Rootrainers are what I've chosen for years to grow overwintering onion sets, garlic and spring planted onion sets. Onion sets (small immature onions) allow me to have onions all year round. But, originally manufactured to grow tree seedlings, why would I use Rootrainers for onion sets?
Growing onions from seeds can be a hit and miss affair, with weather, birds and the like all affecting the young seedlings’ growth outdoors, particularly those seedlings that have to overwinter. I am not after prize winning onions anyway or even huge onions so don't need to use onion seeds. I prefer sets. However, whenever I planted sets direct into the soil outdoors, within a few days, I would find many of the sets had been pulled up from where I had planted them and scattered all over the soil. Who or what was responsible?
Some detective work - root zones and rhizospheres...
The root zone is the name for the region of the soil around plant roots as they grow. The roots produce secretions that help and protect them as they force their way downwards through the soil. The immediate area around the root is called the rhizosphere. There will be many rhizospheres within the root zone of a plant.
The rhizosphere is the most dynamic environment in the soil, or a microbe ‘hot spot’, the fast food areas of the soil! The roots are also continually shedding old tissue and sloughed-off plant cells. The root secretions and dead plant cells are a feast for microbes living in the soil. In return the soil microbes provide nutrients for the plants, which encourage plant growth. More plant growth means more roots. A win-win situation! Hence soil microbes themselves congregate around the roots. Microbes in turn are a beacon for earthworms, who sense the root secretions and microbes in the soil and target such ‘hot spots’ to feast upon them!
In my garden its the blackbird. Blackbirds deliberately pull up the onion sets to seek out earthworms and other such tasty morsels, living and feeding in the root zone underneath the onion set.
By planting onion sets in Rootrainers, the roots will grow in and around the compost, binding it together. Then when planting them outdoors, unless the blackbird has fed on at least 3 Weetabix (!) that morning, it will not have the energy or strength to pull out the onion set with its well rooted fibrous compost ball, which is a lot heavier than an onion set on its own. Result: no more scattered onion sets!
Due to their size onion sets are great for children and school projects too. Handling them is easy. Planting requires less skill as sets are easier to handle than seeds. Together, sets and rootrainers make the exercise easy and interesting for children to do for themselves. One major advantage for schools is that the children can grow AND harvest them all within the school year.