The purpose of a pre-emergent is to prevent seeds from germinating. Therefore, any product marketed as a pre-emergent should keep seeds from germinating without harming the juniper.

However, to be sure, always read the label on the back.

You can start by looking at a product such as Preen. Preen has several products, in order to get the best results, make sure you get the one for gardens (versus the one for lawns, for example).

One factor to consider in the success of using such a product is the germination cycle. Different seeds germinate at different soil temperatures, so applying the pre-emergent at the proper time is important. Apply it too early and it might be gone by the time the seeds germinate; apply it too late and the seeds have already germinated.

Another factor is whether you truly have annual plants (which come only from seed) versus perennial weeds (those which sprout back from roots in the spring. If you have perennial weeds, then this will only handle the new ones that sprout from seeds.

Some chickweeds are annuals and some are perennial, depending on which one you have.

Here is a link that might be useful: Preen products

I had chickweed all over my lawn last year. We had it treated by ChemLawn and it was gone. I think you can treat for certain weeds without harming other plants. But I'm not sure where junipers fit in with that. A pre-emergent will keep them from coming up, but once they are up it's too late. In the lawn at least, they tend to be overtaken once the grass starts growing. But in a shrub bed, they can last longer.

A little trick that works well in all sorts of situations where you've got weeds mixed in with other plants. It's called "whisking". Put on some rubber gloves. Get a spray bottle of Round Up type herbicide. Spray your gloved hand. Then CAREFULLY grab the weeds where possible to coat the leaves without touching your other plant material. Be careful to spray your gloved hand over a tarp or something. THis has been very effective for me in getting Poison Ivy out of Liriope and in lots of other situations.

Thanks for this tip travelergt4. I'm going to try it to get rid of the honeysuckle vine growing in the middle of my favorite shrub.