It’s common to notice a white residue, green algae, or light mold forming on the top of your seedling’s grow medium—especially in indoor environments. All three are caused by a similar combination of moisture, light, and nutrients, and are generally easy to manage.
Hydroponic grow medium is designed to absorb water and nutrients as they cycle through your Farmstand. While most nutrients are taken up by the plant or recirculated, some can settle on the surface. This can result in:
- White nutrient residue: a harmless buildup of dried nutrients
- Green algae: forms when light interacts with moisture and nutrients
- Mold: can develop in areas with excess moisture and limited airflow
Algae and residue are common and not harmful to your plants, though algae can slightly slow root development by limiting airflow in the grow medium. Mold is also manageable with a few simple steps.
How to Treat Mold or Algae
Step 1: Spot Treat the Surface
Dip a cotton swab (Q-tip) directly into hydrogen peroxide and gently swab the top of the plug where buildup appears. (You can purchase this locally or find it in our Complete Plant Defense Kit.)
Step 2: Apply a Diluted Solution
Mix a solution of 1 part 3% hydrogen peroxide to 3 parts water (or 6 parts if using our 6% peroxide). Using a spray bottle, lightly spray the affected area about twice per week. Be sure to spray the grow cup or plug—not directly on the plant.
Tip: Always mix a fresh solution before each use, as diluted hydrogen peroxide breaks down and loses effectiveness over time.
Step 3: Improve Airflow + Adjust Watering
- If buildup persists, consider slightly reducing how often your system runs to limit excess moisture