Better Propagation: Learn How to Air-Prune your Seedlings and Cuttings

Air-pruning plants is such an easy technique, and yet most growers haven't even heard of it! It's all about getting your young plants to totally fill their propagation media without root-circling. Think of air-pruning like 'revving' an engine in a car. You're priming these plants so that when they are transplanted into a hydroponics system they are literally bursting out of their starting blocks!

Propagation is arguably the most important stage of plant development to master. Get this right, and your plants will-lo and behold-grow themselves! Or that's the way it'll seem anyway! With the right foundation, your plants will leap from the starting blocks and truly become the majestic creations their genes predestined.

On the other hand, if you rush propagation, it will seem like your plants never really get to the thriving stage-they will just "exist and get by"-the plant equivalent of a long-term career employee at the Internal Revenue Service just waiting to reach retirement age.

A lot of plant problems later in life start with poor propagation.  Novice growers are prone to overexcitement when they see the first signs of roots emerging from their rockwool cubes or propagation media. As such, they are all too apt to move them prematurely to the next stage-be it a hydroponics system or a simple plant pot-without first taking the time to establish the plant properly. This really is growing 101 guys, so beginners really need to listen up! The following advice could transform your fortunes!

Pre-soaking rockwool blocks in a mild nutrient solution at pH 5 forms part of crucial prep when working with rockwool. Photo credit: Aquaculture Hydroponics.

Take your Time!

Rockwool blocks are a great choice for propagation media-perfect for creating batch after batch of healthy plants for all types of hydroponic techniques. Pre-soak rockwool blocks in a suitable nutrient solution before use - just as you should with any rockwool product. This will balance the alkalinity and pre-load the substrate with nutrients. You can use a good quality low strength hydroponic 'grow' nutrient but better results will be achieved using young plant nutrients that have higher micronutrient content.  Whichever nutrient you use be sure to pH adjust the nutrient solution down to the lower end of the acceptable hydroponic pH scale 5.5-5.8. At this point adding a liquid beneficial microbe product to the nutrient solution is a good strategy to help with root establishment and disease resistance. Leave the block to soak for 12 to 24 hours.

Once the blocks have soaked allow them to fully drain and shake out most of the excess solution before planting.  A light squeeze is ok but be sure not to be too heavy handed and compact the rockwool fibers.  Some growers even go outside and swing the cubes around above their heads in a pillowcase to drain the excess moisture!

Air-pruned cucumber plant - ready for transplantation into a hydroponics system. Photo credit: Aquaculture Hydroponics.

Hardening Off

Plants don't like extreme changes in environmental conditions so when you bring your cuttings out of their warm and humid propagator into a cooler and drier growing area they could receive a bit of a shock. To minimize this you should try to gradually accustom your plants to the different growing environment over a few days. This process is called 'hardening off'. So, after planting the cuttings into the pre-soaked rockwool blocks put them back in the propagator with the vents open. After a day leave the lid of the propagator at an angle to allow more airflow and slowly reduce the relative humidity and temperature inside the propagator. After a few days the lid can be completely removed as the plants will have become more acclimatized to their new environment.

Growing On

Maintaining a favorable environment is very important for young vegetative plants. Aim to keep daytime temperatures between 73 - 79 ºF (23 - 26 ºC) and try to keep the night-time temperature differential as small as possible, this will promote vegetative growth and keep the plants short and stocky. You should aim to keep the relative humidity (RH) between 65-75% which will encourage a decreased transpiration rate and allow the plants to concentrate their energy on root and leaf growth. It's easier to maintain these environmental parameters using fluorescent lighting.


During the propagation stage plants are very sensitive and should be watered with care. You must ensure you provide them with adequate amounts of water and nutrients while not over or under-watering them. You must never leave the blocks standing in water, nor must you water them every day regardless of whether they need it or not. A good method to check if they need water is to lift the blocks, if they feel fairly heavy, don't water them. Wait until they feel half their original watered weight before giving them anymore.  Some growers even weigh their young plants to determine this accurately!

And Finally ... Air-Pruning!

When the plants are hardened off it is vital that you concentrate the transplant's root growth within the rockwool block. The last thing you want to do at this stage is to place your rockwool blocks on a bed of moist growing media to encourage roots to grow out. All this will do is produce plants with a few long roots coming through empty rockwool blocks. Plants grown in this way will be more prone to root disease through damage caused when the blocks are picked up and moved.  They are also more likely be set back when planted out as the lack of root mass within the block will not be able to cope with frequent irrigations and the large volume of water held by the block. Inevitably, this will cause the plant's health and growth rate to suffer through over watering. This scenario is very common with growers using nutrient film technique (NFT). Don't rush those plants into the NFT system!  The aim of the game is to create a batch of plants with an abundance of roots filling the internal structure of the rockwool blocks before planting out. The best way to achieve this is using a technique called 'air-pruning.'

Air-pruned tomato plant. Photo credit: Aquaculture Hydroponics.

Essentially air-pruning is a technique that stops root growth from extending beyond its growing media by exposing the roots to relatively dry air. When propagating with rockwool blocks the plant's first primary roots will grow down through the block until the tips grow out the bottom. Once the primary root tips are exposed to the relatively dry air they will stop growing and die back.  Sounds like a bad thing?  Read on!  As these primary roots now have nowhere else to grow it stimulates an abundance of secondary roots to branch out from them, these secondary roots will then spread through the block until they get air pruned and then stimulate more root growth. The overall effect of this growth and pruning cycle will create a plant with a well developed root system throughout the entire block. Air-pruned plants will literally explode with root growth when they are transplanted into their final hydroponic system.

To make an air-pruning propagation system all you need to do is create an elevated rack on which you can place the rockwool blocks to enable air to flow underneath them. The first time I experimented with air-pruning I took a shelf out of the oven, propped it up with a plant pot on each corner and hey presto! My first air-pruning propagation tray was born.

Now I run a more automated air-pruning system in which I have made a metal mesh tray to fit inside a ebb and flow table. This enables me to irrigate all the plants at once by flooding from the bottom. As the flood level rises up past the mesh it reaches the lower half of the blocks, as the nutrient solution drains away it exposes the bottom of the blocks to the air to allow the roots to get pruned. Simple yet highly effective.

Air-pruned chilli peppers growing on an air-pruning rack - this allows air to reach underneath the blocks (really important!). Photo credit: Aquaculture Hydroponics.

Plants that undergo this air-pruning technique may take slightly longer in the propagation area but it will pay off massively when you see them take off in their final hydroponic system.