How to prevent rot when growing herbs in just water

Question

I've read that it's possible to grow herbs like mint and basil in just jars of water, and articles like this and this seem to suggest that rooted cuttings can thrive in just water for a long time (the author in the first link mentions 6-12 months in the comments). However, I find in my attempts that no matter how clean my tools and water are, the wounds on the cuttings (where I've made the cut, and where I have removed leaves) begin to turn black and rot within a couple of days of being submerged in water. I have been changing the water every 1-2 days. My cuttings now have roots, but the rotting is still progressing, and I will probably have to pot them soon to stop the rot taking over.

I was wondering if anyone has had success growing cuttings in still water for more than a couple of months, and in your case, what did you do to stop the rot from developing?

Answer 1

Of course Hydroponics growers do this all the time, growing plants in liquid medium, but they take special care to provide a suitable environment. First the liquid contains nutrients specially balanced to feed the cuttings. Second there is are special measures to ensure that the roots have access to air. This can be done a couple of ways, first by increasing the air dissolved in the water and second by having long roots partly in air and partly in the liquid medium.

In theory you can have just cuttings in plain water - the growing tips will then steal nutrient from the lower parts of the cuttings causing those leaves and shoots to die off, and while this can go on for a while eventually you end up with a mess. Water may start off aerated to some degree but will soon become stagnant unless replaced regularly with fresh.

I have some willow cuttings in plain water which have been surviving for about 3 months. In this case they don't mind having roots entirely underwater for a while but as the water level drops due to transpiration so some roots find air until I fill up again. Nutrients are stolen from the woody cuttings and so no real rot is visible, but at the same time they don't make the kind of top growth that their cousins out in the field achieve.

Probably the key to preventing rot is to choose as woody material as you can find, then there is little to rot. It will be a case of balancing hardness with ability to produce roots, which is probably why mint is a good candidate.