When to transfer indoor hydroponic seedlings in rockwool into net pots
You've successfully grown some seedlings in some rockwool cubes, and they're growing every day - great! But when and how do you go from...
The process itself is relatively straightforward, as we will be keeping the plant in the rockwool cube and simply transferring it to a true hydroponics system. If you haven't yet, separate the individual rockwool cubes (while being careful not to tear apart any roots that may have grown already), and simply place the cube in a net pot. Don't worry too much about where the roots are at this point - if everything is done right the roots will reach down and away from the net pot into the nutrient solution on their own.
Then, place the net pot in a deep water culture tub, and fill the tub with enough nutrient solution so that the bottom 0.5" or so is submerged. This is to ensure that until the roots start growing downwards, the seedling can continue to have access to water and nutrient. You can (and should, to save space) put multiple seedlings in a single deep water culture tub, but be careful with placing young seedlings with more mature plants. Mature plants have a much greater water and nutrient uptake, and they may drain the deep water culture solution faster than the young seedling can grow roots down into the nutrient solution. In general, continue checking on the plant to ensure that the roots have contact with the nutrient solution either through the rockwool cube or new root growth.
The question of when to transfer seedlings from sitting in a tray to an individual net pot and deep water culture tub is not a critical one, but there are many different things you will want to consider.
If you allow your seedling to spend too much time in the seedling tray and not in the deep water culture tub, the root system will have limited space to grow and sprawl. This can limit the potential nutrient uptake of your plant as it tries to mature. Additionally, the roots can grow into adjacent rockwool cubes, and since you will need to separate the cubes eventually, these roots will need to be cut off. Not only does this harm the seedling, but now you have rotting roots to deal with in the other seedling's rockwool.
If you've been introducing nutrient solution into the rockwool seedling trays, then this may not have much of an affect. If you haven't however, the earlier you transfer your seedlings to the hydroponics system, the faster it will begin is growth spurt. That being said, seedlings have a lower tolerance for too much nutrient concentration, so you'll need to keep that in mind as well.
Growing in seedling trays is much more space efficient than in individual net pots. So as long as you haven't yet hit a point where you are overcrowding, you may be better off letting your seedlings grow a bit more in their seedling tray before transferring them into their own net pot.
Rockwool cubes grow algae very easily due to their composition and proximity to light. Until you transfer them to a net pot, it is hard to cover them up and prevent light from hitting the rockwool material. Therefore, the earlier you transfer the seedling to a net pot and over the rockwool, the lower the chance of growing algae.
As you can see, it's a bit of a balancing act with these factors. It isn't usually a critical decision, but it can help with the long term health, yield and success of your indoor hydroponics system. As you get more experienced, you can begin to have a more systematic approach in planning which seedlings go where, and at what time.