One thing we try to improve every single year is the strength of our seedlings, especially those that are susceptible to pest damage, such as slugs eating them.
The stronger the seedling or plug plant; the more chance it has of surviving any slug attacks or weather in the earlier parts of the season. Plus with drought periods becoming a norm in the UK, the stronger the roots of a plug plant the better.
So how do we get the seedlings to be stronger, well one of the most important factors in a strong seedling are the roots. If the roots are a good strong system they will flourish when put into good soil outside, however if the roots are wrapped around a pot or cell... weak and have become root bound then the plant doesn't have nearly as much chance in the ground.
The best way to get strong root systems is Air Pruning.
What is Air Pruning?
Air pruning is a natural occurence when roots are exposed to air without high humidity. The roots are effectively “burned” off by the air exposure, causing the plant to constantly produce new and healthy branching roots. If roots are not exposed to air, they continue to grow around the container in a constricted pattern... eventually leading to root-bound or pot-bound plants.
A solution to getting strong root systems in the past has been soil blocking, and this is still a fantastic solution if you want to avoid as many plastic trays as you can.
Soil blocking allows you to mix your seed or potting compost with water to get a good consistency and then compact the compost using a soil blocker. The result is a small block of compost that you can seed directly into and the seedling will grow throughout that block. When roots reach the edge of any part of the soil block they will be air pruned and new roots will be encouraged to grow.
You still require a surface or open trays to put the soil blocks on however there is a lot less plastic used. Getting the compost and water mix right takes some practice but can be mastered fairly quickly when you consistently use the same compost.
Soil blockers come in a range of sizes and are available here if you wanted to check them out.
More than 25 years ago Aart Van Wingerden from Holland, where soil blocking was developed, used his knowledge of the well known benefits of soil block air pruning and designed the Winstrip Tray.
Roots obviously grow down and in every nursery tray there is only a round drainage hole at the bottom of the cell. Roots hit the bottom of the tray and circle around the edge and around that hole. A circle on the bottom means circling roots.
The Winstrip tray doesn not have a circle on the bottom, rather it has a strip that extends across the entire bottom of the cell and slits on the sides that reach the opposite corners of the tray. These features air prune the roots, whilst the roots try to circle they hit the air in those corners and stop.
More and more roots grow and the plug / seedling becomes stronger as it waits to get transplanted. It is all about that Winstrip shape at the bottom of the cell. It holds in soil and air prunes at the same time without the requirement for the soil blocking process.
These trays significantly increase the productivity of seeding compared to soil blocking. They are however plastic trays and so we stock both solutions for whatever you would prefer to use.