NOt for aphids but great for adding Mg to the soil twice a year.

Kitchen chemists mix 1-2 drops of liquid dish detergent to 24 oz water in a spray bottle. Aphids shouldn't be a problem in 100F temps, though. You can also use 1 oz baking soda to 1 gallon of water to control aphids. Add 1 tbl bleach OR ammonia to control powdery mildew. Again, shouldn't be a problem in 100F temps.

Spider mites, OTOH, are here and can be controlled by spraying the undersides of rose leaves 2-3 times a week with a hard jet of water. You can purchase a multi-port water wand or trade out the water wand head with a brass hose nozzle and adjust it for a hard spray. Always hose off your roses right after a wind storm. Mites like dry, dusty leaves.

i was told marigolds keeps aphids and a few others away pretty well so i incorporated them into my garden. but roses are taller so im not sure if that would work.

i found a link for you, it's 10 natural or homemade aphid repellants. i love google!

Here is a link that might be useful: 10 eco friendly aphid pesticides

Gardenias love epsom salt. I sprinkle the soil around mine as soon as noticed older leaves turned yellow. No yellowing anymore. I got my gardenia veitchii this spring, it is blooming and so pretty. I can not believe it is doing so well when 95F outside. I have to water every day (it is in the plastic pot) sometimes twice a day. Water rinses Magnesium fast from the soil, so adding epsom helps.
Do you mist your Gracie gardenia? Mine started to open flowers after I start misting it a couple times a day with distill water....

The old fashion, small flower, French Marigold is the variety that is referenced when discussing it repelling ability. Most swear by them but some say no but I think the noers are using the modern MG.