There is no foolproof method of repelling deer. Anything you see advertised or described as a deer repellent is overstating its effectiveness. At best a product can deter them. That's an important differentiation. When hungry enough, deer will eat plants they would normally avoid and will venture into areas that pose a threat. By making a garden less appealing, you hope to veer their wanderings in another direction. That's where deterrents come in.
I have a deer problem and would prefer that they avoid my plants and look for more appealing meals so I experimented this year with soap as a smell deterrent; I also recruited fellow gardeners to help in the experiment. My wife makes custom, luxury soap and I asked her to make a batch using specific ingredients designed to maximize its effectiveness as a deer deterrent.
There are certain smells that are more offensive to deer than others. University of Nebraska studies showed that animal-based fats are more effective than natural oils so my deer soap is made with pig lard rather than the olive oil, coconut oil, and shea butter that my wife normally uses. Mint is a plant that often deters deer so part of the fragrance base of my custom soap is mint. The more odorous the soap the more effective it is, so the overall fragrance is amped up. The result is a soap that I can smell from a good distance which means deer will be unable to avoid the odor.
My research uncovered recommendations for placing soap from 18 inches (.5 meter) to 10 feet (3 meters) apart. Encouraged by persuasive university studies, I placed my deer soap from three to four feet apart (1 - 1.2 meter), hanging from a fence next to my sunflowers. Though sunflower is often listed as a plant that deer don't like, they love to nibble on my young plants. I varied the height of the soap from two feet (.6 meter) to four feet (1.2 meter) above the ground.
My experiment also confirmed a limitation of soap as a deer deterrent. Deer are smart animals and will begin to learn that we're playing tricks on them. They begin to recognize that the surprising and offensive odor is now normal and part of the landscape. At about the six-week point, I noticed deer damage to plants at the periphery of my test area. The plants within a two-feet radius (.6 meter) of the soap bars were still untouched. I suspect that sun and rain have lessened the strength of the fragrance and that the weaker aroma is not as effective at the edges of the bed.
One solution is to switch to another smell deterrent before the deer get used to the old stuff. If a garden always smells "bad" with varying offensive aromas, it is more likely that the deer will pass by looking for a meal that is more appealing. I have more soap with different fragrance components to swap out for the earlier batch.
It is also a good idea to increase the number of soap bars and place them as close to the protected plants as possible. Smells diminish with distance so concentrating the odorous soap keeps the smell strong throughout the protected zone.
I had my soap custom made and it lasted a month and a half before losing its peak efficiency. Almost any fragrant soap will work as a deer deterrent. Many people have reported success with soaps like Ivory and Irish Spring. There are many other smell deterrents that work with varying results. Predator urine, human hair, citrus peels, and rotten eggs have all been shown to deter deer but lose their effectiveness after a good rain.
Soap lasts longer than many of these deterrents and doesn't need to be replaced as often. Varying the type of soap to introduce new smells is the best approach to keeping deer away.
Deer soap does pose a potential problem for gardeners. It can attract voles. When the soap dissolves in a rain, the ground beneath it becomes more appealing to voles. If you have a vole problem (I do) and live in a wet region (I don't) you may want to consider changing the location of the soap periodically so it doesn't build up on the soil and become an attractant to another garden pest. I haven't noticed any increase in vole tunnels near my test bed, but I haven't had very much rain.
I'm quite pleased with the results of my deer-deterrent experiment. Soap will continue to be a part of my arsenal in the battle against deer. It doesn't harm them in any way and it keeps my garden intact. While my soap is hung from a fence, hanging it directly from sturdy branches allows the same effectiveness to extend to fruit trees and decorative bushes and shrubs.
With enough soap the entire landscape can become a deterrent to deer.