Will adding 0-0-5 fertiliser (high rate of potassium) to the water in which I'm keeping cuttings of Ficus help them develop roots faster?
How much is too much?
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You've not said which variety of Ficus you're trying to root in water - I know it works with Ficus benjamina and its cultivars, but not sure it does with all varieties. I wouldn't add anything to the water, especially not fertilizer of any sort - for one thing, the cutting cannot use any fertilizer until its got roots anyway, so it won't help. The cuttings will either root or they won't, but the water they're in should be topped up or changed every few days or so. Appearance of root initials can take anything between 4-10 weeks in my experience, but more usually around 6 weeks, assuming you chose non woody cuttings of the right length and removed most of the leaves (leaving only two or three at the top) in the first place. Once roots start appearing, pot them up into potting compost, preferably seed and cutting or multi purpose rather than a stronger mix.
I've never tried to root Ficus lyrata in water, but found a thread here http://acuriousgardener.blogspot.co.uk/2013/02/caring-for-and-propagating-fiddle-leaf.html which, whilst short on detail, shows Ficus lyrata cuttings in water - they're pretty large and do look woody at the bottoms - said to take some months. Also found confirmation from a proper hoticultural site that adding fertilizer to ficus cuttings does not promote root growth, and gives other methods to root F. lyrata http://mrec.ifas.ufl.edu/foliage/resrpts/rh_90_6.htm
Why are you rooting your ficus in water? The roots that are formed are water roots not soil roots. I would forgo any fertilizer, purchase great starting soil, provide a cover or dome for moisture and stability in warmth/humidity and plant right in the soil.
Do not add fertilizer until the third set of leaves. Once plants have emerged from the soil you have to have great grow lights. Rooting in water is not a good idea at all. You should be able to try to transfer your water rooted vegetative ficus chunks to great sterilized soil for starts. Never use garden soil! Go to a place that caters to pot growers! Truly the best place to get soil and testers and lights and filters and fans and and...the people there are usually more knowledgeable than those at Lowes or Home Depot!
I’m no ficus expert, though my green thumb is something I’m proud of. As for not rooting in water...I guess it very well depends on the specific type of ficus you are talking about. I have 5 benjamina ficus cuttings that were given to me, all about a foot tall. I didn’t trim them down, didn’t do anything except put them in a water mixture. I kept them in a 50/50 ratio of water and all purpose plant food mix (plant food was mixed with water and then diluted 50% with more water)for a month. Changed out the water every couple days. Not only did the plants all get tons of new growth during their time in the vases of water they were in, as soon as I planted them in pots, their growth has been amazing. Not sure if the roots were water roots or what...it took the roots a good 2 weeks to grow. They were thick and not very long when I planted them in dirt. I guess what I’m getting at is that to me, it just made sense to put them in water with an all purpose plant food (again, diluted to 50% strength), as the plants are in shock when you cut them. Water gets sucked up fast when plants are cut and a quick start or plant food additive can help provide the nutrients it needs. I’ve done about 30 cuttings now in the past year this way...not all ficus. And haven’t lost a one. From foot tall cuttings to small little ones. I don’t know the science behind why you should or shouldn’t do the cuttings in water. All I know is that it’s always worked for me and that I get new growth on my plants during their 3-4 week water phase. I would think that new growth would mean the plant is getting nutrition and thriving. Just my experience though. Not saying anyone is right or wrong, just what has worked for me.