5.3 Weed Biology, Life Cycles & Identification
Key Takeaways
- Weeds are classified by life cycle as summer annuals, winter annuals, biennials, or perennials, and each has a distinct control window.
- Grasses have round hollow jointed stems and parallel veins, sedges have solid triangular stems, and broadleaves have net-veined leaves and usually showy flowers.
- Pre-emergence herbicides must be in place and activated before germination and do nothing to established weeds.
- Perennials that spread by rhizomes, stolons, or tubers are best controlled by a translocated systemic herbicide applied when carbohydrates move toward the roots.
- Winter annuals such as chickweed and henbit germinate in the fall in Ohio, which is why fall is the strongest timing for many broadleaf lawn programs.
5.3 Weed Biology, Life Cycles & Identification
Weeds are simply plants growing where they are not wanted. Because a weed is a plant, herbicide timing is dictated by the weed's growth stage and by where the plant is moving its carbohydrates - not by the calendar and not by how bad the stand looks.
Life Cycles
| Life cycle | Pattern | Ohio examples | Best control window |
|---|---|---|---|
| Summer annual | Germinates spring, grows and sets seed summer, dies at frost | Crabgrass, foxtail, common lambsquarters, common ragweed, prostrate spurge | Pre-emergence before germination, or post-emergence on very small seedlings |
| Winter annual | Germinates late summer or fall, overwinters as a rosette, flowers and sets seed in spring, dies by early summer | Common chickweed, henbit, purple deadnettle, annual bluegrass, shepherd's purse | Fall post-emergence on small plants, or fall pre-emergence |
| Biennial | Vegetative rosette the first year, bolts and sets seed the second, then dies | Wild carrot, common burdock, bull thistle, poison hemlock | First-year rosette stage |
| Simple perennial | Lives more than two years, spreads mainly by seed, regrows from a taproot or crown | Dandelion, broadleaf plantain, curly dock | Fall, when carbohydrates translocate to the root |
| Creeping perennial | Lives more than two years, spreads vegetatively by rhizomes, stolons, tubers, or bulbs | Canada thistle, quackgrass, yellow nutsedge, ground ivy, field bindweed | Systemic translocated herbicide at bud-to-bloom or in fall |
The exam's favorite implication: a pre-emergence herbicide does nothing to a weed that has already emerged, and a contact herbicide burns off the top of a creeping perennial while the rhizomes regenerate.
The Three Plant Families You Must Separate
| Feature | Grasses | Sedges | Broadleaves |
|---|---|---|---|
| Stem cross-section | Round, usually hollow, with swollen nodes | Triangular and solid, no nodes | Varies, often round and solid |
| Leaf arrangement | Two-ranked, alternating on opposite sides | Three-ranked, in sets of three | Usually opposite or alternate, not ranked |
| Venation | Parallel | Parallel | Net-veined |
| Growing point of seedling | At or below the soil surface (protected) | At or below soil surface | Exposed at the shoot tip |
| Typical example | Crabgrass, quackgrass | Yellow nutsedge, green kyllinga | Dandelion, clover, ground ivy |
The mnemonic is old but reliable: "sedges have edges, rushes are round, grasses have nodes from their tips to the ground." Sedges are not controlled by most grass herbicides and require sedge-specific chemistry.
Grass identification often turns on the vegetative key characters visible without a flower: the ligule (membranous, hairy, or absent), the auricles (clasping, rudimentary, or absent), the collar, the sheath (split, closed, or compressed), and vernation (rolled or folded in the bud).
Vegetative Reproductive Structures
Knowing which structure a weed uses tells you whether tillage spreads it and whether a systemic herbicide can reach it.
- Rhizome - horizontal underground stem (quackgrass, Canada thistle, johnsongrass). Tillage fragments it and multiplies the stand.
- Stolon - horizontal above-ground stem rooting at nodes (ground ivy, bermudagrass, creeping charlie).
- Tuber (nutlet) - swollen underground storage organ (yellow nutsedge). Dormant tubers survive a single herbicide application, which is why nutsedge takes multiple seasons.
- Bulb / corm - wild garlic, star-of-Bethlehem.
- Taproot - dandelion, curly dock. Regenerates from crown fragments.
Herbicide Timing Logic
Pre-emergence. Applied before weed seed germination, forms a chemical barrier in the germination zone. It must be activated by rainfall or irrigation and it does not move upward to reach emerged weeds. Disturbing the treated layer after application - core aeration, deep raking, tillage - can break the barrier. In Ohio turf, crabgrass pre-emergence timing is commonly anchored to soil temperature and to phenological indicators such as forsythia bloom fading.
Post-emergence. Applied to emerged weeds, and further split:
- Contact - kills only tissue it touches; requires thorough coverage; excellent on annuals, poor on creeping perennials.
- Systemic (translocated) - absorbed and moved within the plant; slower symptom development but reaches rhizomes, stolons, and taproots. Effectiveness depends on the plant actively translocating toward the roots, which is why fall applications on perennials outperform spring applications.
Selective vs. non-selective. A selective herbicide controls certain plants while leaving others essentially unharmed at labeled rates; a non-selective herbicide injures most plants it contacts. Selectivity is a matter of rate, timing, placement, and plant physiology, not magic - a selective product applied at the wrong rate or stage still injures desirable plants.
Field Habits That Prevent Herbicide Complaints
- Identify before you spray. Sedge, grass, and broadleaf require different chemistry.
- Match the stage. Small annual weeds and fall-treated perennials are the two highest-percentage timings in Ohio.
- Watch the label's rotational and replant restrictions. Residual herbicides carry plant-back intervals that create liability when a customer replants.
- Watch temperature and volatility. Ester formulations of growth-regulator herbicides such as 2,4-D volatilize in heat; amine salts are far less volatile.
- Record wind direction, velocity, and air temperature. Ohio's OAC 901:5-11-10(A)(2) requires it, and herbicide drift complaints are the most common Ohio pesticide complaint category.
A customer's lawn is infested with a weed that has solid, triangular stems and leaves in sets of three. What is it and what does that mean for control?
Why is fall generally the strongest timing for controlling simple and creeping perennial broadleaf weeds in Ohio?
An applicator applies a pre-emergence herbicide to a lawn where crabgrass has already emerged and is tillering. What is the likely result?