I have a beautiful 12" flowering stem that broke off my mandevilla. Can I save it by placing it in water or back in the ground? Thank you!
Flowering hormone inhibits rooting hormone. Likely it will fail to root, but you can try and pluck the flowers and root it.
Here are some articles that will help you to root cuttings: https://www.gardeningknowhow.com/garden-how-to/projects/rooting-plant-cuttings.htm
https://www.gardeningknowhow.com/garden-how-to/propagation/cuttings/root-stimulating-hormone.htm
https://www.gardeningknowhow.com/garden-how-to/propagation/cuttings/honey-root-hormone.htm
I want to overwinter a mandevilla. It is rootbound extensively. Can I cut the roots back along with trimming back the mandevilla?
Yes, you can root prune it so it will fit better in the pot and add new potting soil as well.
Cut back the top growth to a more manageable size.
This article will explain how to overwinter your Mandevilla plant: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm
There are several ways to resolve the problem of a rootbound plant. You can divide the plant, trim the roots back or repot the plant. This article should help: https://www.gardeningknowhow.com/plant-problems/environmental/root-bound-symptoms.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
it out and put it in a container, and if so, when is the best time? I've had it for a long time; the location is good and I don't know if it's geting too much water when the sprinklers go on. I don't know how to take a photo and send it. Can I get an answer without a photo?
Pruning should help, but if it is being over watered and you can't change the irrigation, then you may be right about relocating or potting. In your SoCal location you can do that now this month or wait until spring, March/April. You could even cut it to the ground to rejuvenate it before new spring growth begins.
Here is info on pruning:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/prune-mandevilla-vines.htm
Older plant has never been trimmed. West coast Florida climate. Currently brown leaves on lower part of plant but very green and blooming on top. Should I wait until March to cut back?
Wait to cut it back in late winter/early spring before new growth starts. There is no need to keep such long vines becasue it blooms on new growth. Without trimming, all the flowers will be up high.
Also, the brown leaves should be removed, as well as any on the ground in case an infection caused those. Here are helpful articles:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/prune-mandevilla-vines.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-disease-problems.htm
A neighbor gave me a mandevilla she had last year in a pot. I kept it in a sunny window. It is still growing. My question is can I divide it and give her some back?
If there are multiple stems, and you can get roots with each, then yes.
An easier method, may be to take a tip cutting of the new softwood. You can grow a brand new plant from tip cuttings. This article will give you more information on growing cuttings: https://www.gardeningknowhow.com/garden-how-to/projects/rooting-plant-cuttings.htm
I have several small 6” mandevilla plants that I started from cuttings last fall. I am now thinking of transplanting them to a 12” pot in preparation to placing them outside in about a month. My question. How many of the plants would I put in the 12” pot. I.E. 1 or 2 or more plants per pot.
One. They will compete for space and nutrients with any more than one.
I would go up to an 8 inch, then 10, finally 12 as needed. You will likely run into problems with root rot going sizing up that large that fast.
Here are some articles that will help:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/repotting-mandevilla-plants.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-mandevilla-indoors.htm
The older leaves on my mandevilla are developing a green spot which is turning into a dry brown blob. How can I treat this please?
These links will help.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-disease-problems.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/yellow-leaves-on-mandevilla.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm