BITTERCRESS (hairy bittercress, lesser-seeded bittercress, popweed)
Bittercress is a small winter annual that will germinate almost any time in container production. It grows upright when the seed stalk starts to form and is easy to hand-weed at that stage, but as a seedling it is very difficult to remove easily. It produces hundreds of seed per plant, and when the capsule matures it expels the seed some distance from the original plant. Remove this weed before it flowers to reduce seed production and new infestations. Herbicides that are effective against bittercress are those that contain isoxaben, oxadiazon, or oxyfluorfen. Sanitation is also important to reduce the spread of this weed. Because the seed of this weed adheres to soil on the outside of the pots, wash pots before reuse or moving them from an infested area. The seeds are also easily carried in irrigation water. Avoid overwatering or allowing water to runoff from an infested area to a clean one.
Common groundsel is probably the most difficult weed to control in container nurseries in California. It is a hardy weed that grows rapidly and germinates anytime during the year in container nurseries, whereas in the field it usually germinates in fall and early winter. Preemergence herbicides suppress the rooting, making the weed easier to pull. Also, because the seedling is smaller after the use of a preemergence herbicide, competition with the desirable plant is not very significant. Remove this weed before it flowers because its seeds are easily spread by wind. Preemergence herbicide combinations containing dichlobenil, dimethenamid-P + pendimethalin, flumioxazin, isoxaben, napropamide, or oxyfluorfen have given good control.
CREEPING WOODSORREL (Oxalis)
Creeping woodsorrel is a perennial plant that grows in a prostrate manner and forms roots along its stems where nodes contact the soil. It is a prolific seed producer. When its seed pods mature, they burst open and forcefully expel seeds, which may land several feet from the plant. Because the seeds are rough, they adhere to clothing or the surfaces of machinery and are easily dispersed. The primary method of managing creeping woodsorrel is to hand-pull established plants before they set seed, being careful to remove as much of the creeping stolons as possible, and to control germinating seeds with mulch or the preemergence herbicides indaziflam, isoxaben, oryzalin, oxadiazon, oxyfluorfen, pendimethalin, prodiamine, or trifluralin.
Cudweed is an annual that germinates in fall and grows through the winter and spring. It is a whitish, hairy plant that has small inconspicuous flower heads. The preemergence herbicides dichlobenil, indaziflam, isoxaben, and oxyfluorfen have been effective in controlling the seedlings of this weed as they germinate.
Liverworts are nonvascular, primitive plants that reproduce vegetatively and through spores. Their flat leaf-like structure is called a thallus and their root-like structure is a rhizoid. These plants can form dense colonies in ornamental containers resulting in crop damage and reduced marketability. Preemergence herbicides containing flumioxazin or oxyfluorfen provide limited control of this weed, but are not labeled for use in greenhouses. There are no selective postemergence herbicides available.
Pearlwort is a low-growing winter annual that roots on the stems and forms mosslike mats in the container. It reproduces by seed. If oxadiazon has been used repeatedly without rotation to other herbicides it becomes a dominant weed in the nursery. A preemergence application of isoxaben, napropamide, oryzalin, pendimethalin, prodiamine, or trifluralin will control pearlwort.
Creeping and prostrate (spotted) spurge are low-growing annuals that grow rapidly and quickly produce seed. They are more easily removed when older, but by then the seeds have usually been produced and fall off the plant into the container when the weed is removed. Mulching reduces establishment. The preemergence herbicides dichlobenil, indaziflam, isoxaben, oryzalin, oxadiazon, oxyfluorfen, pendimethalin, prodiamine, and trifluralin will control spurge.
There are at least two species of willowherb found in nursery containers, Epilobium paniculatum and E. ciliatum. Willowherb seeds profusely and the seed blows in the wind. Preemergence herbicides that have been effective in controlling germinating seeds include dimethenamid-P + pendimethalin, flumioxazin, oxadiazon, and oxyfluorfen.