What's the best way to clean algae or moss off a concrete planter box?

I’ve been tending to my concrete planter boxes for years, and I’ll be the first to admit—I used to battle green invasions like a frustrated knight with a bucket of bleach. But after accidentally killing a few plants and staining my favorite terracotta-toned box, I learned the hard way that not all cleaning methods are created equal. So, let me tell you what actually works, from someone who’s scrubbed, sprayed, and even cried over stubborn moss.

Step 1: Don’t Panic—And Don’t Reach for Bleach

The first thing I learned: never use harsh chemicals like bleach or ammonia, especially if your planter still houses plants (or will soon). These chemicals seep into porous concrete and can poison roots for months. Instead, embrace the power of patience and natural tools.

My Go-To: White Vinegar + Water + A Stiff Brush

For light to moderate algae or moss, mix equal parts white vinegar and water in a spray bottle. Wet the affected area, then scrub with a stiff-bristled brush (an old broom works wonders for large surfaces). The vinegar’s acidity breaks down the green film without damaging the concrete. Let it sit for 15 minutes, then rinse with a garden hose.

For Stubborn Growth: Baking Soda Scrub

When the moss feels like a carpet—thick and unyielding—I sprinkle baking soda directly onto the damp surface, add a few drops of dish soap, and scrub with a coarse sponge. Baking soda is mildly abrasive, so it lifts the gunk without scratching the planter’s beauty. Rinse thoroughly afterward; you can even pour boiling water over the area to kill any remaining spores.

The Pressure Washer Trick (Use with Love)

If you have a pressure washer, set it to low pressure (1,200–1,500 PSI) and keep the nozzle at least 12 inches away from the concrete. I use this only for empty planters, as the force can dislodge soil. It’s satisfying, but be gentle—concrete can chip if you get too close.

Prevention = Less Cleaning

Here’s what changed my life: I started applying a concrete sealer twice a year. It creates a barrier that makes moss slip off like water off a duck’s back. Also, place your planter in a spot with morning sun, not all-day shade. Algae and moss are like introverts—they love dark, damp corners.

A Final Personal Note

Cleaning algae off concrete isn’t about making it “perfect”—it’s about honoring the relationship between your plants and their home. I now look at a little moss as a sign of life, not a mistake. But when I want that crisp, clean look, vinegar, baking soda, and a gentle scrub always have my back. Your planter will thank you, and so will your plants.