What time of year should I spray Roundup?

Roundup is the brand name for a popular herbicide that contains glyphosate as its active ingredient. Glyphosate is a non-selective herbicide, meaning it will kill most plants and weeds it comes into contact with. Knowing the best time of year to spray Roundup can help ensure it is most effective at controlling weeds in your lawn, garden or landscape.

When are weeds most vulnerable to Roundup?

Weeds are most vulnerable to Roundup and other herbicides when they are actively growing. The key times to spray are early in the season, as weeds first sprout and establish, and later when mature weeds are preparing to flower and go to seed. Repeated applications are usually needed for full control.

Spring application

Early spring is an optimal time to spray Roundup and other glyphosate products. Cool-season annual weeds begin germinating in late winter and early spring. Warm-season annuals sprout later in spring. Perennial weeds also activate growth in early spring.

Spraying Roundup when weeds are small and young allows good coverage and kills them before they become established. It prevents season-long growth and seed production. Early spring weeding reduces competition for soil moisture and nutrients needed by desired plants. It also improves control before dense growth limits spray coverage.

The exact spring spray timing depends on your climate and weed emergence:

  • Northern areas: Apply in March, April or May as weeds become active.
  • Central and southern zones: Spray from February through April.
  • Coastal climates: Spot treat weeds January through May.

Avoid spraying during cool, cloudy weather. Wait for sunny days when daytime temperatures exceed 45 to 50°F for best results. Glyphosate absorption improves under bright, sunny conditions.

Summer application

Summer spraying targets established perennial weeds as they prepare to flower and set seed. It also controls persistent annuals and new flushes of opportunistic weeds.

The optimal summer spray period is usually June through August, though timing varies by region:

  • Northern zones: June, July and early August.
  • Central zones: June through September.
  • Southern zones: May through September.

Apply on actively growing weeds with adequate soil moisture. Avoid drought-stressed plants. Absorption diminishes when plants are under moisture stress. Follow-up fall treatment is often needed for full control of deep-rooted perennials.

Fall application

Fall brings declining day length, cooling temperatures and moisture, signaling the end of active growth. Weeds respond by translocating carbohydrates and nutrients downward to overwintering root systems.

Spraying glyphosate products in fall intercepts this downward translocation. It delivers the herbicide to roots for more complete kill of deep-rooted perennials. The effects may not appear until the following spring when treated plants fail to emerge.

Time fall Roundup application from:

  • Early September through November in northern zones
  • October through December in central and southern areas

Actively growing plants with healthy green foliage absorb herbicide most effectively. Avoid early fall application when plants remain vigorous or have new growth. Late fall spraying may diminish absorption and translocation at cooler temperatures with weaker daylight intensity.

What weeds does Roundup control?

When applied correctly, Roundup is effective on a broad spectrum of grass and broadleaf weeds. It controls annuals, biennials and perennials by disrupting enzyme function, leading to a shutdown of plant growth and eventual death.

Roundup is labeled for control of more than 200 listed weeds. Examples of susceptible annual weeds include:

  • Crabgrass
  • Foxtail
  • Lambsquarters
  • Pigweed
  • Ragweed
  • Velvetleaf

Among the more than 100 perennials on the Roundup label are:

  • Bermudagrass
  • Bindweed
  • Chickweed
  • Dandelion
  • Dock
  • Goldenrod
  • Ground ivy
  • Horsetail
  • Milkweed
  • Nutsedge
  • Plantain
  • Poison ivy
  • Quackgrass
  • Thistle
  • Wild violet

Repeated applications at lower rates are often needed on hard-to-kill perennials. Spot treat or use a shield when spraying near desired vegetation to prevent damage.

How to apply Roundup for best results

Proper application technique is key to achieving good weed control with Roundup:

Spray coverage

Thorough, uniform coverage of all leaf and stem surfaces is essential. Dense leaf canopies shelter lower growth, preventing contact. Avoid applications under breezy conditions that cause uneven or off-target drift.

Flat fan nozzles operated at 15 to 40 psi deliver a consistent broadcast spray pattern. Select spray volume based on weed density. Typical rates range from 20 to 40 gallons per acre. Adjust pressure, speed, nozzle type and spacing to ensure adequate coverage without runoff.


Apply concentrated solutions for smaller weeds. Increase dilution for larger annuals and perennials. Most applications use a 0.75 to 1.5 percent solution. That equates to a dilution between 4 to 8 fluid ounces of Roundup per gallon of clean water.

Higher concentrations improve control of harder-to-kill weeds. But avoid using more than the maximum label rate – excessive product wastes money with no benefit.


Spray adjuvants enhance the performance of Roundup. Common options include:

  • Surfactants – Improve spray retention and glyphosate penetration on waxy or hairy leaf surfaces.
  • Ammonium sulfate – Increases herbicide absorption, especially under stress conditions.
  • Drift control agents – Thicken the spray solution to reduce off-target movement.

Follow label directions when adding adjuvants. Overuse can damage ornamental plants.


Ideally, spray Roundup when daytime highs exceed 60°F. Cool, overcast weather slows absorption. Avoid application during hot, dry conditions that cause rapid moisture loss and dry spray deposits.

For best results, spray when weeds are actively growing and free from environmental stress. Weeds are most susceptible at the seedling stage. Larger plants with established root systems require higher application rates.

Second application

Repeat spraying is often necessary, especially for perennials with deep root reserves. Make a second application if regrowth occurs or new weeds emerge. Follow the product label for the required interval between treatments.

Precautions when using Roundup

While Roundup offers broad-spectrum post-emergent control, proper precautions protect you and avoid damage to desired plants:

  • Wear protective clothing, gloves, goggles and an approved respirator.
  • Avoid contacting or breathing concentrated spray mist.
  • Do not eat, drink or smoke during application.
  • Spot treat or shield ornamentals to prevent contact damage.
  • Avoid drift to sensitive crops like grapes or tomatoes.
  • Wash exposed skin immediately after exposure.
  • Clean equipment after spraying to avoid residue transfer.

Keep people and pets off treated areas until the spray has dried. Follow label instructions for re-entry intervals for larger commercial applications.

Alternative application methods

While spraying is the most common method, Roundup and glyphosate products are also available in several alternative formulations:

Ready-to-use wipes

Pre-moistened, single-use wipes allow targeted application by wiping leaves. They are less messy and time consuming than spraying for spot weed control in gardens and landscape beds.


Higher concentration formulations are designed to dilute with less water for smaller spray jobs. They treat up to 25 gallons and are ideal for backpack sprayers.

Granular formulations

Dry pelletized products are mixed with water and applied through spray equipment. Granules are also available for broadcast spreading with a spreader.

Gel formulations

Thicker gels stick to leaves and stems to resist wash off. Specialized applicators reduce drift during targeted spraying.

Natural alternatives to Roundup

Glyphosate is controversial, and some gardeners prefer to avoid chemical weed control. Organic alternatives include:


Household vinegar kills young seedlings and tender new growth but is less effective on established weeds. Use horticultural vinegar with 10 to 20 percent acetic acid for better results.

Corn gluten

A natural herbicide, corn gluten inhibits root formation during germination. It provides effective pre-emergent weed control with spring application timed before seeding crops or lawns.

Soap-based herbicides

Soaps disrupt cell membranes and metabolism, desiccating and killing foliage. Look for potassium salts of fatty acids as active ingredients. Reapply frequently for ongoing control.

Boiling water

Pouring boiling water on pavement weeds and ugly sprouts provides instant local control, but re-growth often follows. Use carefully to avoid scalding or killing desired plants.


Simple mulches like wood chips, leaves or straw suppress weeds by blocking light from emerging seedlings. But take care, some mulches can introduce or spread weeds.

Hand weeding

The original non-chemical method, hand weeding remains an effective way to remove unwanted vegetation. Use a sturdy weeding tool to uproot plants or hoe small seedlings.


Roundup and other glyphosate products provide an efficient, cost-effective method for controlling a wide range of common weeds. While non-selective, careful application allows selective removal of unwanted vegetation from lawns, gardens, flower beds and other areas.

To maximize control, make applications when weeds are young and actively growing. Early spring and fall offer prime windows for reducing established perennials. Mid-summer spraying targets annuals and prevents seed production. Repeat as needed on persistent regrowth.

Always read and follow label directions for proper product use, timing and safety precautions. Consider more natural alternatives if chemical weed control raises environmental or health concerns.

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