What is the best way to get root cuttings from annual geraniums? 

Generally speaking the best way to get root cuttings for geraniums (Pelargoniums) is to take a softwood cutting in early spring, late summer or early fall. This would be considered a softwood cutting. We can provide you with the appropriate strength of rooting hormone. The label will indicate the appropriate strength for the plant or for the type of cutting.

I recommend taking a cutting above a node, preferably a new growing point where there is actively growing tissue. Remove all flowers and buds. This will force the energy into root production. Be sure to select from a plant or at least a portion of the plant that is healthy, disease and pest free. I recommend having your medium and rooting hormone prepared in advance and processing the cuttings right away to prevent moisture loss. Do not dip the cutting directly into the container of rooting hormone, rather remove a small portion and apply to cutting. This will keep your rooting hormone clean. Dibble a hole in the soil with a pencil will do. This prepares a hole for the cutting so that the hormone is not removed from the cutting by the abrasion of inserting into the soil. Use a soil that is a good mix of peat and perlite for proper drainage. You can purchase mix that is for roots or seed starting, either would be fine. Your chief concern would be that the soil is clean and has proper drainage.

You may even want to consider dipping your cuttings into a fungicidal to ensure that they are clean. These cuttings can then be planted in a flat or small clean planter. For at least the first week I would recommend putting a clear cover (humidome) over the flat so that it seals in the humidity since the plant will initially lose quite a bit of moisture through leaf respiration. Be sure that the soil doesn't dry out but is not soggy. Watch them closely during this time. They will also need adequate temperature (approx. 70 degrees F. soil temp.) Placing them on a heating mat is best to maintain soil temperature. Ample light is also important you can use a grow light or a florescent light and put it on a timer so that they are getting an adequate amount of light during the day. You can also put them in a sunny windowsill if available. It should take approx. 4 weeks to root and be ready for transplant, perhaps even sooner. (This time frame is a bit of a variable.)

After about the 2-week mark you can gentle remove the cuttings from the medium to see if there are roots initializing. This will provide you with a pretty good gauge on how they are progressing. Then return them to their medium. I recommend doing more than one cutting just in case you lose some. Your cuttings should be approx. 2-3 inches length with a additional 1 inch base in the soil medium of optimum success. After rooting has taken place they can be potted up into larger containers.