First you need to know that all stems and roots come from a leaf node (where a leaf was or is). On zonal geraniums there is usually a ring around the stem where the leaf was and 2 tiny dried up things that look like tiny leaves. If you cut above those rings the plant will send out new shoots. It will also send up new shoots from the roots. 1/3 is not a tried and true rule. I usually try to shape the plant when I'm cutting back the stems. In other words if you have a tall gangly stem you can cut it back by more than 1/3.

Be careful not to overwater. Geraniums don't like to be overwatered at any time but especially when they have no leaves. Give the plants the best light you can. If you have grow lights set the plants under them.

You can start new plants with the cuttings. This is taken from my file on plants

TAKING GERANIUM CUTTINGS

First you must make sure everything you use is very clean.

Take your cuttings: With a sharp knife or razor blade cut a 4 or 5 inch
piece from the growing tip, cutting just below a leaf. Remove all but the top 3 or 4 leaves. Let your cuttings
callus over--Lay them out somewhere on newspaper. Mine sit on my spare room
floor. Let them sit for about 5 days. The leaves will look wilted and dead but
they're not. The cut end will shrivel and dry. Letting them callus over and
watching the water cuts down on blackleg a deadly fungal disease especially to
cuttings.

Now plant your cuttings. In a clean pail mix your soil with the water so you
make sure the soil is damp right through. Use a soilless mix like pro mix or
one that is formulated for seeds and cuttings. Just make sure it has no
fertilizer in it. You want the soil wet but not too wet. Squeeze a handful and
some moisture should come out but not a lot. Fill your pots and make a little
hole in the center with your finger or something else that will make a small
hole.A small pot will not be as easily overwatered. I use six pack tray inserts to start them and transfer to larger pots as they grow.

Dip about 1/2 inch of the cut end in rooting hormone. It comes in powder or
gel, either is OK as long as it's for soft wood cuttings. I use Stim-root NO.
1. After it's dipped put it in the hole and cover it in, and you're done.

Keep the soil moist but not overly wet. They will look wilted at first but
that will change as it starts to grow roots. You will know it's rooted when it
starts to show new growth. If you are going to plant it outside, wait until it
is rooted well before you set it out.

Last year I used a heating mat under my cuttings and I only lost 1 or2 of my cuttings to black leg. Black leg is a fungal disease of geraniums. The stem turns black and the cutting is dead or soon will be.. As with most fungal diseases it thrives in cold and wet. Adding the heat mat keeps the soil warm and the nasties away.

Good luck

Thank you. That helps!