Garden Detective

Garden Detective: No primrose path in eradicating invasive plant

We hired a company to landscape the back and front yard last spring. The plants were planted around May 1. In October, I noticed that one of the plants – Oenothera berlandieri – was getting leggy and ugly, growing very large. I decided to get rid of it, so dug it up, not realizing how deep the roots had become.

Four of that plant were put in different parts of the yard. I noticed that some new sprouts had come up in other nearby plant sites. When I called the owner of the business, he said, “Don’t worry; the roots will die when the plant is gone.” I decided to research the Mexican evening primrose and upon Googling it, found out that it is like a weed and can take over your garden. I looked at the other three plants and found they had also sprouted new growth in nearby plant sites. ...

I dug out the site where I removed the first plant, going down 10 inches and removing every root I found. It had spread 4 feet by 3 feet. I took out all the soil 8 inches down and put it in a wheelbarrow because I was afraid I had not gotten every single root. ... There are two more areas where I got most of the roots out, but I left the soil in the ground. I have one plant still in the ground where new sprouts are appearing as we speak. I need to know what is the best plan of action to eradicate any signs of that plant.

– Joan Conklin, Sacramento

According to UC master gardener Anna Symkowick-Rose, the Mexican evening primrose is very difficult to eradicate. Many people choose to place it into landscapes because it is a pretty pink flower that will continue to show beauty for several seasons.

One way to deal with the plant is to keep it but make it more appealing by trimming after each bloom cycle so that it will not appear “leggy.” Full sun exposure and avoiding excess irrigation also helps to keep plants bushy. The more water it gets, the more this usually low-water plant seems to grow.

However, if you want to try to eradicate the plant, here are some suggestions. First, continue doing what you have already done, digging out the plant and sifting through the soil to remove roots in open areas. However, this could cause some damage to other plants. The process will also likely need to be repeated.

If you keep all sprouts cut back, eventually the roots will die due to lack of the plant’s ability to produce food. This could take some time, and would require diligence on your part.

Secondly, the plants could be cut down to a 1- to 2-inch stump that could be brushed with a non-selective herbicide such as Round-Up that will kill anything it comes in contact with. This, too, could kill other plants in the area. (Roses, for example, are very sensitive to Round-Up.) Make sure to follow the application instructions on the product.