Weeds after the first rain

Why weeds show up after rain

Most of the weeds that take over Valley yards in winter are winter annuals. Their seeds sit in the top layer of soil through the dry summer, waiting for water and cooler soil. When the fall rains come, they sprout all at once. UC's integrated pest management program describes cheeseweed (little mallow), for example, as beginning to grow "with the first rains in the fall."

It doesn't stop after one flush. Annual bluegrass keeps germinating through the winter, so you can see several waves of seedlings in one season. Wind also carries in seed from the street and from neighbors' yards. Common groundsel seed blows in easily, even after you've cleaned up your own.

The good news is that these are annuals. A plant you remove before it sets seed is gone for good, and each season you keep them from seeding, the seed bank in your soil gets smaller.

The weeds you'll see

These four account for a lot of the winter weeding in Valley beds and lawns. Each one has a weak spot.

You'll also see burclover, prickly lettuce, wild oat, and wild barley, which UC lists among the common annual weeds in California landscapes. The same rule applies to all of them: remove them while they're small and before they seed.

The six-week window

The clock starts when the seeds sprout. Common groundsel flowers about 6 weeks later, and annual bluegrass can form seed heads at six weeks old. That gives you roughly a month and a half from the first green fuzz to the first seeds.

1. The first couple of weeks after a soaking rain: watch for the first seedlings in bare soil, along edges, and in cracks.
2. While they're tiny: this is the easiest weeding you'll do all year. A shallow pass with a hoe or a few minutes of hand pulling clears a bed.
3. Every week or two through winter: walk the yard and catch the next flush. Each rain can start another one.
4. Before anything flowers: get it out. Once you see flowers, bag the plants and remove them instead of leaving them on the ground.

Pulling and hoeing

For most home yards, hand pulling and hoeing are the main tools, and fall is when they work best.

• Work when the soil is moist but not muddy. A day or two after rain, small weeds come out with their roots. Rock-hard summer soil snaps them off at the crown.
• Keep the hoe shallow. UC recommends cultivating less than 1 inch deep. Digging or tilling deeper brings buried seed up to the surface, where it sprouts.
• Get the taproot on young plants. Cheeseweed is easy at the four-leaf stage and stubborn once the root goes woody. Groundsel seedlings can be cut at the taproot with a shallow hoe.
• Be careful around shallow-rooted plants. Hoeing close to some ornamentals can damage their roots. Hand pull there instead.
• Don't leave flowering weeds in the bed. Seed can still mature after the plant is pulled. Bag them and take them out.

Mulch and landscape fabric

Once a bed is weeded, mulch keeps light off the soil surface so new seedlings struggle to get established. It also holds moisture and cools roots, which our plant care guide covers in more detail.

How deep

• Coarse mulch like wood chips or bark: up to 4 inches deep for long-term control.
• Fine mulch packs tighter: about 2 inches.
• Very large pieces (over 1½ inches) need to go on very thick, often 6 inches or more, to work.
• Don't go deeper than about 4 inches with most mulch. Too much can keep soil wet and starve roots of oxygen.

For cheeseweed, UC suggests at least 3 inches of bark or wood chips, kept at that depth. For groundsel, coarse, medium-sized mulch works better than fine mulch, which can hold moisture on the surface and help seeds get started.

Landscape fabric

Fabric works best around trees and shrubs in beds that will stay the same for four years or more. It's a poor fit for annual beds, areas you replant, or ground covers that need to root and spread. Overlap and pin the seams, cover it with about an inch of mulch, and pull weeds that land on top while they're small so they don't root through. Fabric only blocks seeds already in the soil. Wind-blown seed can still sprout in the mulch on top.

Oxalis: the one you can't just pull

Bermuda buttercup, the oxalis with clover-like leaves and yellow flowers, comes back every fall from bulbs. Each plant makes about a dozen small bulbs a year. Pulling or cutting the top leaves the bulb in the ground, and it's very hard to dig out every bulb, so new plants usually appear.

• Keep removing the tops. Repeated pulling depletes the bulbs' reserves. It works, but it takes years, not weeks.
• Cut before it flowers and forms new bulbs.
• Don't move infested soil to other beds unless it's been carefully sifted.
• Smother heavy patches. UC notes a suggested approach of cardboard topped with a thick layer of organic mulch, left until both have rotted.
• Weed killers only burn the top. The herbicides that kill the leaves don't kill the bulbs, so expect regrowth, and they can injure most ornamentals.

Weeds in the lawn

A thick lawn is its own weed control. Weed seeds need light to germinate, and dense grass shades the soil. Thin, stressed turf is where winter weeds move in.

• Water deeply and less often. Keeping the surface constantly moist, especially in shade, invites annual bluegrass.
• Fill bare spots promptly so weeds don't get them first.
• Bag clippings while annual bluegrass is seeding, and clean the mower after mowing an infested area so you don't spread seed.
• Don't fertilize or aerate during peak germination of annual bluegrass. Both help the weed as much as the lawn.
• Pre-emergent and overseeding don't mix. If you put a pre-emergent on the lawn in fall, you can't reseed it until spring.

In winter, cut irrigation way back and turn it off for several days after a real rain. The plant care guide has a season-by-season watering schedule.

What weed killers can and can't do

Many people reach for a spray once the yard turns green. Before you do, know what each type actually does.

Pre-emergents

Pre-emergent herbicides stop seeds as they sprout. They don't kill weeds that are already up. For winter weeds, they go down in late summer to early fall, before germination, and they need rain, irrigation, or cultivation to move into the soil. If your weeds are already up after the first rain, a pre-emergent won't help with this flush. Some products are available only to licensed commercial applicators, and UC lists no effective home-use pre-emergent for common groundsel or cheeseweed.

Post-emergents

• Glyphosate is nonselective and works on most annual and perennial weeds, but it's ineffective on cheeseweed, and it doesn't kill oxalis bulbs.
• Contact products made from pelargonic acid or acetic acid kill small annual weeds, especially small broadleaf weeds. On perennials they only burn back the leaves, and larger plants need thorough coverage.
• Nonselective means it kills whatever it touches. Spray on a calm day with low pressure and large droplets, and shield nearby plants.

Whatever you use, follow the label exactly. For most yards, catching weeds early by hand and keeping mulch topped up does the job without spraying.

Foxtails and pets

Foxtails are the barbed seed heads of certain grasses, including wild barley. They grow along roads and in backyards. Once the grasses dry out, the seed heads can work into a dog's or cat's nose, ears, mouth, or skin and keep moving inward. UC Davis veterinarians see most cases in California in summer, starting around April.

The grasses that cause trouble next summer sprout with this winter's rain. Pulling them while they're green and before they form seed heads is the simplest way to keep them out of your yard. If your pet starts shaking its head or sneezing persistently after time outdoors, see a veterinarian promptly.