Corn Speedwell
Corn speedwell is a low-growing winter annual weed in US lawns and gardens. This guide covers identification, life cycle, and control in turf and beds.
Key takeaways
- Corn speedwell is a winter annual that germinates in fall, overwinters as a rosette, and sets seed in spring before dying
- Identify it by tiny blue flowers (spring), small rounded leaves with scalloped edges, and low creeping stems
- Pre-emergent herbicides applied in fall are the most effective control — timing is everything
- In lawn turf, corn speedwell thrives in thin, compacted areas — improving turf density is the long-term solution
- Hand-pulling before seed set in spring removes plants but misses fallen seed; combine with fall pre-emergent next season
What corn speedwell is
Corn speedwell (Veronica arvensis) is a low-growing winter annual weed that appears in lawns, garden beds, and disturbed soil across the US. It completes its life cycle over fall through spring — germinating in fall, overwintering as a flat rosette, and flowering in March through May before dying as temperatures rise.
The common name “corn speedwell” comes from its historical presence in European grain fields (cornfields), not from corn the vegetable. It belongs to the genus Veronica, which includes both garden ornamentals and weedy species.
Identification
Key characteristics
Leaves:
- Small, roughly 0.25-0.5 inches long
- Rounded to oval shape
- Distinctly scalloped or toothed edges
- Slightly hairy on both surfaces
- Opposite arrangement on stem (pairs along the stem)
Stems:
- Very low, creeping
- Often root where nodes touch soil
- Hairy; typically reddish-tinged near base
Flowers (early spring):
- Tiny (1/8 inch across), 4-petaled
- Pale blue to lilac, with a white center
- Appear in leaf axils along the stems
- Often the first blue flower visible in lawns in early spring
Seeds:
- Heart-shaped seed capsules that split open when ripe
- Each plant produces hundreds of seeds, which disperse in spring before the plant dies
Look-alike weeds
| Plant | Difference from corn speedwell |
|---|---|
| Persian speedwell (V. persica) | Larger leaves and flowers; seeds on long stalks |
| Ivy-leaf speedwell (V. hederifolia) | Larger leaves, distinctly ivy-shaped lobes |
| Birdeye speedwell (V. chamaedrys) | Larger overall; more flower stalks |
| Hairy bittercress | White flowers, not blue; belongs to mustard family |
Life cycle
Understanding corn speedwell’s life cycle is the key to effective control:
| Season | Stage | What’s happening |
|---|---|---|
| Late summer – fall | Seed germination | Seeds left from last spring sprout as soil cools below 70°F |
| Fall – early winter | Rosette growth | Plant grows as a flat rosette, close to ground |
| Winter | Dormancy or slow growth | Overwinters; survives light frost; may grow slowly in mild winters |
| Early spring | Rapid flowering | Warm spring days trigger fast growth and flower production |
| Late spring | Seed set and death | Sets hundreds of seeds per plant, then dies as summer heat arrives |
The critical window: Spring control, while satisfying to see, does not prevent the next generation of weeds. Plants have already set seeds by the time visible flowers appear. The effective control window is fall, targeting seeds before they germinate.
Why corn speedwell spreads in lawns
Corn speedwell colonizes specific conditions:
- Thin turf: Dense turf physically prevents seed-to-soil contact and suppresses seedling emergence. Thin turf offers open soil.
- Compacted soil: Corn speedwell tolerates compaction. Lawn grasses struggle in compacted soil, creating weak spots where weeds establish.
- Low mowing: Corn speedwell stays below mower blade height naturally. Low-mowed turf removes competition without affecting speedwell.
- Disturbed areas: Edges of beds, footpaths, and areas of foot traffic are prime speedwell habitat.
Long-term solution: Address the conditions that favor speedwell. Aerate compacted soil, overseed thin areas, and mow at the recommended height for your grass type (typically 3-4 inches for most cool-season grasses).
Control options
Pre-emergent herbicides (most effective)
Pre-emergent herbicides form a barrier in the soil that kills germinating seeds before they establish. They must be applied before germination — applying after seedlings are visible provides no benefit.
Timing: Apply when soil temperatures drop below 70°F in fall — typically mid-September to mid-October in most US zones.
| Active ingredient | Product examples | Notes |
|---|---|---|
| Prodiamine | Barricade, Regalkade | Long residual; most common |
| Dithiopyr | Dimension | Also has some post-emergent activity on very young seedlings |
| Pendimethalin | Scotts Halts, Pendulum | Widely available; effective |
Follow label rates exactly. Over-application can harm turf; under-application reduces effectiveness.
Critical note: Pre-emergent herbicides also prevent grass seed from germinating. Do not apply to areas you plan to overseed until after the grass has established (typically 3 months).
Post-emergent herbicides (spring)
Spring post-emergent control kills existing plants but does not prevent seed germination the following fall. It is useful for reducing seed production but is not a standalone solution.
Effective options for turf:
- Triclopyr + 2,4-D + dicamba (broadleaf weed killers like Weed B Gon, Trimec): effective on actively growing speedwell
- Carfentrazone: fast-acting; good contact activity on speedwell in spring
Apply when plants are actively growing and temperatures are between 55-85°F. Most broadleaf herbicides are less effective in cool spring temperatures.
Avoid these conditions:
- Newly seeded areas (wait until after third mowing)
- Days with wind above 10 mph (spray drift damages ornamentals)
- Rain within 24 hours of application
Hand-pulling
Hand-pulling in early spring before seed set removes plants but not the seed bank already in soil. It is practical for small areas and garden beds.
- Pull after rain when soil is soft
- Remove the entire root system
- Bag and dispose of pulled plants — do not compost (seeds may still be viable)
For comprehensive guidance on removing weeds without chemicals, see our how to get rid of weeds naturally guide.
Vinegar-based controls
Vinegar and other contact organic herbicides kill speedwell foliage but do not kill roots or prevent seed germination. They are effective on young seedlings in fall when plants first emerge. Repeated applications in fall can weaken plants before they establish.
See our natural weed killer with vinegar guide for application rates and timing.
In garden beds
Corn speedwell in garden beds is controlled by:
- Mulch: 3-inch layer of wood chip or bark mulch blocks light to germinating seeds and physically impedes establishment. The most practical long-term solution for ornamental beds.
- Hand-weeding in fall: Remove rosettes before they overwinter and produce spring seeds.
- Pre-emergent in beds: Products labeled for use in ornamental beds (not lawns) containing prodiamine or pendimethalin can be applied around established perennials and shrubs.
Non-toxic and chemical-free approaches
For gardeners who prefer to avoid synthetic herbicides:
- Flame weeding in fall: A propane torch briefly applied to emerging seedlings kills them instantly. Effective before plants establish.
- Boiling water: Poured directly on rosettes in early spring kills plants by contact. No residual activity.
- Corn gluten meal: A natural pre-emergent that inhibits root formation in germinating seeds. Less reliable than synthetic pre-emergents but effective with consistent use.
- Thick mulch: The single most effective non-chemical control in garden beds.
For a full overview of weed control products, see our best non-toxic weed killer guide and homemade weed killer guide.
Sarah Mitchell is a Master Gardener and garden writer based in the Pacific Northwest. She has been growing food and flowers for over 15 years across USDA zones 7 and 8.