Hello, and thanks for using the Ask an Expert System,

I also have a purple leaf shamrock, Oxalis. I usually let my plant go dormant in the winter in my unheated, attached garage. I have never had a problem with aphids.  However, I do think that you can eliminate the aphid problem by allowing your plant to go dormant.  Stop watering and put it in a cool place. Allow the leaves to wither. Then remove all the dried, above ground plant material. Thoroughly clean the top edge and outside of the pot with soapy water.  Allow the plant to rest until spring.  That should eliminate the aphid problem.     

If you have further questions you can contact your local the Penn Extension Office.

Cambria Country Penn State Extension
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Thanks for the advise!  I only recently discovered that this plant could go dormant.  I have kept it blooming all year long.  I have had a problem with aphids the last two years.  They seem to be impossible to eradicate once found.  Do you have any suggestions other than the standard soapy water spray or neem oil?   Do aphids winter over in soil?    Thanks again for your help, much appreciated.

One other question.   Your plant is in an unheated garage, but I am assuming that it does not get below freezing correct?   Thanks again.

Aphids can overwinter in soil, although most of them overwinter on plant material.  I think part of your problem is that you have not allowed your plant to go dormant and thus it is available to aphids year round.  Allowing your plant to go dormant and removing as much of the above ground material should drastically reduce any remaining eggs.  Carefully monitor your plants in the spring and spray if there are any signs of aphids.  

My garage is a unique environment.  For the most part, it does not get below freezing.  Sometimes when it is extremely cold the temperature gets below freezing briefly.  I suspect that prolonged periods of subfreezing temperatures may kill Oxalis.  Dormancy also comes about because I stop watering the plant when I bring it into the garage.  I think you can get the same results by keeping it unwatered in a cool basement.  I have had my plant for 10 years, and it has never been actively growing in the winter.  These plants are hard to kill.

I am attaching a link to a fact sheets on all aspects of aphid management from the University of California.  Much of the information may be of no interest to you as it covers aphids on all kinds of plants.  There is a lot of information on various control methods including chemical, nonchemical and cultural control methods.

To access the fact sheet [click here].   

Thank you very much for the link.  Good information.   This is the first time my oxalis has had aphids, and I have moved it to my mother's garage that has some heat so it will not freeze.  I will follow your advise and remove all the dirt and repot in the spring.    I have kept potting soil from several very large pots which I had planned to mix with compost and reuse in the base of pots with fresh potting soil on the top 10 to 12 inches.  The potting soil is stored outside.   Will aphids winter over in this material?   Sorry for the constant questions!   I can not seem to find any information that says freezing the little critters will kill them.  Thanks so much for your help!  Sue

Hello Sue.  Do not remove dirt.  Once the plant goes dormant you should remove all the dead leaves.  There are many different strains of aphid.  Many tree species overwinter as eggs on the branches woody branches.  Some do overwinter in soil, but many of the aphids on herbaceous plants do not overwinter in cold environments.  I really do not expect your aphids to survive the dormancy of their host plant. As I mentioned above, as a precaution, keep a close watch once you take the plant outside in the spring and treat any aphids at the first sign of any activity.  

 Thank you !   Have a Merry Christmas  :)