Mulches on the soil surface are known to suppress weed emergence, but the quantitative relationships between emergence and mulch properties have not been clearly defined. A theoretical framework for describing the relationships among mulch mass, area index, height, cover, light extinction, and weed emergence is introduced. This theory is applied to data from experiments on emergence of four annual weed species through mulches of selected materials applied at six rates. Mulch materials, in order from lowest to highest surface-area-to-mass ratio, were bark chips, Zea mays stalks, Secale cereale, Trifolium incarnatum, Vicia villosa, Quercus leaves, and landscape fabric strips. The order of weed species' sensitivity to mulches was Amaranthus retroflexus > Chenopodium album > Setaria faberi > Abutilon theophrasti, regardless of mulch material. The success of emergence through mulches was related to the capacity of seedlings to grow around obstructing mulch elements under limiting light conditions. Mulch area index was a pivotal property for quantitatively defining mulch properties and understanding weed emergence through mulches. A two-parameter model of emergence as a function of mulch area index and fraction of mulch volume that was solid reasonably predicted emergence across the range of mulches investigated.

Nomenclature: Abutilon theophrasti Medicus ABUTH, velvetleaf; Amaranthus retroflexus L. AMARE, redroot pigweed; Chenopodium album L. CHEAL, common lambsquarters; Setaria faberi Herrm. SETFA, giant foxtail; Quercus alba L., white oak; Quercus montana Willd., chestnut oak; Secale cereale L., rye; Trifolium incarnatum L., crimson clover; Vicia villosa Roth, hairy vetch; Zea mays L., corn.