The Ultimate Guide to Hydroponic Growing Mediums

Picking the right hydroponic growing medium for what you need it to do is really important, but gosh, can it ever be an anxiety-inducing rabbit hole of research.

There are dozens of choices on the market, each with their fans and detractors, with their benefits and their flaws. Picking the wrong one can be disastrous. Choosing one can be paralysing from over-analysis.

Why is selecting the right growing medium so important?

A growing medium is there to nurture your plant from seed to harvest (or just parts of that cycle), including holding the plant’s roots in place, providing it stability, keeping the roots moist but with access to oxygen.

The wrong growing medium could just inhibit your plant’s growth or outright kill it. The right growing medium can compensate for a hydroponic system’s flaws and help your plant grow up big and strong.

So how do you choose the right growing medium? That’s why I’m here to help.

First, you’ll learn what to consider when you’re choosing your growing medium and then we’ll go over the growing mediums currently available on the market, their pros and cons.

By the end, you’ll have a good idea which growing medium to look into more.

What to Consider When Choosing a Hydroponics Growing Medium

Water-retention (its ability to absorb water)

Water absorption and water retention are especially important when you’re using systems with pumps, like NFT, flood and drain, and drip. Any mechanical failures will leave your crops’ roots to dry out. If the growing medium absorbs nutrient solution and water, it can buy you precious time.

Water absorption is also really important in the wick system, as your growing medium’s ability to absorb the water passed along can make or break your system.

Air-holding capacity (AHC, its ability to hold oxygen to prevent overwatering)

Plants need air around their roots as well as water. If there’s not enough oxygen around your plant roots, then your plants will be overwatered and your system will turn anaerobic, meaning the organic matter (like roots!) decomposes. It smells bad, attracts insects, and will kill your plants.

Along with air-holding capacity is the space left in between the growing medium particulates or fibres. You need that space for roots to grow and water to flow through. If the space is too small, you’ll get blockages where water won’t flow.

Sustainability (how environmentally friendly it is)

Hydroponics is all about growing healthy vegetables in local, environmentally friendly ways. So sustainability is an important consideration to take into account and a factor that’s becoming more discussed among growers.

Unfortunately, there’s no one entirely eco-friendly growing medium. You’ll need to make compromises between the most sustainable, what you need it for, whether you can reuse it, and price.

Antifungal and anti-pest properties

Indoor growing avoids a lot of pests and fungal problems found in outdoor crops, but if they take root in your hydroponics systems, they can get really bad, really quickly.

Price and reusability

Price matters. And while I would urge you to spend more to get a higher quality growing medium (especially important with coco coir) as you will get better results, depending on your financial circumstances, that’s not always a feasible option.

Keep in mind that some growing mediums, while initially more expensive, may save you money in the long run if you can reuse them.

Learn more about hydroponic growing media with this video:

7 Types of Growing Medium

Coco Coir

Coconut coir is made from the ground coconut husks leftover from coconut milk and coconut water production. Coir is the thick fibrous layer between the inner seed (which contains the milk) and the outer coat.

The husk naturally acts as a growing medium for coconuts. They protect the coconut seeds from saltwater damage and then provide an ideal growing medium for the new seedling.

Coco coir is made out of three components: coconut peat (which absorbs moisture), coconut fibre (which makes air pockets for roots), and coconut chips (which does a bit of both).

Getting the right balance between these three components makes a huge difference in how easy it is to grow and how much labor you’ll need to spend. Spending more money on a better brand pays off when it comes to coconut coir.

Pros

• Coco coir is very beginner-friendly.
• The husk is designed by nature as a growing medium and is rich with growth hormones.
• Coir is a sustainable renewable resource that would otherwise be a waste product. It’s also usually organic. A great environmentally friendly replacement for peat moss.
• Coir is naturally antifungal and anti-pest.
• It retains moisture, absorbing up to 10 times its weight in water. The long strands of coconut fibre add air pockets which allow roots to grow and breathe.
• Because coir is compacted in the package, you’ll save money on shipping.
• It feels the most like growing in soil.
• The husk is a supportive environment for added beneficial bacteria, which you can add to aid plant growth.

Cons

• Because coconuts are manufactured in the tropics, coir is transported over long distances.
• If the balance of components is off, the coir might absorb too much moisture and drown plants.
• Depending on the manufacturer, you will need to thoroughly rinse the coir to remove the salt. Otherwise, too much salt will build-up in your system. Higher-quality coconut coir will already be rinsed thoroughly and the coconut peat aged properly to avoid bacterial contamination.
• You will need to either get a nutrient solution specifically for coconut coir or use a supplement with calcium and magnesium, such as Calimagic calcium + magnesium supplement.
• You will need to soak and rehydrate the bricks before use.

How To Use

1. Hydrate your coir following the manufacturer’s instructions.
2. If your manufacturer did not already rinse to remove the salts, then you’ll need to rinse the coir until the water runs clear.
3. Break up the hydrated coconut coir until it’s nice and fluffy. Otherwise, it will be an airless clump.
4. Then plant your seeds and seedlings.
5. Keep moist during plant growth.

As mentioned above, you can get a nutrient solution specifically for coconut coir, or go with a regular nutrient solution and add the calcium and magnesium supplement. You can also introduce beneficial bacteria to your coco coir, which will help your plants grow.

We cover Coco Coir more in-depth here

Expandable Clay Pellets

Clay pellets, also known as Hydroton and LECA (Light Expanded Clay Aggregate) are porous clay balls that expand to about the same size as marbles.

Because they’re round and porous, they strike an excellent balance between oxygen and water so you’re not overly drying or drowning the roots.

They’re made by heating the clay to over 2000 degrees in a rotary kiln, which is what gives the clay its air bubbles.

Pros

• They’re inexpensive to buy and you can reuse clay pellets for years. When you can no longer use them for hydroponics, add them to your outdoor garden to add aeration to the soil.
• They are naturally pH-neutral so you don’t need to adjust their pH before use.
• They have excellent drainage. Because there’s space in between pellets and in the large pores, the nutrient solution can flow through unimpeded, even if there’s a buildup of algae and microbes. The water is not going to get clogged or blocked like with smaller particles like perlite.
• The clay pellets absorb water and nutrients solution, helping to keep your plants fed in between water cycles or if there’s a mechanical failure.
• Like coconut coir, insects won’t nest in them.
• Their porous sides hold some air bubbles, keeping plant roots oxygenated. They have the excellent air-holding capacity (AHC).
• Because they’re loose pebbles, you can easily transplant new plants and pull plants out after harvest, which saves you a lot of time and labor.
• If you are using aquaponics, clay pellets can host microbes that can make nutrients from organic sources like fish food available to plants.

Cons

• They dry quickly. If it’s hot where you’re growing, and you lose a lot of water to transpiration, then clay pellets will be difficult to use.
• They are heavy, so aren’t suitable if you’re using very large grow beds that you need to move if your grow space has weight limits, or you’re unable to move heavy objects.
• They are produced by strip-mining. On the other hand, it’s a medium where supply vastly outstrips demand, unlike with other growing mediums.
• You must rinse and soak before use and that will get red dust everywhere. If you don’t properly rinse them, the dust will clog up your mechanics.
• The pebbles may float until they are thoroughly saturated, so they may be sucked into drain lines and cause blockages.
• Their heavier weight means higher shipping costs.

How To Use

Clay pellets are best used for ebb and flow and drip systems, but you can also use them with the Kratky method. Some people are now growing houseplants with a clay pellet Kratky method.

You can use clay pellets for either the germination stage or for raising plants to maturity. If you are using them for germination, crush a few pebbles to increase water retention and use that to plant your seeds.

1. Rinse thoroughly until the water runs clear to remove all the red dust.
2. Soak for 24 hours with pH-balanced water. Clay pellets are pH neutral, but if the water they soak up is not pH neutral they will throw off the pH balance of your system.
3. Keep the Pebbles moist!!!
4. Every once in a while, remove the plants from the clay pellets and rinse them with pH-balanced water. Otherwise, the nutrient solution they soak up can build up to toxic levels where plants won’t be able to grow.
5. To reuse clay pellets, rinse and sterilize with isopropyl alcohol or hydrogen peroxide.

Growstones

Growstones are pretty much like clay pellets, but they’re made out of crushed glass bottles which are melted and mixed with calcium carbonate to become something that looks like lava rocks.

Pros

• Growstones are one of the more sustainable growing mediums because they reuse recycled materials.
• Growstones have an excellent air to water ratio, which helps avoid over-watering in drip hydroponic systems.
• They are lighter weight than clay pellets.
• Because roots grip the grow stones, plants grow more stable with stronger roots. If you want to grow large vegetables like melons, cucumbers, and tomatoes, then grow stones are a great choice.

Cons

• Their irregular shape may make them more difficult to work with, as they may fall into the reservoir or need to be mounded up higher on the plant.
• They are hard to clean after use because bits of roots stick to the growstones after harvest.
• Because plants grip the growstones with their roots, it makes it very difficult to transplant from growstones without damaging the plant. Growstones are best used with permanent beds where you grow plants from seed (or seedling) to harvest.
• Growstones have a naturally high pH so they need to be soaked with pH Down to make them pH neutral.

How To Use

• Rinse growstones before use to get rid of any particles leftover from manufacturing.
• Soak in water treated with pH Down to lower the pH of the growstones.
• Use like clay pellets.

Rockwool

Rockwool is one of the most well-known and popular growing mediums. Some growers consider it the perfect growing medium, but as we see down below, there are a lot of cons to using Rockwool.

It’s made by melting volcanic rock at high temperatures and then spinning that into long fibres that then cool into scored mats.

Pros

• The main pro to Rockwool is high water retention, just like with the Oasis Cubes below, and can compensate for mechanical failures in systems like NFT.
• Rockwool holds up to 20% of oxygen between its fibres, keeping your roots oxygenated and preventing overwatering.
• Best used for germination and propagation.

Cons

• Rockwool requires a lot of energy to make. It has to be melted at high temperatures to even be produced, so it’s not great for sustainability.
• Rockwool is bad for the environment. You can’t reuse Rockwool and after one use, they will end up in the trash and exist in the garbage dump into infinity.
• They’re bad for your health. The fibres and dust can harm your eyes, nose, and lungs. You can mitigate this by wearing a mask and safety glasses when handling dry Rockwool and soaking Rockwool to keep the particles from flying up, but there are better growing mediums that aren’t hazardous to your health.
• Rockwool needs to be soaked for a full day before they are used, adding labour.
• Rockwool has a high pH, so you will need to soak them with pH Down before first use, and then continue to check the pH balance as it can shift pretty quickly.
• They’re relatively expensive.
• Not useful past the seedling stage.

How To Use

To use, follow the same procedures as the Oasis Cubes (below). Wear a mask when handling dry Rockwool!

We cover Rockwool more in-depth here

Oasis Cubes

Oasis cubes are similar to Rockwool but are made out of a plastic styrofoam material for a fraction of the price. They can only be used for germination and seedlings. They are a proprietary growing medium, meaning you can only buy them from one brand.

Pros

• Oasis cubes are cheaper than Rockwool.
• You don’t need to soak them ahead of time.
• They’re best used for germination and propagating cuttings.
• They are pH-neutral.
• They retain 30 to 40 times their own weight in water, keeping your seedlings always hydrated.
• You can reuse them, which makes them more sustainable.

Cons

• You can’t grow plants to harvest. You can only grow them up to seedlings, and then you need to transplant them.
• While reusing them can make them more sustainable, they’re not eco-friendly since it’s essentially made out of styrofoam.

How To Use

1. Gently remove the paper sleeve or the cardboard liner containing the Oasis Cube sheets.
2. Place a sheet of cubes in an industry-standard 1020 carry tray with drainage holes. Don’t place them directly into a solid-bottomed tray because you need to be able to drain excess water.
3. Place the seeds or seedlings in the pre-cut hole. You can do this before or after the next step.
4. Then soak the Oasis cubes thoroughly with nutrient solution. Don’t try to dunk the Oasis cubes underwater! Either place the tray in the tub of complete nutrient solution and let it soak up from the bottom or you can water from above, or you can do both. If you are sowing seeds then you will want to lightly water the top of the sheet to set the seeds into the holes.
5. If you are sowing seeds, place the trays and a dark place until the seeds germinate. This will generally take at least 48 hours, but it depends on what you’re growing. Move the seedlings to a lighted area as soon as the first few seeds have germinated, otherwise, they will grow stretched.
6. Keep misting or watering every day once the seeds have germinated.
7. Once seedlings have two true leaves (as opposed to the initial cotyledon leaves), they are ready to transplant. This should take generally between 12 to 18 days, but varies due to the season (shorter in summer, longer in winter) and the strength of your light source.
8. Any leftover Oasis Cubes should be stored in a closed carton in a dry location away from direct sunlight.

Perlite

If you’ve ever used potting mix before and spotted tiny white stones in the mix, then you are familiar with perlite. Perlites are tiny white stones created by using heat on volcanic glass to pop them into a light and porous material, kind of like popcorn.

In general, you don’t use perlite by itself. The stones are so small that they can slip through net pots and into the lines, causing blockages. Perlite is best used with another growing medium to add more air pockets.

Pros

• They are the best growing medium for air-holding capacity. If you’re having trouble with a lack of oxygen (creating anaerobic conditions that decompose your plants’ roots), adding perlite will help.
• It’s inexpensive. You can get perlite for less than other pH-neutral mediums. As you usually only use a small amount, you stretch out the savings even more.
• They are lightweight so they are easy to haul around and have lower shipping costs.
• Perlite comes naturally sterile and pH neutral. Because it is created by superheating the rock material, there’s no chance for bacterial, fungal, or insect pests to get into it. This helps you avoid a lot of problems.
• You can reuse perlite so long as you sterilize it between use, either with heat or with a hydrogen peroxide solution.

Cons

• Because they are lightweight, they can easily plug up systems, so they’re best used in combination with something else.
• Perlite is strip-mined and is not a renewable resource.
• Perlite is essentially tiny bits of glass. Never use them with aquaponics, as the perlite dust can get into the gills of a fish and cause real damage. The same goes for your lungs. Wear a mask when handling dry perlite.
• Perlite holds no water whatsoever, so if you have a mechanical failure, your plants will die much quicker than with a growing medium that absorbs moisture.
• Roots and algae build-up can cause blockages. Because perlite is so much smaller than clay pellets, there’s not as much room in between the individual particles for roots to grow. If the roots take up all the space or if algae and bacteria build up on the surface, the liquid won’t be able to flow through.

How To Use

Mostly, you’re going to be using this to add to other growing mediums to add more air. The amount you add will depend on which growing medium you use.

1. Before using, rinse the perlite to remove the small particles. Make sure you’re wearing a mask!
2. While damp, mix in with the other growing medium.

We cover Perlite more in-depth here

Starter Plugs

Starter plugs come with a variety of names, including seed starter plugs and seed pod sponges. Probably the most well-known starter plug is Jiffy peat pellets.

The materials they’re made out of depends on what brand you buy. Jiffy peat pellets are made from compressed peat held together by a thin plastic netting that need to be rehydrated to expand to full size. You can also get something similar made out of coco coir. Other plugs are made out of compressed composted materials.

Whatever you choose, most starter plugs can only be used for germinating seeds. Jiffy peat pellets are especially small. But if you have a small plant, like basil, you can get away with just keeping it in a starter plug.

Pros

• Depending on what you buy, they can be the best growing medium of all in terms of organic and sustainability, since they’re made up of just organic compost. Jiffy peat pellets come in a plastic mesh that can photo-degrade (biodegrade when exposed to light) but will generally just become waste.
• They’re compact, which saves you money on shipping, and they store easily.
• They’re quick to rehydrate.

Cons

• Starter plugs are more of an expensive option.
• You can only use them for starting seeds or propagating, so you will need to transplant them into another growing medium to grow until harvest.
• If the starter plugs are kept wet all the time, you may have problems with fungus gnats.

How To Use

1. Soak the starter plugs in water or nutrient solution to rehydrate. (Expandable pellets will expand from discs into their full size.)
2. Once rehydrated, set them on a seedling tray with no drainage holes.
3. Place the seed in the center hole of the plug.
4. Keep the starter plugs moist by bottom-watering.
5. Once seedlings grow their first set of true leaves, they’re ready to transplant.

Conclusion

Phew. That was a lot of info.

But now that you know your own growing needs and an overview of the pros and cons of each growing medium, you’ll be able to narrow down the list of possibilities to find the right growing medium for you.