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Dahlias Need Help! #878597

Asked July 24, 2024, 2:30 PM EDT

Hello, I have some questions regarding soil pH from my recent soil test in Hennepin County. We built a new house and although I have been gardening for many years, this is my first year gardening at the property. According to my soil test, I was told I have a pH of 7.8 in my garden soil. Nothing on the soil test indicated that would be particularly problematic for a flower garden, as well as no information that it should be remedied. However, many of my plants are struggling, especially my cut flowers and dahlias, and through further research am now finding that my dahlia plants (and other cut flower varieties) are probably struggling with nutrient uptake due to the high alkalinity of my soil. I have over 500 dahlias planted, and not one of them is doing well. I was probably naive to not pay much attention to my pH, but I've never gardened at a property where my pH was a problem. My dahlias were growing very well, but as soon as I pinched them the week before the Fourth of July, they seemed to become stunted and discolored. I have some questions about how to remedy my high alkaline soil: 1. I plan to apply elemental sulfur this fall. Is elemental sulfur something that I should probably plan to apply on a yearly basis? 2. Is there a limit of how much e.s. one should apply at any given time? 3. Once I put down the e.s. is it a good idea to plant a cover crop over it, or will it expedite the break down of the e.s. if I periodically till? 4. As for my dahlias already planted, would there be a benefit to put down iron sulfate/ammonium sulfate this year? By putting it down now, would they begin to uptake nutrients, or has the damage already been done? If so, at what rate would I apply if I am trying to get to a pH of 6.5? 5. I have begun foliar feed my dahlias weekly to try to get them the nutrients they need. Am I wasting my time? 6. When we moved in, our well water was tested and we found out the pH of our water is 7.6. If I'm trying to get my soil pH down to 6.5, am I wasting my time if my water is already that high? Will water pH bring my soil pH right back up to those high sevens? Thank you! Kristen

Hennepin County Minnesota

Expert Response

Good Morning Kristin, 

Thank you for contacting the U of M Extension Service. 

i am not sure the pH is the first problem. Since this is the first year you are gardening at this location, do you have good soil?  Here is one link: https://extension.umn.edu/manage-soil-nutrients/quick-guide-fertilizing-plants

From your photos, it does appear the dahlias are struggling. Just a side note, I was at the Minnesota Landscape Arboretum on Tuesday and their dahlia test garden was not what it usually is at this time of year. 

Please click on the following link about dahlias: https://extension.uga.edu/publications/detail.html?number=C576&title=dahlias#Soil

Here is another link about lowering pH: https://hgic.clemson.edu/factsheet/changing-the-ph-of-your-soil/#:~:text=Two%20materials%20commonly%20used%20for,it%20dissolves%20in%20the%20soil.

This information discusses the difference between aluminum sulfate and elemental sulphur applications. The directions on the package of the products will explain how much to apply. Also, the above article explains it and has a chart. 

I would apply elemental sulphur in the fall. I am not sure about the question about your water, if it will raise the pH again. 

Again, I feel the soil quality is your first concern. Was the soil "garden ready" when you moved in or did you have soil delivered and installed in your gardens? Is the quality of the soil deep enough for your dahlias? 

Could you have introduced a pathogen on the plants when you pinched them? Did you use a pruning tool and did you disinfect it between plants? 

Here is another link that discusses dahlias briefly: https://hgic.clemson.edu/factsheet/summer-and-fall-flowering-bulbs/

There were no other issues in your soil test that pH? 

Read the labels on the aluminum sulfate and also the elemental sulphur. You may need to apply elemental sulphur in the fall every year. Soil tests will give you the results. 

I am not sure what you meant by "foliar feeding". Please ensure you are not over fertilizing. 

Good Luck!

Maureen Graber Replied July 25, 2024, 9:03 AM EDT

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