Hi Jacqueline, 

Thanks for your question! The good news is that whiteflies are warm-weather pests that likely won't survive the winter in our area.

I would recommend holding off on cutting back your plants. Plants like your geraniums use this time to store energy in their root system for next year's growth. Wait until our first frost kills back the top foliage naturally. Once the broadleaf foliage turns brown and dies back, cut the dead growth down to ground level and clear away the debris to remove any overwintering insect eggs or pupae. Dispose of affected foliage in the trash. 

Plants like geraniums are preparing to go dormant at this time of year. So, moderate to minimal leaf damage this late in the season will not kill the plant or damage its root system for next spring. You can just wait for it to frost and take no other steps to deal with the whiteflies until then. This is likely the best course of action to take. 

If you do not wish to wait, you have two other options: 

1. Before using the insecticide spray, try using a garden hose with a nozzle set to a firm spray setting to thoroughly blast the undersides of infested broad leaves. This physically knocks off adults and nymphs without any chemical residue.

2. If you don't see any improvement with the hose and would like to use the insecticide spray, make sure you get the ratio water to soap ratio correct. The recommended ratio depends on the strength of the brand that you purchased. There are various ratio amount recommendations available online, but I would recommend starting with smaller soap amounts until you know how your plants tolerate it. You may also want to test the mixture on a small part of the plant(s) first to see how it reacts. Spray in the early morning or late in the afternoon (to avoid direct sunlight), and specifically target the leaf undersides. Repeat only once every 5-7 days if the whiteflies are still present.

Hope this helps tackle the whitefly issue. I know they can be annoying!

Best,

Kellie | DC Master Gardener