Thank you for your question.

Unfortunately, you are dealing with the persistence of nature. The trees on this lot will continue to produce seeds for as long as they are standing. The only recourse to this is to remove all the trees. There is no magic spray that you could apply to prevent them from seed production. In the same vein, when these seeds land on mulch or any other favorable surface, they will germinate and produce seedlings. One option would be to distribute a pre-emergent herbicide to all areas of your mulch. This would work in the short term but unless you did repeated applications, the effectiveness of this herbicide would diminish with time. Pulling the seedlings up would work as you remarked in your question, but this would require a constant effort your part. I suppose that you could apply a broad-spectrum herbicide containing glyphosate to your mulch. While this would kill the seedlings, it would also kill all other vegetation in the area. There does not exist a herbicide that will destroy just these seedlings and leave everything else unscathed.

In short, I don’t think there is a quick and easy solution to the issue that you are presenting. It is just the reality of living in a natural world. Thanks for consulting with us.

Thanks for your input. A few follow-up questions: 

1. Can you help me identify the seedlings? 

2. I have tried glyphosate sprays in the past with limited to no success; the seedlings just carried on. So I've been reluctant to try a pre-emergent. Should both work against tree seedlings? I would try again, if it's not a futile effort. 

3. Related, how often and when would pre-emergent need to be applied? Once every season? Early in the spring? In the fall? 

4. This year, I used a hoe to disturb all the seedlings, essentially raking up and redistributing all our mulch -- which was labor intensive! I'm hoping the disturbed seedlings don't just start to sprout again from their new location in the mulch. Thoughts? 

Thanks for your response.

1). As far as I can determine, these are Green Ash seedlings (Fraxinus pennsylvanica). If my identification is correct, you should be able to find mature Green Ash trees in the vicinity of these seedings. See:

https://www.dnr.state.mn.us/trees/green-ash.html

https://campustrees.umn.edu/green-ash

2). Glyphosate (e.g., Roundup) is effective when it is taken up by the vascular system of plants. It will not prevent new seedlings from emerging. Perhaps in the past when you applied it to the leaves, newly emerging seedlings led you to believe that it was ineffective? See:

https://edis.ifas.ufl.edu/publication/AG259

3). Pre-emergent herbicides operate on a different basis. These have no effect upon emergin seedlings. Rather they prevent ash seeds from germinating. For this reason, a pre-emergent herbicide would have to be applied in the very early spring before the appearance of any seedlings. Since seedlings may emerge continually during the spring, multiple applications of the herbicide may be necessary. For maximum effectiveness, follow the instructions on the herbicide label. See:

https://wieseacres.blogspot.com/2013/04/pre-emergent-herbicides-and-mulch.html

4). Unfortunately, these ash seedlings can be persistent. It is quite possible that after hoeing things up, some seedlings may reestablish themselves.

Here is one other possible remedial approach you might take. Let these seedlings grow until they are four or five inches tall. This shouldn’t take too long. Then go in with a weed-wacker and cut them down. You could just leave the cut seedlings on the mulch as they will decompose rather quickly. If additional seedlings emerge from the roots remaining in the ground, cut these seedlings down as well.

One final thought. Green ash seeds are released in the fall. If you watch things at that time, you should be able to rake up most seeds that fall on your mulch. This would be the most proactive step that you might take.

As I previously mentioned, there really is no easy solution to your situation. As long as mature ash trees are present, new seedlings will emerge every spring.

Good luck. Thanks again for consulting us.