Aquatic weed seasons by US region: a regional treatment calendar

A treatment plan that works in Tampa fails in St. Paul. Aquatic weed biology is governed by water temperature, day length, and ice cover — all three vary dramatically by region. Below, the seasonal cycle in each major US climate zone.

Florida and the Gulf Coast

Growing season: January through December. No true dormancy.

• December–February: vegetation at annual low; water 60–70°F; cheapest assessment and removal window. Primary work: pre-season harvest, shoreline buffer planting (Jan–Feb is ideal), permit applications.
• March–April: rapid greenup as water passes 70°F. Hyacinth populations begin doubling on a 14-day cycle. Last call for proactive harvest before peak.
• May–August: peak growth. Hyacinth doubles every 8–12 days; hydrilla extends 3–4 inches per day. Reactive removal only for most homeowners. 30–50% pricing premium vs. winter work.
• September–October: still warm but slowing. Second-best window for proactive work — biomass has built but doubling rates are decreasing.
• November: transition month; clean-up before next year.

See our Florida-specific timing post for the detailed monthly plan.

Southeast Piedmont (GA, SC, NC, TN, AL)

Growing season: March through November. Brief winter dormancy in zones 7 and colder.

• December–February: light dormancy; water 45–55°F. Cattails and emergents go dormant; submerged invasives (hydrilla, milfoil) slow but don't fully stop in zone 8.
• March–May: spring greenup; water passes 65°F by late April. Optimal proactive treatment window.
• June–August: peak; hydrilla and hyacinth at maximum growth. Hyacinth often winter-kills in Atlanta-and-north, so summer infestations come from southward expansion.
• September–October: slowing growth; second proactive window.

The expanding northern range of water hyacinth (now overwintering in coastal NC and TN) is the biggest emerging issue in this region.

Mid-Atlantic and Northeast (MD, VA, PA, NY, NJ, NE states)

Growing season: May through October. Hard winter dormancy.

• November–April: ice cover or near-freezing water. Vegetation dormant. Drawdown season for dam-controlled ponds.
• May: ice-out and greenup. Curly-leaf pondweed peaks early — May/June — and dies back in summer.
• June–August: peak. Eurasian milfoil and hydrilla both active; treatment window is narrow and crowded.
• September–October: slowing; final knockdown opportunity before winter.

The compressed window means mechanical contractors are heavily booked June–August. Schedule months ahead.

Midwest and Great Lakes (MN, WI, MI, IL, IN, OH)

Growing season: late May through September. Severe winter dormancy.

• November–April: ice cover; full dormancy. Drawdown is the dominant winter strategy.
• Late May–early June: ice-out and bloom. Curly-leaf pondweed peaks first.
• June–August: Eurasian watermilfoil dominates; hybrid milfoil increasingly common in Wisconsin and Minnesota.
• September: rapid slowdown after Labor Day; treatment effectiveness drops as water cools.

Climate-driven note: Great Lakes ice-out has shifted 2–4 weeks earlier since 1980, lengthening the active season.

Texas and the Lower Plains

Growing season: March through November. Mild winter — most invasives don't fully dormant.

• December–February: vegetation thinned but not dormant. Hyacinth and hydrilla persist in southern Texas year-round.
• March–April: rapid recovery; treatment window opens.
• May–September: peak. Giant salvinia is the dominant invasive concern in East Texas and Louisiana — see our identification guide for ID notes.
• October–November: secondary work window.

Drought years compress the treatment window because lower water levels expose vegetation but also concentrate herbicide doses unpredictably.

Pacific Northwest (WA, OR, ID)

Growing season: June through September on lowland lakes; July–August at elevation.

• November–May: cold dormancy on most water bodies; some Puget Sound lowland lakes stay active year-round.
• June: greenup begins; Eurasian milfoil and parrot feather active in Columbia River system.
• July–September: peak. Brazilian elodea (a regional invasive) shows up in coastal lakes; hydrilla now established in some Washington reservoirs.
• October: rapid slowdown.

Cool summers limit growth rates compared to the Southeast — a hyacinth doubling time of 25 days in Seattle vs. 10 days in Tampa.

California

Growing season: depends entirely on elevation and latitude.

• Coastal Southern CA: year-round, similar to Florida pattern but drier.
• Central Valley: March through November; high temperatures (water exceeds 85°F) actually slow hyacinth somewhat in mid-summer.
• Sierra and Mountain lakes: June–September only; freeze-out winters.

The Sacramento-San Joaquin Delta is the worst-affected aquatic invasive system in the US outside Florida — hyacinth, water primrose, and Brazilian waterweed all chronic.

Mountain West and high-elevation lakes (CO, UT, MT, WY, ID)

Growing season: June through August only.

• Compressed window means most lakes have light vegetation pressure relative to lower elevations.
• Eurasian milfoil is the main concern — established on Lake Powell, Lake Tahoe, and many Rocky Mountain reservoirs.
• Treatment window is 60–90 days; mechanical contractors book up months ahead.

How to use this for treatment planning

1. Identify your region's peak window (when water is above 78°F).
2. Schedule major remediation 30–60 days before the peak — pre-season harvest holds longer than peak harvest.
3. Schedule a follow-up at the peak's end (cooling water, slowing growth — knockdown holds through winter).
4. Avoid mid-peak treatment unless reactive — it's expensive and short-lived.
5. Match doubling time, not calendar months — see growth rates by species for cadence math.

For Central Florida specifically, January and February are the cheapest months to schedule. Crews are not in peak-pricing, biomass is at its annual low, and a winter harvest produces visibly cleaner water through the May greenup. Reach out through our contact page to schedule before March.