Is Too Much Perlite Bad for Plants? (Tips on How to Use It)
Perlite is a popular substrate used by gardeners indoors, outdoors, and in hydroponics. I’ve spent so much time in the garden with perlite that I can consider it an awesome friend. As an avid gardener, I’ve spent a significant amount of time experimenting with various growing mediums, so I’ll show you what sets perlite apart.
Perlite is generally used as a growing medium amendment in raised beds, hydroponics, and container gardening. The ideal perlite-to-soil ratio is 1:4. Too much perlite will cause rapid water drainage, which will be detrimental to plants because they will have less time to access water.
One of the main aspects of perlite that has caught my interest over the years is its application outside—in a raised bed garden. Is this a common practice among gardeners? This is a question that will be addressed before the end of this article, so keep reading!
Can You Add Too Much Perlite to Soil?
An excessive amount of perlite in the soil is undesirable because it shortens the time it takes for water to drain. When this happens, roots have less time to absorb sufficient water resources that help them transport nutrients throughout the plant.
The Perfect Perlite Ratio: One of the most widely accepted “ideal” perlite-to-soil ratios is 1:4. You can do this with a cup, as shown in the illustration below, or you can use a shovel or pot if you are mixing in bulk.
So if you are using perlite either in a gallon, a raised bed, or a pot, take note that having 20% perlite is an ideal mix.
In the following sections, we will delve deeper into the applications of perlite, so grab a pen and paper and take notes! I’ll be giving out fantastic advice!
How is Perlite Used in Gardening? (3 Horticultural Uses)
Perlite is used in gardening as an amendment to improve soil structure, a growing medium for hydroponics, and as a substrate for root cuttings and seedlings.
1. Soil Amendment to Improve Drainage
Perlite is a material that is commonly used to improve soil structure, water retention, and air space. This is due to perlite’s high water holding capacity and capillary action. Its large particle size is used to improve drainage by providing spaces. It’s commonly combined with other growing mediums like garden soil, coco coir, and peat moss.
You may be wondering, “how does perlite improve drainage?”
The answer is in its large particle size. Consider marbles in a jar. When you run water through them, you will notice that it flows easily due to the small spaces between each marble.
The same thing goes with perlite. Because it has bigger particles, unlike sand or loam soil, it can create those little spaces where water and air can pass freely.
This is also the main reason why it is combined with more compact growing mediums such as loam soil, coco coir, and peat moss.
2. Growing Medium for Hydroponics
The porosity of perlite makes it suitable for use as a hydroponic growing medium. It is commonly used to anchor hydroponic lettuce, tomatoes, and strawberries while also providing air space for the plants’ roots.
Perlite’s ability to balance out the water and air spaces makes it a good growing medium for hydroponics, wherein systems are usually submerged in water for a long time.
Explore more on growing mediums in our article about how to choose a hydroponic medium.
3. Seedling and Rooting Substrate
Perlite, specifically the fine grade, is used as a substrate to start seedlings and grow root cuttings. The air space provided by perlite is advantageous as it gives the plant’s roots a space to breathe and grow.
This is usually done on seedling trays before transplanting. An advantage of perlite for starting seedlings and root cuttings is its neutral pH.
When plants are exposed to neutral pH, they are less stressed in adjusting to extreme pH levels and can devote all of their energy resources to initiating roots and leaves.
As we dig deeper into perlite’s characteristics, the following section will reveal three of its characteristics that influence its use in gardening.
3 Perlite Features That Affect Its Usage
Perlite’s use in gardening demonstrates its features such as being lightweight, reusable, and inorganic.
Learn more about this in our article on perlite facts.
1. Lightweight
Perlite is a low-density growing medium. It floats in water and can be blown away by air. Given this, it is necessary to combine it with other growing mediums when used with outdoor plants that are frequently watered.
As seen in the illustration below, a cup of perlite only weighs around 19 grams!
So some may ask…
Can Perlite Be Blown by Air?
Perlite is usually compared to styrofoam balls. It also moves like styrofoam balls. This occurs because perlite is produced by a popping process when water enters a volcanic rock.
If you want to visualize this in a real-life example, consider cooking popcorn! It is just like that!
Does Perlite Float in Water?
Because perlite is a lightweight growing medium, it tends to float when watered. Perhaps you’re wondering how you can exert control over this. The best way to achieve balance is to combine perlite with other growing mediums such as garden soil or peat.
How to Mix Perlite
Because perlite is lightweight, it must be mixed with other heavier growing mediums. For you to establish why, imagine this with me. Consider using styrofoam balls as your sole potting medium. What happens if the wind blows? What happens when you water them?
They will fly away. They will float and scatter in your garden. The same thing will happen if perlite is used as the sole growing medium in a pot.
Sections back, I discussed using perlite with garden soil. Now, let us look at how to use perlite with peat.
Pro Tip: A 1:1 ratio of perlite and peat would be ideal. This mixture should be placed halfway through the pot, with peat filling the remaining space. (I will reveal more tips on mixing perlite later!)
This strategy would fit plants that require daily watering. Therefore, if you are growing a cactus that is not frequently watered, the floating issue may not be a big deal. However, for hydroponic systems that are submerged in water for extended periods, this may be a concern—so using a perlite mix is the best option.
2. Reusability
Since perlite is inorganic, it has no expiration date. Perlite does not biodegrade. This means that perlite can be used and reused repeatedly, some perlite can last for years.
Now, how is this advantageous for a gardener?
For outdoor gardeners, this means perlite will not decompose in your garden bed or pots. While for indoor gardeners, you can reuse it over the years. Thus, making it a good investment.
3. Sterility
The sterility of perlite as a substrate is one of its advantages. This means that they will not be exposed to pathogens during the early stages of plant growth. This is the reason why perlite is used as a seedling starter and root cutting medium.
Pathogens are one of the biggest headaches for every gardener. Once your plants are affected in their early stages, they could carry that while growing up.
Unlike perlite which is sterile, soil and other mediums can harbor pathogens that could be detrimental to your garden!
Using Perlite Outdoors
Perlite is mixed with garden soil and used in raised vegetable beds as a soil amendment. It can also be used in outdoor potted plants in a 1:4 perlite-to-soil ratio.
If you are planning to use perlite in your raised bed, I summarized the suggested perlite requirement in the table below to save you time in calculating.
| Amount of Perlite (gallons) | Space |
|---|---|
| 2 to 3 | per cubic meter |
| 0.10 to 0.15 | per cubic foot |
| 1.5 to 2 | per cubic yard |
As you may have noticed, mixing perlite is the only approach that we have discussed thus far in this article.
You may be wondering…
Can you use perlite as mulch?
The simple answer is no, perlite cannot be effectively used as mulch. Primarily, this is because perlite is a very lightweight material. As a result, if it is used as mulch for outdoor gardens it will easily fly away.
Using Perlite Indoors
For indoor gardening, perlite is often used as a sole medium for succulents. Meanwhile, it is mixed with other growing mediums for houseplants and hydroponic plants.
For an easier view, I have summarized the recommended amount of perlite per growing medium in the table below.
| Indoor Gardening | Perlite to Other Growing Medium Ratio |
|---|---|
| Succulents | 100% Perlite |
| Potted Houseplants | 20–30% Perlite : 70–80% Soil |
| Potted Houseplants | 50% Perlite: 50% Peat Moss |
| Hydroponics | 50% Perlite: 50% Coco Coir |
Let us use the soil brand below as an example.
How to Use Perlite With Fox Farm’s Happy FrogⓇ Potting Mix
- Fill the bottom 1 inch of the pot with perlite.
- In a separate container, mix 1 part perlite with 4 parts of the Happy FrogⓇ potting mix.
- Use the 1:4 potting mix to fill the pot, leaving an inch above.
- Water the potting mix.
- Let it settle before planting/transplanting.
Important Note: The 1-inch allowance above the pot is a safety precaution against floating perlite. Perlite may float when plants with this mix are watered, but because there is an inch allowance, none of the perlite will spill. Instead, it will still return to the soil once the water is absorbed.
Where Can You Buy Perlite?
Amazon has a large selection of perlite products that one can buy in bulk or small quantities.
In Bulk
GROW!T perlite by HydroFarm is available on Amazon in 4 grades (#2, #3, #4, and #8). With this option, you have a wide array of grade choices—from fine to extra course—depending on which is best for your garden.
Viagrow horticultural perlite on Amazon is available in 1-pack, 2-pack, and 30-pack options, thus, if you are developing a huge garden, this option might be the best for you!
In Small Amounts
This Harris horticultural perlite comes in an 8-quart package and is available on Amazon. Gardeners patronizing this product praise its versatility as it is a medium grade, therefore, you can use this in cuttings, soil, and even hydroponics.
If you want an excellent choice for water retention and aeration in your outdoor vegetable and herb garden and houseplants, this coarse and chunky perlite by xGarden might be a fantastic choice.
If you are curious about other growing mediums, head to our article perlite vs vermiculite.
FAQs
Should you soak perlite before using it?
There’s no need to soak perlite before using it in your garden, but dampening it will help gardeners to avoid inhaling the dust that comes with it. Remember that inhaling the dust of perlite might trigger some allergies, most especially for people with highly sensitive noses.
Will perlite hurt earthworms and snails?
Perlite is not dangerous for your earthworms and snails. This is because it is an inorganic material, just like pebbles, which are naturally occurring in the soil. Even with the presence of perlite, these organisms can still fulfill their ecological purposes.
Summary of Using Perlite in Your Garden
Perlite must be used in a controlled ratio because too much will quicken the drainage rate and will not allow the water to be properly absorbed by the plants. Thus, a perlite-to-soil ratio of 1:4 is a good choice for general gardening.
Common uses of perlite in gardening include being a soil amendment, hydroponic growing medium, and seedling and rooting substrate. This is because it is a lightweight, reusable, and sterile growing medium.
Furthermore, perlite can be used as an amendment in raised beds for outdoor gardening, but it is not suitable as mulch. Indoors, it can be used as a sole medium for succulents; however, for houseplants and hydroponics, it is best to mix it with other growing mediums such as peat moss, soil, and coco coir.
Sources
- “Inorganic and Synthetic Organic Components of Soilless Culture and Potting Mixes” by Papadopoulos, A.P. et al. in Soilless Culture
- “Perlite Gradation and Peat/Perlite Mixtures” by Matkin, O.A. in The Schundler Company
- “The Water-Holding Capacity of Perlite” by Perlite Institute in Hess Perlite
