Weed Management in Horticultural Crops

PROPAGATION OF WEED SEEDS

PROPAGATION

Propagation is the process of multiplying or increasing the number of plants of the same species and at the same time perpetuating their desirable characteristics. There are two general methods of plant propagation: sexual and asexual propagation.

Reproduction by seed

Reproduction by seed is called sexual reproduction. It requires pollination and fertilization of an egg which results in seed that is capable of producing a new plant. Seed production varies greatly among and within weed species in part due to environmental variability between years, competition from neighboring plants, and genetic variability. For example, while Canada thistle has been observed to produce as few as 680 seeds per plant, Curly dock often produces more than 30,000 seeds per plant.

Vegetative reproduction

In vegetative (asexual) reproduction, a new plant develops from a vegetative organ such as a stem, root or leaf. Several modifications of these organs are common in perennial weeds, such as underground stems (rhizomes), above-ground stems (stolons), bulbs, corms, and tubers. Although vegetative structures generally do not survive as long in the soil as do seeds, very small structures can result in a new plant. Canada thistle, for example, can produce a new plant from as small as a 1/4-inch section of root.

Vegetative reproduction can be as prolific as seed production. Yellow nut-sedge (Cyperus esculentus) has been reported to produce more than 1,900 new plants and more than 6,800 tubers in 1 year.

Some of the vegetative propagules are described as follows:

• Rhizome: A horizontal, underground stem which can produce adventitious roots and shoots (i.e., new plants) at the nodes. A rhizome can be distinguished from a root because rhizomes have nodes, internodes, and scale leaves (rudimentary leaves). Roots do not have nodes or leaves. Johnson grass is example.
• Stolon: An above-ground stem that grows flat on the ground and can produce adventitious roots and shoots (i.e., new plants) at the nodes. Bermuda grass is example.
• Tuber: Enlarged terminal portion of rhizomes, possess extensive storage tissues and axillary buds. Yellow nut sedge is example.
• Bulb: Specialized underground storage organ consisting of fleshy leaves with a short stem at the base. Food storage in the leaves. Wild garlic is example.
• Creeping roots: Horizontal roots modified for food storage and vegetative reproduction (can give rise to shoots). Often deep in the soil. Carolina horse nettle is example