Mulch layers that are thick enough tend not to have weeds growing through them, with the exception of some of the tougher perennial weeds (Canada Thistle, for instance). A two- or three-inch mulch layer is typically sufficient to stop weeds without risking suffocating the roots of desirable plants. The mulch in the photo appears to be relatively thin, so if last year's mulching degraded this much by winter/spring, then perhaps a slightly thicker layer is needed this time around or a mid-year top-dressing of another inch would be beneficial. Pine bark and cedar mulches tend to biodegrade more slowly than shredded hardwood, but decomposition rate also depends on your soil's microbial population plus weathering and other environmental conditions, so can be hard to predict.

The weeds pictured appear to be chickweed, which is a winter annual species. This means that germination largely took place back in autumn as the weather cooled and the daylength shortened, so prevention methods for these types of weeds are best done in late summer. Prevention for summer annual weeds (crabgrass, for instance) is done in spring since that's when they're germinating. Winter annual weeds already present will either need to be manually removed (which is relatively easy since their root systems aren't as extensive as those of perennial weeds) or treated with an herbicide.

Post-emergent herbicides kill weeds already sprouted and growing; pre-emergent herbicides (Preen being one type) prevent germination and won't impact weeds already growing. Pre-emergent herbicides shouldn't be disturbed after application because the chemical barrier at the soil surface that they form is key to how they halt germination, so any digging, hoeing weeds, or raking mulch afterwards can render an application ineffective. Therefore, in a situation where weeds are already present, they should be removed first (whether by hand or with an herbicide), then new mulch applied, and then (if appropriate or needed) a pre-emergent used to prevent any new weeds from appearing. Usually, though a fresh mulch layer is enough without the added need for pre-emergent.

As for using an herbicide at this point, you could select either a systemic (likely overkill for an annual weed) or a contact product, the latter category of which encompasses any organic options. While each option could potentially harm adjacent desirable plants, the risk mainly stems from foliage contact, so dormant deciduous plants like Spirea will probably be unharmed. Contact products don't kill roots, only above-ground growth, so repeat treatments might be needed. Given the more limited root energy stores that annuals have compared to perennials, a contact treatment is probably all you'd need for something like chickweed -- equal in efficacy to just pulling it up or hoeing it out manually.

Newspaper underneath mulch isn't an ideal solution and mulch by itself should be just as effective, if not more. Landscape fabric (weed barrier fabric) doesn't biodegrade and can be difficult to remove if desirable plant roots become entangled in soil below and above it (as mulch decomposes into organic matter atop the fabric, weeds can then germinate in that layer, plus it can tempt root growth from the desirable species). Landscape fabric has useful applications, but mainly as a temporary measure to smother plants needing removal or in other situations different than this (such as when barriers to root growth are needed at the bottom of a raised vegetable bed).

Miri

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