An introduction to soilless growing; semi-hydro
With the rise of houseplant popularity there has been an increase in the number of people turning to soilless mediums to grow their prized plants to top instagram-able condition.
Read on for an introduction to semi-hydro growing; its terms and common techniques.
For tried and tested tips we caught up with Fair of Fairest plants to understand why she classes herself as a soilless convert and get a 101 on how to grow without the use of typical potting mixes.
Fair’s Monstera thai constellation and Monstera aurea. Image @fairestplants
Fair is a stem cell scientist and plant fanatic with an enviable collection of over 300 plants in her London flat, she sells cutting of her rare and unusual plants on @asian_aroids and has been using soilless mediums for two years.
Fair’s favourite plants to grow are Monsteras, Philodendrons, Anthuriums and Syngoniums. Some grown by windows, some in an IKEA MILSBO cabinet with the IKEA VAXER LED bar and some in a grow tent with Acadia Jungle Dawn LED grow bars. (See our grow light blog for recommendations by top plant fans).
What is semi-hydro?
Semi-hydro is also known as passive-hydro, it refers to the act of growing plants in a soilless planting medium that is inorganic and inert, where the plant is self-watered via a water reservoir.
Semi-hydro potting mediums are highly porous. Commonly used mediums are LECA, Perlite and LECHUZA-PON. Water is drawn up through the potting medium due to its porous nature, this is known as ‘wicking’.
This differs from hydroponic growing that does not include the use of a growing medium, where the plants are suspended directly into a solution of water and nutrients, and pumps are used to provide the plant’s roots with necessary oxygen.
Anthuriam magnificum in semi-hydro. Planted in LECHUZA-PON in clear plastic pot with water reservoir. Image: @fairestplants
Like many, Fair started out planting in soil. Then, through propagating cuttings in water and realising they can continually grow in there she started experimenting with soilless mediums. That was two years ago and Fair has gone on to covert the vast majority of her plants to semi-hydro, for ease of maintenance and for the reduced risk of root rot.
By using a water reservoir there is less frequent watering required, the plant takes up the water at the rate that it needs.
Reduced risk of root rot
By eliminating soil-born organic microbes and by allowing the plant to take up the amount of water it needs there is less risk of root rot. Soilless mediums don’t clump or become waterlogged like soil does when over-watered, providing more aeration for the plant which aids healthy root growth.
No more fungus gnats?
An added bonus, and one many people cite as the main reason for converting to semi-hydro is to rid the home of fungus gnats. Worth noting though that Fair has experienced fungus gnats when using Perlite.
Re-useable potting mediums
A great benefit of the soilless potting mediums is that they can be reused for a considerable amount of time. They just need to be washed and rinsed and can be reused.
Downsides to semi-hydro?
Accessibility of products
The soilless mediums can be slightly trickier to get hold of than regular compost; they are less widely available than purchasing compost from your average garden centre, supermarket or DIY store.
Soilless potting mediums such as Perlite, LECA and LECHUZA-PON are mined from natural, non-renewable sources and then put through an intensive heating process in order to make them commercially viable, so there is much debate around the sustainability of these mediums. Additionally, Sphagnum moss is often used as a soilless growing medium and although there are renewable and responsible sources of Sphagnum, much is extracted from the vital peat bogs of its natural habitat which are in devastatingly fast decline. Peat bogs are a really important habitat for many flora and fauna and a hugely important carbon store. More on growing peat free here.
Semi-hydro set up: the reservoir method
Semi-hydro planting is achieved through keeping a reservoir of water (and nutrients) below the potting medium, the water is wicked up by the porous potting medium providing the plant’s roots with the water and nutrients it needs. There are a number of ways of setting up the reservoir method, explained below.
a) A pot, cup, glass or vase with no drainage holes.
The water reservoir is at the bottom of the container and the plant’s roots sit above the water level. Eventually the plant will grow roots that are adapted for water into the reservoir.
b) An inner pot and cache pot
An inner pot with drainage holes holds the plant and potting medium. This sits inside an outer cache pot that holds the reservoir of water.
The potting medium wicks up the water and nutrients. Some include a water level gauge so you can easily see when the reservoir needs topping up.
c) Material wicking system
A self watering pot with a water reservoir and a cotton or cord wick that sits with one end in the water and the other end in the potting medium. The material wicks up the water into the potting medium. Some self-watering pots come with a water gauge.
Semi-hydro terms and definitions
In order to explain the nitty gritty of soilless growing we’ll clarify some commonly used terms and products.
Wicking is the act of absorbing or drawing water via capillary action. For example, when water evaporates from a plant through its leaf surface, this pulls water up through the plant from the roots via capillary action. Additionally, water can be ‘wicked’ up through material (a cord or string) from a water reservoir into a potting medium.
The material in which an organism lives and takes nutrients from. In growing, this refers to the potting medium.
Meaning not derived from living matter. Conversely, soil is an organic medium as it is made up in part of composed organic matter (from decayed plants and animal manure). Inorganic material is seen as beneficial for growing because it carries less risk of pest and disease.
In a chemical sense, inert means without the ability to react. In growing mediums this is beneficial as it doesn’t degenerate.
As explained above, this is the method in which a reservoir of water is kept below the potting medium, the water is wicked up by the porous potting medium providing the plant’s roots with water and nutrients.
The shower method is used when a plant is in soilless medium and all the water is drained out of the pot after watering, there is no water reservoir used. This method is commonly used when transitioning a plant into a semi-hydro situation; to gradually get the plant’s roots ready for a water reservoir.
Many soilless fans in fact never use a reservoir, they continue with the shower method, watering as if the plant were in soil, letting it dry out to some extent before watering again. A long-term shower method can be beneficial for plants who like a drier situation such as cacti and succulents.
A thorough rinse of the plant’s roots and substrate to wash away mineral and impurity build up. Carried out periodically to maintain healthy plant growth. Many people also recommend giving your substrate a thorough wash prior to planting to remove impurities, particularly useful to carry out when using LECA.
Allowing a period of dryness where no water is added. This process is advised by LECUZA when using their substrates and is also often used for plants that like a drier substrate such as cacti and succulents.
In semi-hydro context, this is the process of increasing or decreasing the pH with the aim of achieving neutral pH levels. LECA, LECHUZA-PON, Perlite and Sphagnum moss have a neutral pH, however the introduction of fertiliser and water can change the pH from neutral over time.
Soilless mediums explained
There are a number of substrate options for both propagating and growing without soil, below we explain what Perlite, LECA, LECHUZA-PON and Sphagnum moss are and how they are often used in soilless houseplant growing.
Grow greens, herbs, vegetables and fruit all year long
Hydroponics is a type of soilless gardening that can be done either indoors or outdoors. It’s a great option for people with little or no gardening space, or who want to grow herbs and vegetables through the winter.
Hydroponic gardening is space-efficient and takes less water than gardening in soil. Growing in water also means no weeds. With artificial lighting, you can grow hydroponically all year long, even in Minnesota.
Although almost anything can be grown hydroponically. Short-season crops or crops that do not produce fruit such as herbs and leafy greens are great choices for indoor production in the winter. In the summer, strawberries, tomatoes, cucumbers and peppers are all great choices. It’s becoming more common for commercial growers of these crops to grow hydroponically instead of in soil.
Basic components of hydroponic systems
The simplest hydroponic systems to use at home fall into a category of hydroponics called “Deep Water Culture.”
- Plants are suspended above a tank of water and the roots hang into the container where they absorb water and nutrients.
- This is the most common type of hydroponic system for small-scale growers such as people growing for their own use and school demonstration gardens.
- It is also the least expensive and easiest to maintain and expand.
You can buy premade deep water culture hydroponic systems, but it is more affordable and nearly as easy to build your own.
For this type of system, your container for holding water and plants can be as simple as a 5-gallon bucket or a plastic storage bin. Any kind of container that holds water will do for hydroponics, as long as it is clean and made of a material that is safe for food (a material that will not leach harmful chemicals into the water).
Consider the following when choosing a container for your hydroponic system:
The size of the plants you’d like to grow should dictate the size of your container. For example, if you’d like to grow a tomato hydroponically, consider the size of the canopy of a mature tomato plant, and choose a container that is approximately the same size.
The seed packet should tell you how large the plant will be.
If you’d like to grow multiple things in one container such as multiple heads of lettuce, a wider container will be necessary.
Lid or flotation device
The container in your hydroponic system will hold the water and nutrients, but something needs to support the plant.
When using a bucket, the most common support structure for plants is simply the lid of the bucket with holes drilled through it for the plants.
If you do not have a lid, another common practice is to use extruded polystyrene (sheets of insulation). You can either rest the polystyrene sheets over the top of the container or float them directly on top of the water.
If you choose to float the sheets directly on the water, it’s a good idea to provide some extra support (such as PVC tubes) to hold the polystyrene sheet as the plants become heavier.
Cost, aesthetic appeal, and space efficiency
If you would like your hydroponic system to look more attractive than just a bucket, an easy solution is to build a frame around it such as the Hydroponic Salad Table.
If you’re hoping to expand your system to have multiple containers in use at the same time, you can increase space efficiency with adjustable wire shelves, with plastic totes or buckets on each shelf.
These additional factors are not necessary and will increase the cost, but can make your set-up more efficient and attractive.
Pots and substrate
A key component of any hydroponic system is the support structure for your plants.
The most common system for DIY hydroponics is the combination of net pots and a substrate.
- A net pot is simply a pot with holes or slits in the sides to allow the root system to reach the nutrient solution below.
- The net pot should only be partially submerged to allow the developing plant roots to get oxygen.
- Net pots come in a variety of sizes and styles to suit a diverse range of systems.
Rather than filling the net pots with soil, common substrates include perlite, hydroton, pumice, gravel, coconut coir and Rockwool.
The pros and cons of each substrate are listed in the table below. No matter which substrate you choose, soak it in water for 24 hours and change the water before transplanting your seedlings into it. This helps to remove dirt and debris, and in some cases can remove residues that can impact the pH of your system.
If you re-use your substrate, make sure to clean it and sanitize it before planting again to prevent pathogen build-up.
The list of substrates in the table is not comprehensive; new products are released regularly.
|Rockwool||Most popular medium; Superheated rock and chalk that is spun like wool||Comes in a variety of shapes, sizes||Not biodegradable|
|Retains oxygen well||Can irritate skin, eyes, and lungs|
|Pore spaces are a good size for root development||Alkaline – can impact the pH|
|Coconut coir||Coconut husk byproduct||Good water retention – keeps roots from becoming dehydrated||Variable product|
|Environmentally friendly||Some types of coir (bricks) must be rehydrated before use|
|Reusable (a few times)|
|Made of organic material – more potential for insects or pathogens|
|Hydroton / LECA||Lightweight expanded clay pebbles the size of marbles||Larger spaces between pebbles: more airflow and ease of root development||Limited water holding capacity (only a problem if you forget to water or let the water level drop)|
|Reusable||More expensive than other options|
|Easy to work with and clean|
|Perlite||Ore that has been heated in a kiln and puffed / expanded||Holds on to oxygen well||Non-renewable resource|
|Reusable (a few times)||Small particles are more prone to blockages – can accumulate algae and biofilms|
|Inexpensive||Can be harmful to fish – do not use in aquaponics|
|Can be too lightweight, causing it to float|
|Pumice||Porous volcanic rock||Available in multiple sizes; larger sizes allow for more airflow||May be hard to find pumice that is not chemically treated|
|Lightweight||Variable product: sharp edges can sometimes cause root damage to plants|
|No super-heating necessary; may be more sustainable than some of the other rock-based options||Can be too lightweight, causing it to float|
|Gravel||Small rock from a variety of sources||Inexpensive||Heavy|
|Drains well||Can impact the pH|
|Easy to work with and clean||Small particles are more prone to blockages – can accumulate algae and biofilms|
The net pot system needs to be supported in some way. The easiest way to do this is to drill net pot-sized holes in the top of your container. The hole should be large enough to fit the majority of the pot through it, but just small enough for the lip of the net pot to rest on top.
If your container does not have a lid, you can use an extruded polystyrene board (insulation boards) or a wide-lip basket. When using polystyrene, support the board from below by adding PVC tubes to the container. This way, the board can be elevated slightly above the water level, which becomes important as the plant roots begin to develop.
The simplest option for hydroponic lighting is to grow outdoors in the summer. This is a great option for people who have access to a balcony or patio with sun exposure.
For indoor hydroponics, supplemental lighting is almost always necessary. While you may find success growing plants indoors if you have a very sunny south-facing window, you will likely need artificial lights in the winter.
The most common types of lighting available to small-scale growers include LED and fluorescent bulbs. There are pros and cons to using each type.
|Types of grow lights||Pros||Cons|
|LED (light emitting diode)||Very energy efficient||Higher up-front cost than other bulbs|
|Wide spectrum of light|
|Do not produce too much heat|
|Wide variety of styles and sizes|
|Fluorescent||Moderately energy efficient||Do not last as long|
|Cheaper up-front cost||Use more energy than LEDs|
|Some only produce light in the blue-green spectrum, but others have a wider spectrum that includes red light; check label|
|Incandescent||Cheapest up-front cost||Inefficient|
|Do not last as long|
|High pressure sodium||Emit substantial light||Better suited to large-scale systems|
|Release a lot of heat|
|Not all of the light is usable to plants (outside of plant-available spectrum)|
Regardless of the type of light you choose, consider the following factors:
Red vs. blue light:
The light spectrum includes colors ranging from red to blue; some light bulbs produce mostly blue light, some produce mainly red, and others produce a mix of blue and red light.
- While a balance of blue and red light is best for plant growth, you can get away with just blue light if you’re only growing leafy greens.
- Light from the red portion of the light spectrum helps plants to develop thicker stems, which are needed for producing flowers or fruit.
- In experiments with lettuce, less blue light results in more mild tasting lettuce with a flat texture; more blue light results in “spicier” lettuce with a curlier texture.
Check the specifications of your lightbulb to see what type of light it emits. Light bulbs marketed as “white light” or “full spectrum” will produce a balance of red and blue light.
Consider the up-front cost as well as day to day costs to run your lighting system to determine whether it is worth it for you.
For a small hydroponics system with 1 or 2 5-gallon buckets, you would need a single 9-Watt LED grow light with a PPF of 16 micromoles per second ($15 plus $10 for a work light to mount the bulb).
LED lights have a lifespan of about 25,000 hours. If you run that light for 14 hours per day, every day of the year (5110 hours per year), it should last about 4 years and 10.5 months. For approximately 45,990 Watts of electricity per year (9 Watts x 5110 hours run time).
At an electricity cost of 12 cents per kilowatt hour (a common price in 2020), you can expect to pay about 45 cents per month or $5.44 per year in electricity. Add to that $15 every 5 years to replace the bulb.
Passive aeration (the Kratky method)
In a passive deep water system, the aeration is provided by the air gap above the water. This is why it’s important to only submerge the net pot partway as the seedlings grow, and why it’s important to keep the roots only 1/3 to 1/2 submerged once they start to develop beyond the net pot.
If using a single container, an airstone is the most common type of aeration device in hydroponics. An airstone is a synthetic “stone” full of pores. It is connected through tubing to an external pump. The pump pushes oxygen through the stone, which, due to its porous structure, releases the air as tiny bubbles. They are commonly used in aquariums and come in a wide variety of sizes and shapes.
Consider an airstone if you are growing multiple plants with different root lengths in the same container. If one plant’s roots are submerged more than half of their length to accommodate a shorter plant nearby, aeration will help to keep the water sufficiently oxygenated to prevent plant damage.
If you have many containers, it is more common to use a recirculating deep water culture (DWC) system. Containers are connected with tubing, and a large pump circulates oxygenated water between the containers and a reservoir. With this method, you can just pay attention to balancing the pH and nutrients in the reservoir vs. managing each container separately.
You can purchase prefabricated recirculating deep water culture systems from online retailers and hardware stores, but they tend to be costly for hobby growers. There are also some excellent DIY tutorials on YouTube if you prefer to build your own.
Advanced growing systems
While deep water culture is by far the most simple hydroponic set-up, it is by no means the only option. The following methods are more common among commercial growers, but they can be adapted for smaller-scale systems as well.
All of the information listed above related to pots, substrates, and lighting still apply to these more advanced systems. The major downside of these more complex systems is that they rely on pumps and other mechanical parts. If there is a malfunction (such as a power outage), the plants cannot access water, and they can dry out quickly. Larger growers typically have generators as a back-up, but this is less practical for small-scale hydroponic gardeners.
The following is an overview of some of the most common advanced systems; it is not a comprehensive list, and there are variations to all of these systems.
Ebb and flow:
In an ebb and flow system, plants are grown in a tray with substrate. The water and nutrient solution are in a separate but connected reservoir.
- The growing tray either has a porous bottom, in which case it is placed directly above the nutrient reservoir, or it contains a drain, which connects via tubing back to the reservoir.
- Growers use pumps to pull the water from the reservoir into the tray where the plants are. It then slowly drains through the porous substrate, back into the reservoir.
- These systems are available as prefabricated units. They are sold online from hydroponics vendors and at hardware stores.
- They can also be made at home with basic materials such as buckets, plastic containers and plumbing supplies.
In a nutrient film system, plants are usually grown in channels or troughs. The idea is similar to deep water culture in that there is a constant stream of water available to the roots. The water is constantly moving and tends to be quite shallow.
- This type of system uses a reservoir for water and nutrients (such as a large plastic drum) and the water is pulled into the channels, usually with tubing and a pump.
- The channels are angled downwards to allow for water flow. When the water reaches the end it is recirculated to the reservoir.
- These can be purchased as premade units, or homemade with recycled gutters.
- This is an efficient system for larger-scale operations because it allows you to manage the water and nutrient solution from one central location.
Drip systems are very similar to the nutrient film technique but, instead of having a continuous stream of water flowing through a trough or channel, the water flows through a hose system.
- The hose has holes spaced in accordance with your plants. This is essentially drip irrigation.
- Some drip systems have a trough or channel that allows excess water to flow back to the reservoir and others do not.
- This set-up is becoming increasingly common in high tunnels, where farmers grow tomatoes and strawberries in long, tubular bags filled with a substrate like coconut coir, and use hoses with holes or drip lines directly at the base of each plant.
Plant selection is key to successful hydroponic gardening. In order to choose a plant that will be successful in your system, consider how much light and heat you will have access to.
Plants that do well in hydroponic systems
All year long:
- Lettuce, herbs, and plants in the Brassica family, such as mustards and kale, grow well year-round in hydroponic systems.
- Other leafy greens such as amaranth and swiss chard can also work well.
- Look for seed companies with varieties identified as doing well in hydroponic systems.
Outdoors in the summer:
- Almost anything can be grown hydroponically outdoors in the summer. Tomatoes, cucumbers and strawberries are popular options.
- While it is possible to grow these plants indoors with supplemental heat and lighting, the amount of added heat and light would be cost-prohibitive for most gardeners.
- If you choose to grow these heat-loving plants hydroponically, look for terms like “grows well in pots” or “container variety” when selecting a variety.
Starting seeds and transplanting into a hydroponic system
Start seeds separately and then transplant them into the hydroponic system after germination.
Starting seeds in starter cubes
- You can purchase plant starter cubes or grow plugs online or from some gardening stores.
- The cubes are made from materials such as rock wool or coconut coir and can be directly set into the larger system when your seeds have germinated.
- Starter cubes should be soaked in water before using them and should remain partially submerged (but not fully).
- When the roots have pushed through the bottom of the cube, they are ready to transplant.
- Since different starter cubes are made of different materials, read the instructions on the package before you start.
Starting seeds in soil
- You can also use potting soil and start seeds the way you would if growing them for your garden.
- With this approach, rinse the roots when you transplant them to remove any remaining soil.
Just like when starting seeds for a garden, you’ll need a warm, humid environment. You can start seeds using a container with a lid (such as a plastic storage container) in a south-facing window. If you don’t have a south-facing window, consider buying a heating mat or starting your seeds under grow lights.
This video series from UMN Extension horticulture educators shows three different at-home systems for starting seed.
There are three main water quality considerations when growing with hydroponics. All three of these factors contribute to plant health, and are easily amenable if your water is not naturally in the ideal range.
Plants do best when growing in water with a pH of 5.4-7. A pH of 7 is considered “neutral”; lower than 7 is acidic and higher than 7 is basic.
- If your pH is either too low or too high, plants may be unable to efficiently use nutrients in the water.
- You can test your water with pH test strips or paper; a 15-foot roll of testing paper costs around $7, and lasts for years.
- Keep in mind that certain fertilizers (ammonium or urea-based) can acidify the water in your system, and others (nitrate based) can make it more basic.
- It’s a good idea to test the pH of your water as it comes out of the tap, as well as after you’ve added your fertilizer.
After adjusting your initial nutrient solution, test the pH of your water every few weeks (commercial growers should test more often, but the pH is unlikely to change substantially in a small-scale system).
If the pH is still too low after adding fertilizer, you can add baking soda to make it more basic. If it’s too high, you can add sulfuric acid, phosphoric acid, or citric acid.
There are also commercial products branded for hydroponic systems that can increase or decrease the pH of your water. Make sure that any product you purchase to change the pH of your water is labeled for food use.
- Alkalinity refers to the presence of bicarbonate in your hydroponic water.
- High concentrations of bicarbonate can increase the pH of the water, so the pH in systems with high alkalinity (more than 75 ppm) should be tested more regularly, and adjusted as necessary.
- You can also reduce alkalinity by using an acidic fertilizer or by adding acids such as citric, phosphoric or sulfuric acid, or vinegar to your system.
- While testing for alkalinity can be a bit complicated, if you have “hard” water that tends to form a crust on faucets over time, assume that your water is likely alkaline.
- If this is a concern, consider purchasing water for your system; you can purchase reverse osmosis filtered water at most grocery stores using refillable containers for around 39 cents per gallon.
Caption: pH testing paper. Rip a small piece from the roll and dip it in your nutrient solution. Wait for the paper to change color, and match the color with the color guide to determine your pH.
2 inch diameter) plastic wheel with a roll of paper inside. The top of the wheel has a color wheel with different colors corresponding to different numbers, which equal pH measurements.
There are 17 nutrients that plants require to grow and develop properly.
Only carbon, hydrogen, and oxygen are naturally available to plants in a hydroponic system.
Nitrogen, phosphorus, and potassium are considered macronutrients because plants use them in large quantities. If you have gardened outdoors, you are likely familiar with adding these nutrients to your garden via commercial fertilizer or compost.
Other nutrients are often not a concern in traditional soil-based gardening systems because the soil naturally contains many of these essential nutrients. However, in hydroponics, you need to supply not just the nitrogen, phosphorus, and potassium, but also calcium, magnesium, sulfur, manganese, iron, molybdenum, copper, zinc, boron, chlorine, and nickel.
The good news is, there are many pre-made fertilizers available that have been specially designed for hydroponics.
Liquid vs. dry fertilizers
Liquid fertilizers tend to be easy to use; you simply measure the suggested amount (the label will tell you how much to add per gallon of water) and pour them into the water. However, due to their weight and associated shipping costs, liquid fertilizers tend to be expensive.
Dry fertilizers are much cheaper but are sometimes sold as a multi-part mix. If you order dry fertilizer for lettuce, for example, you may end up receiving 3+ separate bags that you’ll need to mix on-site. This is because the various nutrients have different weights and solubilities, so they store better and face less risk of separating out if they are mixed just prior to use. Other dry fertilizers are available in a pre-mixed form.
Make sure to check the labels to ensure that you have the property quantities and balance of fertilizers.
Determining the correct balance for each crop
The simplest solution to making sure you have the correct balance of nutrients for your specific crop is to purchase a hydroponic fertilizer solution tailored to your crop. For example, you can purchase fertilizer specifically designed for hydroponic lettuce, or for hydroponic tomatoes.
Read the label to see how much fertilizer per gallon is recommended for the crop you’re growing, and keep plants with different fertilizer requirements in different containers.