By: Justin Russell | September 13, 2013
It's early spring, which means my greenhouse is full of seedlings. Some have just poked their heads through the seed raising mix, others are at the cotyledon (seed leaf) stage, while the oldest have grown their first set of true leaves and are nearly ready to be gradually exposed to more light and wind before being planted out in the garden.
For the first couple of weeks after germinating, seedlings are able to exist on nutrients contained in the seed raising mix. But it's important to remember that seed raising mix is hardly a rich growing medium. It becomes depleted of nutrients very quickly and rapidly growing seedlings can start to look weak and anaemic within days of looking fresh and vibrant.
Nutrient deficiencies manifest in different ways, but the most common is a general lightening of the leaf colour. Green leaves start to turn yellow, and red leaves will become pink. In either case, it means the seedlings are hungry, and will become stunted and useless if not given a feed.
The easiest way to provide nutrients to seedlings is in a liquid form. Seaweed extract can help, but it's more like a plant tonic than a fertiliser, so instead I use a fish emulsion based fertiliser diluted to about half the recommended concentration. I apply this with a watering can fitted with a gentle shower hose, but you could also use a pressure sprayer to lightly mist the plants.
You'll find that after being fed your seedlings will perk up pretty fast. But the downside to liquid fertiliser is that it is easily leached from the seed raising mix. To ensure your seedlings get the nutrients they need to become well equipped for the garden, it's a good idea to feed them weekly, always at a reduced concentration to ensure their fine roots aren't burned.
After the seedlings have grown their second set of true leaves, it's time to harden them off by easing them out of the greenhouse and into their final growing conditions. For vegies that require full sun, start by bringing the seedlings outside for a couple of hours of gentle morning sun. Over the course of a week, increase the sun exposure by an hour or two each day until the seedlings are adapted to a full day's worth of sunlight. At this point, your little babies are toughened up for the rigours of the garden and can be planted into their final growing positions.