I have observed that plants like an onion or spinach, after having been harvested for their leaves, will continue to regrow their leaves.
My question is how many times can we keep regrowing them with good amount of production (i.e. not too low than the first harvest)?
I am told that it works only for few harvests (~2-3). But why so? If I 'feed' them well in a an optimal setting, can we get to regrow them for many more harvests?
The common varieties of onions (Allium cepa) are biennials. In the first year they produce a bulb which survives the winter, and in the second year they flower and die.
Of course when they are grown as vegetables the bulbs are harvested and eaten at the end of the first year.
The growth cycle is controlled by day length. Bulb formation starts when the day length exceeds some threshold (between 11 and 14 hours daylight, depending on the variety) and the plant will stop producing leaves when the day length falls below the threshold again.
There are other species which are perennial (common names Welsh onions, Egpytian onions, walking onions, etc) where you could harvest the some of leaves every year, so long as you leave enough for the parent plant to store enough energy in the bulbs to survive the winter.
