What is killing our annuals?

Asked March 28, 2019, 11:46 AM EDT

I am in the DeWitt Garden Club and helping with the cemetery garden and there is a brick walkway that grows mold between the bricks. Along the walkway they used to plant impatients. Then they got that disease so they planted other annuals and they all died. They tried again for a couple years later and continued to all die. One year a member sprayed the walkway with round up to kill the mold – we think maybe they used the really bad round up. Do you think it’s the round up or the impatient disease causing the annuals along the walkway to die? Is there a soil test that could help us figure out what to do?

Clinton County Michigan flowers desalinization soil test results soil compaction

5 Responses


First, impatiens downy mildew can stay in the soil, but it doesn’t kill other flowers except impatiens. Next, if fungi or moss, or mold is growing on brick then you know that area is staying damp-and probably shady. Is it growing on the brick or between bricks in soil? If it is growing between bricks, then the soil is probably compacted. Moss can grow in compacted soil in full sun, moist or dry.

Roundup is a brand, and they make several kinds of products. The longest lasting one can affect growth about 9-12 months.

If you would give me some details on what the area is like, I may be able to suggest some plants to try.

Tell me how many hours of bright sun does it get,

how much water is applied and how often is it watered,

and does the soil seem sandy or more heavy and sticky(clay)?

What annuals have you tried? What flowers did you plant in the past that did well for you?

Does the walkway get salted in winter? (salty soil won’t grow flowers well)

In any case, you should have a soil test done for this area. It will tell soil pH, what type soil you have, and what nutrients may be lacking. A MSU soil test kit can be ordered online or Clinton MSU Extension office may stock them—-989-224-5240


I will watch for your reply to this message. Thank you for using our service.

Thanks so much for the advice. I sent in a soil sample for the soil along the walkway and one for the inner part of the garden. It should be coming back any time now. But for now I have answers to your questions. The area is in full sun
There is a irrigation system. It is watered 5 min everyday and twice a day for 5 minutes in the middle of summer July, Aug.
In the past they've grown Blue Victoria Salvia, Petunias,
Verbena, Sweet William, and Impatients now none of these grow.
The walkway does not get plowed or salted.
I will let you know what kind of soil when the test comes back and the PH If there's anything else you'd like just let me know.

Thanks for the additional information. Since this is full sun but mold or moss still can grow, I suspect two things- too much water for the type of soil( usually it will be a clay soil), and/or compacted soil. Compacted soil doesn’t have enough air in it for roots to survive. Consider aerating the beds before planting. When the soil test comes back, it will state whether you need to add organic matter or not, Let’s await those results and see how to proceed.

Hello Laura,
The soil test came back. Soil type is sandy loam but when I was obtaining the samples I really had to use some force to get deep enough. Along the walkway did seem compacted compared to the the rest of the garden.

Organic matter came back good at 6.2%

I had an added sodium and chloride test done because you asked if the walkway was being salted. To our knowledge it wasn't but the soil test came back at 205 ppm (high) Sodium. So it must have been salted because what else could cause that?

Is there anything we can do to lower the sodium level other than make sure it doesn't get salted in the winter? If not, are there any annuals that can tolerate higher salt levels?

Thanks for your help!


I am glad you tested for salt, and that you discovered the soil compaction issue.

Here are some references -

There are a few Ornamental plants that can tolerate salty soil-


Safer for plants de-icers-


For the compaction, the soil should be worked with a garden fork( a pitch fork like tool with tines that are about 1/2 inch wide) or spade, broken up and turned to a depth of 6-12 inches. Smooth and plant the area. This will have to be checked and repeated each spring, as I suspect over winter foot traffic and bicycle traffic is present. In fall after cleanup, cover the area with 2-3 inches of mulch, which helps reduce compaction over the winter.

For the salt, all you can do is flush the soil repeatedly with water and in time the salt will be rinsed away. Do not apply gypsum unless your soil pH is below 6.0 or so. Otherwise, removing the soil and replacing it is the other option. To prevent salt buildup in future, check if the service that is applying deicer would switch to using a plant safe product, as listed in the second link, above.

Lastly, until the soil salinity is corrected, consider using a row of planters or large pots to line the walkway, and plant in them.

Best regards, Laura