Choosing the wrong growing medium will stunt the growth of your plants and even kill them? What is s growing medium in the first place and which one is best for your plants?
A growing medium is a material where plants develop their roots and grow either at ground level or inside a container. The growing medium regulates watering, aeration, and nutrient intake in plants. There are dozens of different growing mediums that can be used alone or combined. The best growing medium needs to match the water and nutrient requirements of the plant.
There are dozens of growing mediums out there. Should you go for soil or a soilless medium? What should you choose among the 10+ available in the market for your plants in a greenhouse? These and more are in the next section.
Table of Contents
- 1 The 12 Plant Growing Medium
- 2 Growing Medium: The 2 Main Types You Can Find in the Market
- 3 Best Growing Medium for Herbs?
- 4 Gardening Soil as Growing Medium in Containers?
- 5 Best Growing Medium For Seedlings and Rooting
- 6 How to Grow Cuttings?
- 7 The Hydroponic World: Water Only
- 8 The Three Soil Type: Sand Silt and Clay
- 9 Related Questions
- 10 Further Readings
In this section, I am going to introduce you to all the different materials that you might find in a growing medium. This is absolutely vital if you want to understand the property of the commercially growing medium you can see in the market.
It is essential to point out that most of them can be growing mediums on their own. However, quite often, 3 or more are mixed together in order to improve the overall performance of the medium so obtained.
The most common growing mediums for plants are:
- Perlite and Pumice
- Peat Moss or Coco Coir
- Sand, silt or clay
- Outdoor garden soil
- Wooden Bark
What is perlite?
Perlite is officially called a “volcanic glass” usually sold in bags of white roughly-shaped spheres of a few millimeters in diameter. They are incredibly light, and a blow can make them fly quite easily. They also float on water. This material is very porous and retains a bit of water as well.
What is pumice?
Pumice is a related cousin of perlite. It is slightly heavier, darker in color. However, it is used for similar purposes of perlite.
Both perlite and vermiculite can be found in retailer shops and gardening shops. Perlite is more known in the gardening world, easier to find, and cheaper.
Perlite is usually mixed with at least one (typically way more) growing medium (like peat, compost, vermiculite).
Why use perlite or pumice in a growing medium?
Because perlite and pumice can:
- Significantly improve aeration: this means that your plant roots can have easier access to the outside air. This is important for those herbs.
- Significantly improve drainage: it allows the water to flow quickly from top to bottom of the growing medium. This is possible because such spheres have many empty spaces inside (like a sponge) where the water can pass through.
- Increase the water retention: indeed, water tends to stick to the surface of the perlite particle.
- Make the planter heavier (pumice only): this aspect might be necessary if you have your potted herb in a balcony where strong winds can be a problem or in case you want to make it more stable in case your furred friend plays with it.
More details on the benefits of using perlite can be found in the perlite vs. vermiculite differences detailed analysis.
What is Vermiculite?
Vermiculite is a mineral mainly composed of clay (more on clay later). This material is again sold in small particles the size (and shape!) of worms of dark orange/golden color. This material is obtained, similarly to perlite, through a heating process.
Here is what even gardeners get wrong: vermiculite is not the same as perlite. They cannot be used as one for another due to their different properties. For instance, you know that vermiculite absorbs way more water than perlite? Check out this article if you are in doubt about their differences.
Why use vermiculite in the growing medium?
Because vermiculite can:
- Significantly improve water retention: Indeed, differently from perlite, vermiculite act like a sponge sucking water within. Hence, this can be an excellent addition for thirsty plants or in case you do not want to water often;
- Slightly improve aeration: this is true when the vermiculite is dry. Indeed, when wet, it tends to collapse on itself reducing its ability to increase airflow in the growing medium is part of;
What is Zeolite?
This is a relatively unknown growing medium for many gardeners. You do not find it often in a local store, and it is more expensive than perlite and vermiculite. Similar to perlite, zeolite has “volcanic” origins, although the chemical composition is different.
Zeolite is sold in bags of grey/green particles similar to perlite (although darker in color) with a very similar density.
Why use zeolite in the growing medium?
Because zeolite can:
- Significantly improve water retention and release: academic sources proved that zeolite can absorb up to 60% of its weight in water. This will increase the water available in the growing medium zeolite is added;
Now it gets even better.
Do you know that your plant roots can absorb water from the zeolite when they need it? Zeolite acts like a portable water bottle for your plants that can access when needed! This will avoid overwatering, by far the most common mistake among beginners gardeners;
- Optimal nutrient releaser: this is probably the best quality. Indeed, several studies have shown that zeolite can absorb Nitrogen and release it only when the herb requires it. This is a fantastic advantage as it will make your life as a gardener easier avoiding over-fertilizing, number 2 mistake among beginners.
What is compost?
Compost is the first material on our list that looks like soil. However, do not get fooled!
Compost is far from being soil.
Ready for the truth?
Compost is no more than a well-rotted organic matter. Do not be disgusted, compost has a good earthy odor, and it is obtained from material that otherwise would be wasted (like vegetable, fruit, and animal waste). You can find it easily, even in a large store with a gardening area.
Here is a video showing how you can produce compost at home step by step (very useful although the decomposition process takes time and you might want it to do it outside)
Why use zeolite in the growing medium?
Because compost can:
- Create the right bacteria environment: bacteria might sound nasty, but in reality, healthy soil has lots of beneficial bacteria that reduce diseases, avoid some pests and, more importantly, allow to break down nutrients for the plant. This is the most important benefit of compost as its nutrient content is small.
- Improve (slightly) the nutrient content: here is the catch. Many gardeners still believe that compost provides “lots” of nutrients to your plants. This is no true!
Compost is not a fertilizer and, although it does have nutrients, these are in a small amount (2 to 4%), as also stated by authoritative sources. Hence, the presence of the nutrient does help, but it is not definitely the main reason to use compost.
- Increase water retention: compost traps water keeping the growing medium moist and providing water when your plants need it the most.
5. Peat Moss, Coco Coir
What is peat moss?
Peat moss can be seen as a very special type of compost. Indeed, peat moss is the result of the decomposition of a kind of moss (Called Sphagnum moss). The use of such material is controversial as it covers a vital role in nature (for water retention) and, as well, it is not a renewable resource. That’s why, in recent years, many alternatives to peat moss have been investigated.
You can easily find peat moss (often under the name of Sphagnum Peat Moss) sold in large bags in the main supermarket, gardening shops, and online retailers.
What is peat coir?
Coco coir is by far the most promising peat moss replacement. This material comes from the waste coconut coir that otherwise would be wasted, so a totally environmental-friendly material.
Like peat moss, coco coir is gaining popularity although sometimes harder to find than peat moss. It might be easier to check online.
Why using peat moss or coco coir in the growing medium?
Peat moss or coco coir is beneficial in a healthy growing medium as they can:
- Significantly increase the water retention: peat moss can take up to 10 times its body weight in water while coco coir very closely to 8-9 times. This is important for water-loving plants, and reducing the frequency of watering, key in hot periods of long light hours.
- Increase aeration: peat moss makes the soil lighter, increasing aeration when added in the right proportion to the soil (a third of the total).
What are sand, silt, and clay?
Sand, silt, and clay are officially called soils. The detailed guide of the differences will illustrate the differences among them in great detail. All “soils” that you might find are just a composition of these three types.
These three basic soils cannot be found easily in shops or retailers. They are mainly used in large/commercial/industrial applications (especially sand in for building purposes).
However, all you need to know for now is that the main difference among the three types boils down to the particle size. From smaller to larger, we have clay, silt, and sand.
Larger particles (like sand) allow water to flow smoothly. On the opposite small particles trap water and gases.
Why use sand, silt, or/and clay in the growing medium?
Because sand, silt and/or clay can:
- Improve drainage and aeration: sand, due to its large particle, allows water to flow easily through the growing medium. This is a fundamental requirement for those plant that like slightly moist/dry roots (rosemary, for instance).
- Improve nutrients retention: clay soil is quite notorious to have properties to retain efficiently nutrients that will stay longer limit to been washed away by recurrent watering. It does also provide anchorage and support for your plants.
What is garden soil?
Well, this is not a single material, but the mix of a large variety of materials depending on where you live and how old is the soil. Garden soil can contain compost, sand, clay, fertilizer, and other materials. This is a growing material by itself.
However, as you can go to any garden and dig out some soil, this can be seen as a basic material for your growing medium.
Why use garden soil in the growing medium?
Because gardening soil is:
- Free: you can take it from your garden (or neighbor, ask first!), and it is available all the time.
- Rich in organic matter and healthy bacteria: the outdoor soil is way richer than typically indoor growing medium. It contains a large variety of bacteria whose efficacy is already demonstrated by the plant growing there.
Read this please before digging your garden soil.
Using gardening soil for potted herbs might sound convenient. However, the disadvantages (as I will detail in a later section) are way more important than the advantages. Hence, no to outdoor garden soil for potting
8. Wooden Bark
What is wooden bark?
Wooden bark is a type of mulch quite used in gardening. It is sold in pieces of a few cm in size (and a few millimeters in thickness) that come from the superficial part of a tree trunk or roots.
It can be easily found in gardening shops and online retailers sold in large bags. It is quite inexpensive.
Why using wooden bark in the growing medium?
Wooden bark, usually placed on top of the soil to cover it covers two important roles:
- Trap moisture: especially in hot weather, the water will tend to easily evaporate, making your plant suffering. However, this problem can be solved with a top layer of wooden bark to trap moisture and keep the growing medium below cool;
- Protect from temperature variations: a top layer of wooden bark also acts as a thermal insulator keeping the roots protected by extreme temperature variation.
Wait, water as a growing medium?
Yes, here is the crazy fact
It is totally possible to grow whole plants in water only! This is a crazy technique, known as hydroponics. I am using just water (with nothing else) to make the roots of my starter mint develop. The good news is that they are growing with no issue for 2 weeks now. For more check out this article on propagation by cutting.
Moreover, there is a whole new branch of gardening called hydroponics, where plants and plants are grown exclusively in water containing all the nutrients the plants might need.
Why use water in the growing medium? A water-only growing medium is cheap and always available. However, its use is particular for hydroponics and root development applications. I do not suggest, for starter indoor gardeners, to jump to hydroponics as way more complicated than traditional gardening.
What is sawdust?
Sawdust is wood in a very fine form, almost powder. This is usually produced as waste when working with wood (from cutting, planing, or any other operation that requires reshaping or cutting wood).
Sawdust is another quite easy-to-find material, although it might not be in the gardening sector. Indeed, sawdust is used for many other reasons (cat litter, for instance). It is sold in several pounds (kg) large bags.
Why use sawdust in the growing medium?
Sawdust is organic material that can help to improve the structure of too heavy growing medium (like clay) or too light (like sandy). Sawdust is placed within the soil which is good practice to add nitrogen-heavy fertilizer. In my experience, sawdust is used for outdoor applications only.
What is Limestone?
Limestone is a rock generated within centuries of marine animals’ skeletons accumulation. It is usually sold in large bags (quite cheap), and it appears commonly as white dust. Its exact consistency change in the function of the type of limestone you are buying.
Why use limestone in the growing medium?
Limestone is a material added to the so-called soilless growing medium to correct the soil pH.
Indeed, in a soilless growing medium, chances are that peat moss and compost are present (these are very common). Such materials have a low pH (acidic), making the growing medium in which they are part, unsuitable to many plants.
That’s why the manufacturer “corrects” this problem adding limestone, which has a higher than average pH offsetting the acidity of those components.
You know all the ingredients you can find in the majority (if not all) of growing mediums in the market. You know the pros of each material.
This is an upper advantage that only experts have. Why?
If you know the “ingredients” of any generic growing medium, it will be relatively easy to have an idea of the quality of that growing medium, and how it can be used.
To make your life even easier, below, you can find a table reporting most of the growing medium commercially available, with an indication of their ingredients, their quality, and or which plants are the most suitable to.
Here is the catch
The problem is not in finding a growing medium. Even the most humble of the retailers sell at least 2-3 types of them. The difficulty is to choose the right one for your herbs and plants.
The good news is that your plants are not very picky in terms of growing medium and, if you avoid some big mistakes, they will do just fine. However, if you have to choose, just go for the best since the start saving you future hassle, money and time.
Do you want good news?
The majority of your indoor herbs and houseplants will be perfectly fine with any top-quality potting mix you can find in the market. Indeed, a potting mix is the application of years of studies centered on identifying the best components and their ratio to promote the best plant growth.
Making your life easy
If your houseplant does not require any special care, you might need to choose just between two growing medium types: potting mix and cacti mix.
- Potting Mix: this is a balance of a moisture-retaining material (very commonly peat moss or coco coir), a medium to increase drainage and aeration (like perlite), and some organic material to provide the first nutrients and making the soil able to produce them (compost). You might also find fertilizer. For more detail have a look at this detailed guide on the best potting mix;
- Cacti mix: this mix is especially recommended to all those plants that are native of arid/sandy soil. This growing medium presents fewer nutrients and higher drainage capability. It is not recommended for edible plants (like herbs) as they usually thrive in moist rather than dry medium. For more, have a look at this detailed guide.
Choose the Potting Mix in 3 Steps?
Check the water requirements of your plant.
- Go in the “Database Plant Search Page.” This is one of my favorite resources, but feel free to check others if you find them.
- Type the name of your plant and check the water requirements.
- If the water requirement is dry, then go for a good quality cacti soil (check this one on Amazon), otherwise for if moist or more than go for a good quality potting mix (check this great one, on Amazon)
Here a table summarising the best growing medium for the most common indoor plants, herbs, and spices. You can also find what is “already available” in the market if you do not want to create your medium from scratch.
|Plant||Ready available||What is made of the growing medium||Why this growing medium|
| – Potted Herbs: Mint, Oregano, Basil, Rosemary, Dill, Chives, Sage, Marojaran, Catnip, Cilantro, Lavender, Thyme|
– Houseplants: pothos, anthuriums,bromeliads, dahlias, fuchsias, Paperwhites, lucky bamboo
– Vegetables/spices: lettuce, carrots, tomatoes, peppers, garlic, hot peppers, onions,
|All Purpose Potting Mix: check this great one on Amazon||Compost, Peat Moss, Perlite, Limestone|
In quality potting mix you can also found: fish emulsion, crab meal, shrimp meal, earthworm castings, sandy loam, bat guano, granite dust, pH regulators
|Nutrient rich, soft, retain moisture and well drained.|
|Dry houseplants: Snake plant, cactus, jader plant, adenium (desert rose), Lithops,||Cacti soil||Potting soil, sand, perlite||Great drainage ability to keep the roots dry and aerated|
Here, is another table showing the best potting soil for specific indoor plants that many of you have or are thinking of having.
The reason for a separate table is that those plants require special care. Indeed, due to drainage, humidity, pH, or other specification, the general potting or cacti mix (a safe bet 90% of the time) is not the best choice.
|Plant||Ready available||Ingredients (Main)||Why|
|African Violets||Check this good one on Amazon||perlite and potting mix||Need some extra aeration compared to regular potting mix as this plant is susceptible to root rot|
|Orchid (like cattleya, Paphs), ginger||Check this one on Amazon||Bark, coco coir, pet moss, perlite and tree fern fibers – Source||The medium need to retain moisture but also provide plenty of air to the roots|
|Blueberries||If you do not find a blueberries specific potting mix: 7 parts of regular potting mix (here on Amazon) and 3 parts of acidic-loving plant mix||Fertilizers (like fish bone meal and feather meal), potassium sulfate||Acidic soil (pH 4.5-5.5) is required|
|Roses, geranium, Begonias||Check this one on Amazon||Loam soil (contain 2 or more sole type) or potting mix|
|Flytrap||Check this one on Amazon||Sand and peat moss||It prefers acidic soil with little nutrients and high moisture retention. Avoid potting mix|
Remember that this is just a guideline to choose a growing medium, but this is just the first step. Indeed, you might want to add some bark, for instance, to trap moisture or totally avoid it.
Pro tip: check the companions
Plants that in the previous table are in the same growing medium might be incompatible to grow close to each other.
Choosing the growing medium is just the beginning: now what?
You might need to consider if adding other materials like bark or sawdust is necessary. For instance, the bark is a good idea for rosemary, not for basil and dahlias. In this case, you need to look for the plant you want to grow and check for some “care guide.”
The fertilizer also covers an important role. Indeed, some plants (like rosemary and cactus) might not need any fertilizer, while others, such as basil, might require the frequent provision, especially in warm summer periods.
You might be wondering.
Could I use a pure soil type (like entirely sand) to grow my plants? Well, although this is possible (excluding, for now, pH and salinity problems), it can be very hard. Indeed, each soil type has a marked “defect” (drainage, aeration, nutrient content) that you might need to compensate for with lots of attention (frequent watering, fertilizer provision, etc.). This also increases the chances of you giving up and your plant dying.
Hence, in reality, for indoor plants, a growing medium entirely given by one soil type is never recommended.
You can use gardening soil in containers, but it is not a recommended practice, and you should avoid it.
Indeed, as also discussed in this step-by-step guide to sterilization, gardening soil comes with a large number of bacteria and potential pests. These in a closed and protected environment (a container on a kitchen counter) might thrive due to the absence of other predators/ insects.
Think about aphids, mealybugs, gnats just to name a few.
A potential solution to this problem is good sterilization. However, this process is not very straightforward. Indeed, the temperature should be maintained high enough to kill pests but not too high to avoid damaging the soil itself (that can become toxic). Have a look at this guide for more info on the procedure and temperature values.
Moreover, as a side effect, you are losing precious nutrients that might be important for your plants.
And that’s not it!
Gardening soil is often described by expert gardeners as “heavy.” This means that it has limited drainage capabilities. This is not a big deal in an open space while it is a problem in a small container where a heavy soil will leave your plants wet for a long time so increasing the chance of root rot significantly.
Seedlings have different requirements in terms of a growing medium compared to a growing plant. Indeed, temperature and moisture levels are key in this case.
What growing medium should you use?
For those of you DIY fan
I would go for a mix of 50% perlite and 50% vermiculite. As discussed in this perlite vs. vermiculite guide. This growing medium has great aeration (due to the perlite) and great moisture retention capabilities (due to the vermiculite). These are two ideal conditions for seeds to sprout. The lack of nutrients (perlite and vermiculite are nutrients deprived) is not a problem at all for seeds. You can find perlite and vermiculite in large bags even on Amazon (check perlite here and vermiculite here)
For those of you looking for a ready option
A good potting mix for seedlings with excellent physical and chemical properties also suggested by other expert colleagues is the Espoma SS8 (you can check its price here on Amazon).
Number one mistake to avoid.
If your plant is in the “cacti” medium, do not use a regular potting mix. If your plant is in the potting mix medium, do not use cacti. Simple right?
Example 1: basil in a cactus soil? It can grow, but it will require more care. Indeed, the lack of water (growing medium constantly dry) will require you to water the basil more often. Moreover, the constant watering is washing away precious nutrients, so you might also need to fertilize more often.
Example 2: basil let it grow onto a rose-specific growing medium. How many gardeners forget that a roses medium of roses is more acidic than normal potting soil. Results: your basil will slowly die to the high acidity level in the soil
Here another takeaway
Use the wrong growing medium and, the least of the consequences, you are going to spend more money and time than you should. More probably, your plant will die.
Below you can see a picture of one of my leeks that I used for a tasty soup. Rather than throw it, I just left it in a mug with some water to keep the bottom wet. Try to guess what’s happened? It is growing!!
Here the crazy part
The leek is producing edible material with just air, water, and sunlight. Of course, do not expect this to happen with many other types of plants. Indeed, the onion family is famous for being really adaptable for that (spring onion, garlic, etc…).
I left in the water a mint cutting, and it developed after 2 weeks of some roots. It is still alive, fresh, and ready to eat if I want to or being planted (as I will do) to grow an endless supply of fresh mint from something that normally goes to the bin.
The takeaway is clear.
Many plants can grow or root or at least root, just in simple water. These are called cuttings (you cut part of the plant and grow it back). If you do it well (here a step by step herb guide).
I discussed the “traditional” way to grow herbs and plants, which is through a growing medium that is solid and looks like (although it might be quite different) regular soil.
But this is not the only way!
However, this is not the only way to grow plants. It is possible to grow plants exclusively in water (with the addition into the water of those nutrients and minerals that might be needed).
For those of you unaware of this technique, this might sound impossible. However, 2000 years ago, the Babylonese used to grow plants floating on water in their famous Hanging gardens (have a look at our good Wikipedia for more details). Hence, nothing news under the sun. Of course, to grow in such unusual conditions, you need more attention to your plant.
Interesting to know that the majority of plants can grow very well in hydroponic.
Do you know that the capability to retain nutrients of a soil type is linked to its surface? This is something that I call the Pit Ball Comparison. Smaller particles attract more nutrients as they tend to stick to an overall larger surface area.
End of the story?
No, smaller particles, by limiting water circulation, prevents aeration, so potentially suffocating your plants.
Hence, how to choose among them?
Head to the sand, silt and clay differences
What are the cheapest growing mediums for hydroponic? Perlite, Rockwool, and grow rock are by far the cheapest option for the hydroponic grower. Moreover, they can be easily found in the majority of large retailers.
What is the best growing medium? There is no single growing medium that is suitable for every plant. However, a regular potting mix or cacti mix is able to satisfy the requirements of the vast majority of houseplants (with a few exceptions). The potting mix ideal for many houseplants, while cacti plants mix is specifically for those plants native of dry areas (like succulents).
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