By Diane Sommers, Master Rosarian
Of all the things I enjoy about gardening, pulling weeds is not one of them. When I consider all the time I have spent over the years pulling weeds, I know there has to be a better way to manage weeds. Some years I am more diligent than others and some years the weeds get the best of me!
The practice of manually pulling weeds can be tedious and time consuming. Early in my gardening “career” I used fabric cloth in my rose gardens. I soon realized that wouldn’t work, as I would be adding plantings and cutting through much of the cloth. Indeed, it is difficult to amend soil when there is a cloth covering it. Of course there are other means to employ in the garden, one of the most popular being the addition of mulch to the garden. Mulch can preserve moisture, improve soil composition and deter weeds from sprouting. I have also used pre-emergent herbicides in my garden, which will be a focus of this article.
How do pre-emergent herbicides work? In answering that question, we need to understand the chemical make-up of products used for pre-emergent weed control. The most common product I see in the market is Preen. I have been using Snapshot for years as it was recommended by my landscaper when we built our house. The active ingredients in both these products include Trifluralin, with Isoxaben added in some formulations. Trifluralin prevents seeds from germinating by binding to proteins used in chromosome production thereby inhibiting root development. It does not discriminate between your flower or vegetable seeds and weed seeds and therefore it is important to not apply in areas where you plan to add such seeds. Very little of the herbicide is taken up into the plant, and it is strongly absorbed to soil particles with negligible leaching. It will not hurt your roses unless you are planting rose seeds!
I asked participants in the ARS Rose Forum about their use of pre-emergent herbicides. Preen was most mentioned. There are different formulations of Preen available, something that I was not aware of. Weed seeds, like other seeds, can germinate all season so you will either want a product with a longer coverage or you will want to be prepared for additional applications during the garden season.
As you can see in the table below, the higher concentration of Trifluralin and the added Isoxaben will lengthen the control period along with the number of different weeds controlled. That added control comes at a cost, however. Also, Preen has formulations with fertilizer in it. While the “do it all at once” approach may sound simpler, I find it is more important to understand what your soil is lacking and then address specifically at the proper time. Fertilizer applications in early spring before the roses are growing will have no impact. Identifying the weeds problematic in your garden and understanding which product can prevent them is also important. For example, I have had a problem with bindweed in my garden. The Preen Weed Preventor will not control it but the Preen Extended Control, Snapshot, & Anderson 2.5 will. The list of weeds controlled can be found online with their product descriptions.
| Product | Trifluralin | Isoxaben | Length of Control | Toxicity | # of Weeds Controlled |
| Preen Weed Preventor | 1.47% | None | Up to 3 months | Caution | 37 broadleafplants |
| Preen Extended Control | 1.5% | 0.375% | Up to 6 months | Caution | 125 broadleaf & grassy plants |
| Preen Garden Weed Preventor Plus Plant Food | 1.47% | None | Up to 3 months | Caution | 37 broadleafplants |
| Snapshot | 2.0% | 0.5% | 6-8 months | Caution | >100 broadleaf and grassy plants |
| Anderson 2.5 TG | 2.0% | 0.5% | 6-8 months | Caution | >100 broadleaf and grassy plants |
When to Apply a Pre-Emergent
Application must take place prior to germination of the weed seeds for the products to be effective. Literature suggests that a soil (not air) temperature of 55 degrees is best for timing of weed seed germination. I think this has been my biggest challenge is in application timing. In spring 2024, following a “warm” Wisconsin winter, I applied my pre-emergent on March 10th, about a month earlier than typical for me. It snowed a week later yet weed control in my gardens that year was good. Lesson learned. If the product that you are using is effective for only three months, then you will probably want to apply it again during the summer – remember weed seeds germinate all season. If you plan to do some planting in your garden (as I do), simply apply the pre-emergent in early spring and then after digging around in your garden. In 2024 I also purchased a simple handheld applicator to make sure I was evenly applying the product and I think that helped with my results for the season. Annuals and perennial plants will be fine to add to the treated garden but not flower seeds. The reseeding of perennials may be limited if you use a pre-emergent.
Safety Matters!
In reviewing the safety data sheets for these products they all indicate that you should wear protective eye covering as well as equipment to protect from inhaling of the product during application. Wear long sleeves and pants along with shoes (no sandals). Read the product information sheets to determine if you can use the product on edibles (herbs and vegetables). I also noticed that these products are toxic to aquatic life.As always, understanding the specific problems in your gardens and then targeting actions and products to address them is the best approach. I hope you find this informative.
Greater Milwaukee Rose Society 