In these uncertain times of social distancing and stay-at-home directives, I have been spending lots of time in my garden. After pruning every possible shrub and planting more vegetables than two people can ever eat, I have started on the weeds. Usually this time of year, I am at war with weeds; this year I am thankful for the opportunity they present as a way to productively use my time. Weeding gives me a feeling of accomplishment, allows me to work outside in our beautiful spring weather, and is a seemingly endless task!
We experienced an exceptionally long period of warm, dry weather in the fall, followed by rainfall in December and recently in late March and early April. Despite our ongoing drought, the lawn and garden weeds are remarkably vigorous, and appear every year on time! In fact, the winter crop of germinating weeds such as annual bluegrass, creeping wood sorrel (oxalis), and dandelion are already flowering and producing seeds for the next cycle. Let me share with you some facts on these three most annoying and prolific weeds and what you can do to get rid of them.
My favorite, annual bluegrass, is everywhere this time of year. Annual bluegrass (Poa annua) is one of the most common weeds of residential and commercial lawns, ornamental plantings, and gardens in the United States. It is native to Europe but is distributed worldwide. Annual bluegrass is a cool-season grass-looking weed that starts germinating in autumn as soil temperatures fall below 70°F. It continues to germinate throughout the winter, allowing several flushes of regrowth at any one site throughout the season. Annual bluegrass grows 6 to 8 inches high when left unmowed. It is a rapid and prolific seeder. Each small plant can produce about 100 seeds, which are viable just a few days after pollination, allowing the plant to reseed even in frequently mowed lawns.
Annual bluegrass has a fairly weak and shallow root system, needing frequent rainfall or irrigation to survive. It grows well in moist areas in partial shade to full sun and tolerates compacted soil conditions. It usually dies off in our hot summers only to return again in the late fall. Once a few annual bluegrass plants become established in lawns or ornamental areas, it can spread rapidly because of its prolific and rapid seed production. Mowing, foot traffic, birds, and cultivation all spread seeds.
Second on my favorite’s list is creeping woodsorrel, or oxalis (Oxalis corniculata). I have been spending some isolation time weeding in Master Gardeners’ demo garden, Ralph Moore Miniature Roses. In these beds, creeping woodsorrel is rampant, reoccurring annually with a vengeance. It is a perennial plant, growing in a prostrate (low and creeping) manner, and forms new roots and stems where old stems touch the soil. It grows in both full sun and shade with adequate soil moisture. The leaves are comprised of 3 heart-shaped leaflets attached to the tip of a long stem. It blooms all year, but spring is a time of heavy flowering (small yellow flowers) and seed formation.
The seedpods are erect, cylindrical capsules with a pointed tip. Each capsule contains 10 to 50 seeds with a potential for more than 5,000 seeds per plant. When seedpods rupture, the seeds are forcefully expelled, landing up to 10 feet from the plant. Creeping woodsorrel grows rapidly, forming a fleshy taproot and an extensive root system that expands outward. If you pull it, the taproot or creeping stems often break off and remain in the soil, allowing the plant to regrow or develop new plants. As seedpods mature and expel seeds, it spreads from container to container, from flower bed to flower bed, or across lawns. It spreads during mowing and other cultural operations. Once established, it is very hard to get rid of because of its year-round growth and extensive root system.
Another favorite is dandelion. Kids, the world round love dandelions because of their seed puffball, fun to blow and chase. Adults, on the other hand, are not so fond of them! Dandelion (Taraxacum officinale) was introduced from Europe and has been used as a medicinal plant since Roman times. It has a high vitamin and mineral content. Mature leaves are often dried and used to make a mild tea. Salads, beer, and wine are also made from the leaves and flowers.
Dandelion is a perennial, growing best in moist areas in full sun, but can survive shade and dry conditions once established. Dandelion grows year round in the central valley of California. It produces a strong taproot commonly 6 to 18 inches deep, but can go to a depth of 10 feet. Buds grow from the uppermost area of the root, producing a crown that can regenerate new plants even after the plant is cut off at or below the soil surface. Sections of the root as short as 1 inch in length are also capable of producing new plants. Flowering stalks are 6 to 24 inches in length and terminate in a compound inflorescence that contains 100 to 300 ray flowers that looks like a characteristic puffball. This puffball contains the seeds which end in a parachute-like structure, allowing the seeds to be transported by wind currents (and children!) for miles.
Dandelion can be a major weed problem for home gardeners. When dandelion infests lawns and ornamental plantings, it forms dense circular mats of leaves (6 to 14 inches in diameter) that crowd out desirable species and reduce the vigor of those plants that survive.
For home gardeners, the primary method of control for any weed is prevention. But, once the weeds are established, the first step in removal and control of germinating seeds is hand pulling or hoeing. This method, of course, is effective as long as it is done frequently. Cleaning landscape equipment after use in infested sites can help prevent spreading weeds to uninfested areas. Because dense seedling infestations are common, open areas where old plants have been removed will often have new flushes of seedling plants, hence the need for frequent attention. Infested sites require constant vigilance and continuous weed removal.
Water permeable fabrics in planter beds can prevent weed seeds from germinating, yet allow for water percolation into the soil. Landscaping fabrics are available at hardware stores and nursery outlets. They are easy to install, but edges should be overlapped so no light is allowed to reach the soil. The fabric needs to be covered with 3 to 4 inches of organic (wood chips or compost) or inorganic (stones or gravel) mulch to block plant growth. Mulching with landscape fabrics can be particularly effective in controlling seed germination. Mulches also prevent tearing or deterioration of the fabric, provide a finished look to a yard, and conserve moisture in the soil. If weed seeds get into the mulch, they can germinate and establish themselves, just as if they were in soil, but can be easily removed by hand or with a hoe. Mulch needs to be replenished periodically to maintain adequate cover and eliminate light penetration to the soil.
A special note on dandelion in the home landscape: a dandelion plant can easily be grubbed out (digging out the entire plant, taproot and all), especially when it is young. Dandelion knives and similar specialized tools are available for removing individual weeds and their roots while minimizing soil disturbance. Because of the extensive root system of established plants, hand-pulling or hoeing to remove dandelion is usually futile unless done repeatedly over a long period of time.
The final control method is the use of chemicals, pre- and post-emergent herbicide products available to the home gardener. Pre-emergent sprays are applied in the late fall or early spring to prevent seed germination in the spring. Post-emergent herbicides are sprayed directly on growing weeds. Many of these post-emergent herbicides are non-selective, meaning they kill or greatly harm all plants on which they are applied, weed or ornamental. Application may be problematic: poor aim or drifting spray will likely cause collateral damage to nearby landscape plants. Lastly, there are selective products available for specific weeds, like oxalis, or broadleaf weeds in lawns and weedy grasses in ornamental beds. Master Gardeners always recommend using prevention and hand weed removal before the use chemicals.
So, my friends, after shopping for toilet paper and food supplies, cleaning the garage, and sewing face masks for your family, head outside and start on those weeds. It is good for your garden, your health and your soul. Take care.
To learn more about weeds and using herbicides in your garden, consult UC Pest Notes, found at ipm.ucanr.edu/PMG/menu.weeds.html
Due to the shelter-at-home guidelines, the Master Gardeners have canceled all public events until at least May 10, but their phone lines are still open: 559-684-3325, Tuesdays and Thursdays between 9:30 and 11:30 a.m.