that hard tap water can build up in pots and on soil surfaces over a season. Every gallon you collect is a gallon you don’t have to draw from the tap.

If you build your own mix, see our super soil guide. If you adjust pH when feeding, see how pH affects cannabis plants.

Rainwater doesn’t change the basics of watering. Plants still need deep, infrequent watering rather than constant sips, and our guide to watering cannabis plants covers the rhythm. Too much water of any kind causes trouble, and our overwatering guide explains how to spot and fix it.

How Do You Set Up a Rain Barrel for a Garden?

A basic setup has five parts:

• a downspout feeding a first-flush diverter
• a screened inlet on the barrel lid to keep out leaves and mosquitoes
• the barrel itself, usually 50 to 60 gallons
• an overflow hose that sends extra water away from the house foundation
• a spigot near the bottom

Set the barrel on cinder blocks or a sturdy stand so a watering can fits under the spigot. The extra height also adds a little water pressure. The EPA’s Soak Up the Rain program and Penn State Extension both have setup guides with photos.

Even a small roof collects more water than most people expect. The University of Nebraska–Lincoln Extension guide G2220 uses a simple formula: roof area in square feet × inches of rain × 0.623 = gallons.

That is why the overflow hose matters. One good storm can fill a barrel several times over.

What Is a First-Flush Diverter?

The first rain to come off a roof carries much of the dust, pollen, leaf bits and bird droppings that built up since the last storm. A first-flush diverter is a short pipe chamber on the downspout. It fills first and holds that early water. Once it is full, cleaner water flows on into the barrel. A small drain hole lets the chamber empty slowly so it is ready for the next rain.

How Do You Keep Rain Barrel Water Clean?

Start with a clean roof and a screened barrel. Keep these habits:

• Clear the gutters in spring and fall.
• Keep the lid closed and the screen intact.
• Use an opaque barrel. Light inside a clear barrel encourages algae.
• Empty and rinse the barrel once a year, ideally in late fall before freezing weather.
• In cold areas, disconnect the barrel for winter so ice doesn’t split it.

How Should You Store and Apply Rainwater?

Use stored water during the growing season, and keep the barrel in shade if you can.

Apply it to the soil, not the plant. Pour slowly at the base of the stem and let it soak in, keeping the leaves and buds dry. That is the practice Rutgers Cooperative Extension and UConn Extension recommend for harvested rainwater in gardens.

A rain barrel fits naturally into a low-input garden, along with the ideas in growing weed naturally and building an eco-friendly grow room. If you are still choosing a spot, our guide to the best places to grow outdoors is a good place to start.