Which Essential Oils Kill Germs Best? A Fun Science Experiment

I love science, it's was always my favorite subject in school. I even majored in biology in college. Luckily my daughter seems to have inherited my love of science. She loves learning about insects, dinosaurs, space, geology, and this year she asked to learn about germs. And so we've spent the last few months growing bacteria on agar in petri dishes and testing all sorts of products. One of our experiments tested the effectiveness of various natural household cleaning products. If you haven't already, you can read about it here. Since our essential oil spray scored the worst on our previous experiment, we were curious if essential oils would work better full strength and which ones kill germs best, so we designed an experiment to find out.

Our Experiment: Which Essential Oils Kill Germs Best?

First we decided which essential oils to test. We went with tea tree, cypress, eucalyptus, clove, lemongrass, and sweet orange and then I had my daughter write a hypothesis. She hypothesized that essential oils would kill germs well full strength and that sweet orange would work best (mostly because it's her favorite oil I think lol).

The petri dishes we use come pre filled with agar so there's no preparation, they come ready to use. You can order them yourself here. We labeled one dish as our control with a permanent marker and then labeled the rest of our dishes with the name of each oil we were testing (we were running low on petri dishes so we divided them in half with a permanent marker so we could test the maximum amount of oils. Probably not the most scientific move I've ever made, but it worked okay).

Then my daughter's favorite part, smearing her dirty fingers all over the agar in all our petri dishes to introduce bacteria. Then we set the lid back on our control dish and set it aside (no oils would be going into this dish). Then we dropped two drops of each oil on it's labeled dish and replaced the petri dish lids.

Next we set them inside our homemade incubator (just a large pot with a lid and warm water bottles for heat inside) and let them incubate for 3 days, changing out the warm water bottles several times a day. After three days these were our results:

Control petri dish: essential oil germ fighting experiment

Germ fighting essential oil experiment. Homeschool science.

Germ fighting essential oils: ckove and lemongrass

Our Results

Clove and eucalyptus appeared to kill all the bacteria they touched. Lemongrass also seems to be a pretty good germ killer. Sweet orange didn't look like it killed anything, so probably not the best choice for cleaning. I'm surprised by the tea tree oil results. I had always heard that tea was supposed to be a pretty effective germ killer but it didn't appear to kill much bacteria in our experiment. I'm not sure if we messed something up or if tea tree essential oil just isn't as good a germ killer as I thought. I'd like to redo tea tree sometime when we get a chance.

To finish up I had my daughter draw pictures of our results and record the number of bacterial colonies that grew in each section and write a conclusion. She was a little bummed that her favorite sweet orange oil turned out not to be an effective germ killer, but hey, it's still good for lots of other things.

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