5 Reasons Why Perlite Changes Color (Can Washing Help?)

The white color of perlite increases its applicability for indoor gardening by adding aesthetic value to every houseplant and succulent pot. But, isn’t it frustrating when your white perlite turns green, orange, yellow, or even brown? Back then, I listed potential factors that could be caused. When I started paying attention to these details, my white perlite stopped becoming discolored!

In general, perlite turns yellow to orange because of excess nutrients and water quality. It becomes brown to black due to soil mixing and turns green due to water-clogging and high humidity. Washing and soaking discolored perlite with diluted bleach for at least 20 minutes can help make it safe to reuse.

What are possible ways to stop the discoloration of perlite? Can washing help? What should I use? Bleach? Hydrogen peroxide? Well, the first tip that I will give you today is to read on! I’ve got a mountain of tips as we stride forward!

1. Excess Nutrients

Perlite can become orange or yellow because water evaporates. Indeed, the excess nutrients such as iron, calcium, and magnesium tend to remain at the surface of perlite given such hues.

In the context of vegetable and herb gardening, adding nutrients is helpful to nourish your plants with sufficient tools to boost their productivity. If you are using perlite as a growing medium (or even a portion of it), the possible discoloration may be a concern for you.

But here is something that I need you to know about—having orange and yellow discoloration is normal and will not be detrimental to your veggies and herbs!

For instance, if you are using perlite as a growing medium for hydroponics, expect that at some point it will turn yellow or orange. This is because, in a hydroponic system, your growing medium is submerged in water continuously.

So, when will this perlite discoloration be a huge issue?

If the point of your garden is aesthetics, this will be irritating. For example, if you are growing indoor succulents or houseplants, it is not pleasing to see some yellowing in your growing medium. Yes, it might be another possible point for conversation, but you do not want your visitors interrogating you about that orange or yellow thing on your pot right?

But whenever you reach that point, here is something to say.

The orange discoloration of perlite might be from excess iron.

Iron is the element that is naturally associated with rust, which is commonly orange in color. If you are growing in soil, it is possible that your soil has a high iron content.

The same thing can happen with iron-rich water. Thus, I suggest using distilled or filtered water instead of tap, soft or hard water.

The yellow discoloration of perlite might be from excess calcium and magnesium.

If you are adding these nutrients in your water, there is a huge tendency of yellow discoloration on perlite.

When you use this solution frequently, remnants of these nutrients will remain on the perlite surface when the liquid part of it evaporates. This happens quickly especially under intense heat or light.

Pro Tip for Houseplants: If you are both adding nutrients in your garden water and using grow lights, it would help to 1) water only every 3 days, and 2) lessen the light exposure as it increases heat and promotes quick evaporation.

Let us talk more about water quality in the next section.

2. Water Quality

When perlite turns yellow or orange, it might be because of poor water quality. Using tap, soft, and hard water is not ideal if perlite is a component of one’s growing medium. These water types carry minerals and salts that can interact with the surface of perlite and change its color.

We had an aquarium when I was a kid, and one of the most interesting things I noticed was the color fading off the vibrant pebbles. I remember asking my mom once “why are our pebbles yellow now?” With very factors around the aquarium affecting pebbles, I realized that it might be because of the water

True enough, little me was right. Water quality is a factor that could cause stone discoloration. Let us take this discussion one step higher by discussing 3 water types.

Perlite Discoloration Due to Tap Water

Chlorine, fluoride, and other elements in tap water can accumulate on the surface of perlite. These nutrients in the water are measured by TDS or total dissolved solids.

In the US, tap water has a TDS of 350 ppm. Although this concentration is safe for human consumption, using it with perlite will still result in yellowing.

Pro Tip: If you’re not confident about the quality of the tap water in your area, attaching an activated carbon filter is a simple and effective way to decrease the TDS level. The activated carbon filter physically strains water, resulting in relatively pure water output.

Perlite Discoloration Due to Soft and Hard Water

Soft and hard water are labeled according to the concentration of the minerals calcium and magnesium. These minerals are more concentrated in hard water than in soft water.

Although these minerals are highly soluble, cases of buildup still happen when there is an increase in temperature. Thus, it is also important to control the temperature that may come from the room growing conditions and even LED grow lights.

Pro Tip: When watering perlite-grown plants, use distilled water rather than tap water. This is because the process of distillation eliminates naturally occurring residues like minerals, preventing perlite discoloration.

   Explore more water types in our article explaining different water types. 

3. Soil Mixing

The surface color of perlite becomes brown to black overtime when mixed with other growing mediums such as garden soil, peat moss, or vermicompost.

Again, let me clarify this—this discoloration is not harmful to your plants. In fact, this is quite normal for perlite in soil mixes. When this happens, you can just wipe the brown/black color off the perlite since it is just on the surface.

The yellow to brown discoloration might be due to tannin leaching from peat moss.

One of the most prominent mixes for perlite is peat moss. They are commercially available in a 4:1 ratio for peat moss and perlite. When peat moss is moistened, it excretes tannins.

Fun Fact: Tannic acid is the phytochemical component that is responsible for the low pH of peat moss. This pH makes peat moss an ideal substrate for acid-loving plants such as pothos, daffodils, and magnolias.

The yellow and/or brown staining of the perlite on its surfaces is due to the continuous fresh air exposure. Under this condition, tannins darken and discolor any object they come into contact with.

4. Water-Clogging

The green color on the surface of the perlite is a result of water-clogging. Environments with high moisture, light access, and nutrients are breeding grounds for algae. This makes hydroponics the number one type of gardening affected by this problem. When they proliferate, algae can compete with the resources needed by plants.

Algae growth is aided by a combination of sunlight and nutrients. In the context of hydroponics where the perlite is submerged in water, there are higher chances of algae infestation.

There are ways to prevent green algae spots on perlite including, but not limited to: providing shade for the growing medium, adding diluted hydrogen peroxide to the water, and cleaning the whole hydroponic system.

1. Provide Shade

Cover every possible spot so that sunlight does not reach your nutrient water. You can accomplish this by:

1. Making use of an opaque nutrient water reservoir, pipes, and tubing
2. Covering the growing medium with perlite, pebbles, or be inventive and use dark garments as a medium cover instead.

2. Add Diluted Hydrogen Peroxide

When algae grows in difficult-to-reach areas in your hydroponics (for instance, the bottom part of the perlite in the net pot), you can mix food-safe hydrogen peroxide and circulate it throughout your system.

To do this, dilute 3 mL of it per gallon of nutrient water once a week to prevent algae infestation.

3. Clean The System

If the system is highly infested with algae, the best thing to do is to clean the whole unit. You can also replace the perlite or wash them (more of this below!).

5. High Humidity

The green discoloration of perlite can be attributed to high humidity as it favors the growth and reproduction of algae on the surface of perlite.

Pro Tip: To lower relative humidity, you can use a dehumidifier like the one below on Amazon. Another way is to grow houseplants like Calathea and Anthurium that absorb humidity.

If you only have a small space that you need to dehumidify, the product below might be the best option for you.

For bigger rooms or indoor gardens, here is a product on Amazon that might help you.

Can Washing Perlite Counter Discoloration? (3 Options)

Washing perlite with diluted bleach can both counter discoloration and disinfect the perlite for reuse. For optimum results, dilute 7 teaspoons of bleach per gallon of water. Other options for disinfection are hydrogen peroxide and vinegar.

   Learn more about this in our article hydrogen peroxide vs bleach.  

To use food-grade hydrogen peroxide, dilute 2-3 teaspoons of it per gallon of water. This option is really an effective option, but the downside is its expensive price.

Conversely, if you are on a tight budget, you can also use vinegar as is. However, a disadvantage would be its really strong odor.

This is the reason why bleach is preferred more than either hydrogen peroxide or vinegar—is because it is both more effective and cheaper.

How to Properly Wash Perlite (5 Easy Steps)

Washing perlite entails the following steps: 1) collection, 2) soaking, 3) rinsing, 4) draining, and 5) storage or use.

You will need the following materials to wash perlite:

• Bucket
• Net bag
• Garden hose (running water will do)
• Bleach

To clean your discolored perlite, follow the steps below.

1. Collect the perlite from your pot.
2. Place them in a net bag. Make sure that the net bag holes are smaller than the particle size of your perlite.
3. Soak the perlite in the diluted bleach water (7 teaspoons of bleach per gallon of water) for 20 minutes. Note: If the perlite has really bad discoloration, soak them overnight.
4. While still being inside the net bag, rinse the perlite well using a garden hose or just running water.
5. Let the perlite sit for a day to drain well and dry out.
6. Store them in bigger net sacks or use them again in your garden.

FAQs

Is perlite always white?

Perlite’s color ranges from white to light gray. This color is brought by trapping tiny bubbles during perlite formation. Under extreme heat, the lava rock obsidian pops just like popcorn, thus its styrofoam-like texture and characteristics.

Can molds grow in perlite?

Yes, molds can grow in perlite under heavily moist conditions. However, this problem is more common in perlite mixed with peat moss, coco coir, and garden soil because they can carry microorganisms like molds due to their organic forms.

Does perlite expire?

Because perlite is inorganic, it has no expiration date and does not degrade over time. This means that perlite can be used and reused numerous times without going bad.

Summary of 5 Reasons Why Perlite Changes Color

Perlite can turn to yellow, orange, green, brown, or black due to environmental factors that affect its surface. These are excess nutrients, inferior water quality, soil mixing, water-clogging, and high humidity.

To prevent perlite discoloration, one can lessen the watering schedule to every 3 days, lower light exposure, install a filtration system, use distilled water in watering, clean the system, and put a dehumidifier near the plants.

Once the perlite has already changed colors, one can simply wash it using diluted bleach water. This can be done by collecting the perlite from the pots, soaking them in bleach water for 20 minutes, rinsing them well in water, and draining them overnight before reuse.

Sources

• “Distillation For Home Water Treatment” by Kamrin et al in Purdue University
• “TDS and pH” by n/a in Safe Drinking Water Foundation
• “Water Softening” by Swistock, B. in Pennsylvania State University