Controlling Annual Weeds

 

The gardener’s war on weeds cannot be ignored at this time of the year, as cool-season annual weeds respond to the current temperatures and recent rains. By popping up in our gardens.

The weeds in front of us always command immediate attention, but they also encourage a broader view of weeds. Let’s review the sources of the problem and the strategies for weed control.

The first step in control involves knowing the three broad categories of weeds: annuals, which are usually the most troublesome, and biennials and perennials, which we will consider on another day.

Annual weeds are those that grow and die in one season. This cycle begins with seed germination and ends with the dispersal of a new generation of seeds.

Annual weeds include two sub-categories: summer and winter annuals, which are also referred to as warm season and cool-season annuals. The names of weeds in each of these groups could be identified, but their botanical or common names are less important than their life cycle.

After Annual Weeds Sprout

Cool-season annual weeds germinate from seed in the late summer or early fall. They grow during the winter, flower, set seed and die from heat in the late spring or early summer. This approximate calendar guides actions to reduce weed growth in the garden. Right now, these pesky plants have already germinated, so the appropriate action is to minimize the production and dispersal of seeds. Do this by removing the flowers as soon as they appear. Depending on the circumstances and other factors, this can be done by plucking the flowers, mowing the weed batch, or pulling the entire plant.

Warm-season annual weeds germinate and grow in the spring, and thrive throughout the summer and into the early fall. They will die by frost, however, in the Monterey Bay area, where frost comes later if at all, these plants can persist for quite a while. Controlling warm-season weeds after germination uses the same techniques outlined above: minimize the spread of new seeds by removing the flowers or the entire plant before, or as soon as, the flowers appear.

Up to half the weeds that are pulled from the garden are still capable of dispersing seeds, so the safest plan is to dispose of them in the green waste. They could be added to a compost bin, but only if your compost maintains a temperature of 160 degrees Fahrenheit or more for three hours or more, the seeds won’t survive.

Before Annual Weeds Sprout

Methods to minimize the production and dispersal of seeds are highly recommended, but inevitably some seeds will make their way into in your garden. Seeds will arrive from weeds that already grow there, or are brought in by the wind, by the dropping of birds, or by hitching a ride on people’s shoes or clothing, animal’s fur, or imported plants, even plants from a garden center. The result of these several sources is the weed seed bank tat exists in all gardens.

All seeds require light and moisture to germinate, so the basic method for discouraging the germination of seeds already in the garden begins with mulch. Two or three inches of organic mulch will deny the seeds light, and most will not be able to sprout.

Viable seeds can remain in the soil for years, waiting for light and moisture. They might be surprisingly deep in the soil. The best plan is to let sleeping seeds lie. The worst plan is to dig up the soil to give the seeds access to light and moisture. If you need to dig up your garden, even to install a new plant, cover the exposed soil immediately after with mulch. Mechanical tilling of the soil will reliably produce a new crop of weeds.

In addition to mulching, close placement of desirable plants also will deny light and moisture to the dormant seeds. In many cases, a closely planted bed is also attractive.

Also, controlling access to moisture with drip irrigation, while controlling access to light with mulch and landscape plants should minimize weed growth significantly.

Finally, let a few weeds grow. You are unlikely with a goal of complete eradication of unwanted plants, and there is some value in the contribution of weeds to the flora and fauna of the garden. In any event, a casual (but systematic) approach to weed control allows you to relax in the garden.

Cold, Rain, and Buttercups

We have had welcome rains, and apparently we should expect more around the time that this column appears. That’s good, but that weather prompts some salient observations.

First, although the drought has been broken for the present, we should regard the present period as a hiatus, rather than an end to dry times. Long-term projections still indicate below average rainfall in future years,

Most importantly, our aquifers were depleted significantly during the drought years, and replenishing them will require many years of at least normal rainfall. We will have to wait for encouraging reports on that front.

Meanwhile, examine your garden for possible negative impacts from the recent cold and rainy days. The plants in my garden are nearly all native to California or other summer-dry (Mediterranean) climates, so they managed well during the dry days.

One exception was a mature Chilean Myrtle (Luma apiculata) that succumbed to the drought, and had to be removed.

That experience recalls a thoughtful comment by Alexander Graham Bell: “When one door closes, another opens; but we often look so long and so regretfully upon the closed door that we do not see the one which has opened for us.” In this case, the tree had grown larger than expected and closer to the house than wanted, so while I regretted its loss, its absence opened space for new plants.

Almost all my succulent plants shrugged off the cold spell, with the exception of a Foxtail Agave (Agave attenuata), which I had been growing in a large container. I planned to plant it in the ground eventually, to provide the space it will need to reach its full size of four feet high and eight feet wide. The plant’s rosette is quite attractive and dramatic, but its distinctiveness is based on its flower stalk, which grows five-to-ten feet, reflexes to the ground and then arches upward again. I learned too late that this plant is unusual among Mexican native succulents for its susceptibility to cold weather. I should have covered it, or brought it inside. It lost some leaves, but now appears to be coming back.

I have been taking advantage of the fine weather between rainstorms by catching up (with help!) on the seasonal weeding tasks. This work includes the annual battle with Bermuda buttercup (Oxalis pes-caprae), a South African native that thrives in California’s coastal gardens, and opens its bright yellow blossoms in late winter or early spring.

Bermuda Buttercups

Bermuda Buttercups

I have been puzzled by the relentless and random spread of this annoying plant. The explanation on UC’s Integrated Pest Management website (ipm.ucanr.edu/) has been incomplete:

“Viable seed never has been documented in California, and rarely has it been seen anywhere else in world. Foliage dies and the bulbs become dormant when temperatures rise in late spring and summer. Bermuda buttercup reproduces vegetatively by bulbs and spreads when plants are divided or soil containing the bulbs is moved to un-infested areas.”

I have learned that this plant is tristylous, an uncommon morphology meaning that it has three flower shapes (morphs). All the flowers on a given plant will have the same shape, but the pollen from a flower on one morph cannot fertilize another flower of the same morph.

Tristyly does not necessarily affect the propagation of the Bermuda buttercup, but it might explain its spread in gardens, and provide a clue to its management. The search goes on!

Meanwhile, we continue with weeding and mulching, and preparing to enjoy the garden in the spring.

Summer Priorities

 

My first summer priority in the garden centers on weeding, and my regular resolution to walk around with a weed identification book. My desire to know the names of garden plants extends to the weeds that know so well how to grow under all conditions, and without nurture.

One new arrival is the Black Nightshade (Solanum nigrum), which is a relative of several desirable plants, including potatoes, tomatoes and peppers. It is common throughout California.

7-17-15 - Solanum_nigrum

This weed grows up to four feet tall. UC’s Integrated Pest Management website describes its leaves as follows: “The first true leaves are spade shaped with smooth edges and the lower surface is often purple. Later leaves are increasingly larger, egg shaped, dark green, often purple tinged, with a smooth to slightly wavy edge, and covered with short non-glandular hairs and some glandular hairs.”

The plant produces small, star-shaped white flowers that develop in blackberries about one-quarter inch in diameter. Some varieties have edible berries, but their taste is not good enough to offset risking that you have a different variety in your garden.

To its credit, the Black Nightshade pulls up easily.

Another member of the Solanaceae, the tomato, is also flourishing in my garden. This year, I planted two cherry tomato varieties from Love Apple Farm, chosen at San Francisco Flower & Garden Show. A volunteer from last season, ‘Sweet Million’, popped up, so with minimal watering, I have an abundant harvest of cherries for salads and casual snacks outdoors.

Then, a friend gave me a seedling of a Black Krim (Solanum lycopersicum), a heirloom tomato from Russia. This plant needs consistent moisture. If the soil is allowed to dry out between watering sessions, the fruit has a tendency to crack.

Tomato fanciers relish its “rich, salty flavor,” so I’m watching for the fruits to ripen enough for a taste. When fully ripe, the tomatoes are a dark reddish purple or brown (not truly black) and with dark green around the top, or shoulder.

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Black Krim Tomato Credit: www.gardenharvestsupply.com/

Another priority for the summer is mulching, which is actually a good practice for any season, whenever the soil is uncovered. Mulching discourages weeds, slows the evaporation of moisture, creates a good environment for soil microbes, and protects the soil from erosion.

Organic mulches are available in garden centers in bags of 1.5 or 2 cubic feet. For larger gardens, chipped material from a tree service is a low-cost alternative. Such material is coarser than commercial mulches, and consequently lasts longer.

A tree service will drop a load of chips on your property without charge, if you can receive it on their schedule. Another option is load a yard or two of chips into your truck, at the service’s premises. Call ahead!

A responsible tree service will not bring or offer chips from diseased trees, but otherwise the chips will be from a randomly chosen pruning project. All chips are good in the garden, but some are more aromatic than others. Think eucalyptus!

Finally, during these warm summer months, we can anticipate significant rainfalls this winter, based on the meteorologists’ assessments of the El Niño ocean conditions. Potentially heavy rains won’t completely end our drought, but it could help a lot.

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Gardening in July

During this hot and dry month, the avid gardener should pursue seasonal tasks to keep the garden looking good and prepare for the change of seasons.

Irrigation should be a high priority to sustain plants that must have a ration of water during the drought. Pass by Mediterranean climate plants, which are accustomed to dry summers. A little moisture will perk up even these rugged individuals and extend their best days, but a better use of scarce water resources would target the garden’s thirstier specimens.

Roses, for example, could produce another bloom cycle during July if treated to a balanced fertilizer and watered deeply. Other candidates for regular watering are plants in containers, which can dry out fast.  First confirm that your water usage is within current restrictions.

If your garden consists mostly of Mediterranean climate and succulent plants, this year’s drought should not cause alarm. On the other hand, if you have a thirsty lawn, consider replacing it with plants of the summer-dry persuasion. The same strategy would be appropriate for plants from tropical, riparian or boggy areas.

Blossoms to enjoy in July include gladiolus, agapanthus and fuchsia, and fragrant Oriental hybrid lilies, e.g., pure white ‘Casablanca’.

Casablanca Lily

I am also enjoying blossoms of Chitalpa tashkentensis ‘Pink Dawn’ trees, which are crosses of catalpa and desert willow. They put on a show reliably around Independence Day, but opened a little earlier this year.

IMG_0291

The Corsican hellebores (H. argutifolius) have finished their winter-to-spring display, and leaned down their bloom stalks to drop seeds all around. The seasonal task is to cut stalks to their bases to make room for the new growth, which has already started.

The tall bearded irises also have finished blooming for this year. They will look best after the flower stalks are cut down, the leaves fade, and the rhizomes enter dormancy. Every four years, during the period from mid-July to mid-September, dig and divide the rhizomes to promote blooming for net spring.

In July and August, plant autumn-blooming blubs, e.g., autumn crocus (C. speciosus and C. sativus), meadow saffron (Colchicum autumnale), and spider lilies: Lycoris squamigera with lilac or rose pink blooms and L. radiata with orange-red blooms.

Control cool-season annual weeds, currently going to seed: bindweed, chickweed, crab grass, knotweed, lambs-quarters, mallow, pigweed, purple deadnettle, groundsel, nettle, purslane, speedwell and spurge, as well as field grasses. Dispose of seeds in the green waste not in the compost bin! The invasive cheery yellow Bermuda buttercup (Oxalis pes-caprae) has already faded, leaving clusters of bulbs to sprout next spring.

Renovating the Garden – More Planning

Gardening and landscaping involves planning before getting your hands in the soil.

Last week’s column, which was about renovating a garden, recommended four preparatory actions:

  1. Draw a Diagram of the Property
  2. Decide on Basic Design Concepts
  3. Establish Objectives for the Finished Landscape
  4. Set Priorities for Development

Once the gardener has completed those actions, he or she still has to additional tasks to complete. Those tasks are the focus of this week’s column.

Remove Unwanted Plants

A neglected garden probably motivated the landscape renovation project. Neglect often includes trees and woody shrubs that have outgrown their space, lack a role in the new design, or are unhealthy. In some cases, this task will require contracting with an arborist or laborers. Check local ordinances before removing trees.

All herbaceous plants that are unwanted are defined as weeds. These include garden plants and grasses as well as common weeds. Pull or dig larger plants, then remove grasses and weeds efficiently with chemical-free solarization. This method covers the target area with clear plastic so that the sun raises the temperature of the soil, killing weeds, pathogens, nematodes, and insects. For details, see the University of California’s free online publication “Soil Solarization for Gardens & Landscapes.”

Removing weed plants with a do-it-yourself approach could be time-consuming and frustrating. Consider contracted services to get the job done quickly and thoroughly.

Remove Unwanted Hardscape

If your garden includes paving, e.g., sidewalk, patio, walls or outbuildings that are not included in the new design, remove them to free your progress. Again, consider contracted services to speed the work. This would be a good time to invite a disinterested friend to comment on your garden accessory collection, and to remove items that are no longer assets.

Analyze Soil Structure

The gardener should know the structure of the garden’s soil. An ideal soil would have 45% minerals (sand, clay, silt), 5% organic material (plant and animal), 25% air and 25% water.

Click to Enlarge

Soil Textural Triangle

The Soil Texture Triangle illustrates various combinations of sand, clay and silt that might be found in a garden. The mineral content of ideal garden soil, called loam, should be about 40% sand, 40% silt and 20% clay. A simple method to analyze your garden’s soil texture, involves placing a sample of your soil in a jar with water, shaking  and then letting it settle into density layers. Here are Organic Gardening’s brief instructions for a soil texture test.  Also, check out this helpful article by the Marin Master Gardeners.

Notice that these preparations do not include buying plants! For many gardeners, the primary strategy for improving the landscape is to buy plants. That approach, without an overall plan, weakens the landscape design and wastes time and money.

Enjoy your preparation! The fun stuff (selecting and installing plants) comes next.

Gardening Ideas for Fall

Recently, we have considered seasonal projects: harvesting annual seeds for planting and planting bulbs. Here are more timely garden projects.

More about bulbs: gophers avoid daffodils, which are toxic to them, so encircle a favored bed with daffodils to produce both a gopher barrier and a pleasing display for the spring. This project works best with island beds and costs least with wholesale prices (as little as 30¢ per bulb). A good source for daffodils by the hundred: Van Engelen, Inc., at www.vanengelen.com or (860) 567-8734.

Renovating Garden Beds

The fall is the ideal time to renovate a bed that has become neglected, overgrown or plain boring. First, clean out everything unwanted, reserving plants small enough to be relocated or given to friends.

Every three years, divide plants with rhizomes, tubers or bulbs. Divide overgrown plants by cutting their root balls into two or more segments, and replant.

Then, add compost, cultivate, and add fertilizer.

If the bed is larger than four feet in any dimension, install narrow paths to provide access to the plants without compressing the soil or stepping on plants.

Then, select plants that are right for your climate and the bed’s sun exposure, and that will grow to appropriate sizes. Also, choose plants that will combine well and please your eye.

Finally, plant, mulch and water. Keep watering until the rains take over.

Controlling Weeds

The early fall is also time to control both annual and perennial weeds.

Annual weeds include bindweed, chickweed, crab grass, knotweed, lambs-quarters, mallow, pigweed, purple deadnettle, groundsel, nettle (common), purslane, speedwell, spurge and yellow oxalis.

Perennial weeds include bindweed, burdock, dandelion, dock, ground ivy, horsetail, Japanese knotweed, plantain, poison ivy, purslane, quackgrass, thistle, ragweed and anything else you might have.

The primary strategy for organic weed control is to remove weed seeds before they mature and are dispersed. Hoeing weeds before they set seed can be effective with annual weeds.

This method certainly helps to reduce the spread of perennial weeds, but it leaves behind root segments that could re-grow. For this reason, perennial weed control includes removing the entire root system by pulling or digging. Persistence is the gardener’s friend!

Other organic approaches to weed control include providing a three-to-four inch layer of mulch between plants, to deny weed seeds the light and air they need to grow. Dense spacing of desirable plants also can crowd out weeds.

Finally, drip irrigation systems deliver water to desired plants and deny water to weeds.

Enjoy gardening in the fall!

More

A helpful resource for organic weed control is the website, Pests in Gardens and Landscapes: Weeds, maintained by the University of California’s Integrated Pest Management Program. This site includes weed photos, articles and fact sheets in individual weeds that are common in California gardens and landscapes.

A Seasonal Pest

For the past several days, I have been pulling oxalis seedlings from my garden. There are 800 species of oxalis, some of which are desirable ornamental plants, but the species in my garden is no prize.

The pest in my garden is Oxalis pes-caprae (Bermuda Buttercup or Sour Grass), a South African plant that has naturalized in the Monterey Bay area’s similar climate. The plant has clover-like leaves and a bright yellow flower. It’s not unattractive, but it reproduces rapidly and could take over the entire garden in time.

Leaves, root, bulblets

Ocalis-pes-caprae

It reproduces by multiplying its small bulbs, which are around one-quarter inch in diameter. One bulb can produce ten in a single season.

It’s theoretically possible to excavate 100% of the bulbs, but this is time-consuming and unlikely to be successful.

The most effective approach is “old bulb exhaustion,” which involves removing the top growth before it can flower. In this area, flowering occurs in February, with sunnier spots blossoming earlier than shadier spots. The idea is to deprive the old bulb of nutrients that would be provided by the leafy growth above ground, so that the new bulbs will die off.

It is best also to disrupt the new bulbs by cultivation, but this might be difficult if the pest has started close to desirable plants.

In any event, this process requires tilling to remove any new growth that appears in about two weeks. A Dutch hoe, which is good for shallow cultivation, would make short work of the new growth.

Several products are marketed as controls for this pest and other broadleaf plants: Weed-B-Gon Chickweed, Clover & Oxalis Weed Killer, Roundup, Finale, Oxalis X, etc. I do not use synthetic chemicals in my garden, because of concerns that they will harm plants, beneficial insects, microorganisms and perhaps myself and other mammals, in addition to the plants or pests they are intended to control.

There are less toxic concoctions for controlling this plant. For example, one foliar spray recipe calls for two cups of white vinegar, one teaspoon of baking soda, and one teaspoon of liquid detergent. Reportedly, this spray will kill the oxalis plant’s top growth but not the bulbs. It should not be sprayed on desirable plants.

Another control strategy is let chickens snack on this weed. Reportedly, they like it a lot.

The gardener engaged in weed control adventures can find confidence in the knowledge that no plant will survive the persistent removal of its top growth.

Finally, a frustrating encounter with Oxalis pes-caprae should not bias the gardener against less invasive species from the wood sorrel family. One of my favorite nurseries, Annie’s Annuals and Perennials, lists seven Oxalis species as desirable garden plants.

Enjoy your weed-free garden.

More

More information on this weed from the California Invasive Plant Council.

 

Interesting Weeds

Yes, even weeds can be interesting. Here are two examples.

First, I’ve written about the “weed seed bank” that exists in all gardens. This inventory of dormant seeds lurks in the top few inches of soil, waiting for life-giving sunlight, air and moisture.

The weed seed bank results from earlier generations of weeds that dropped their seeds under the plant, projected them a few feet away or cast them to the winds for wider distribution. Such seeds might also be brought to the garden as the undigested part of a bird’s meal, tracked in on a visitor’s clothing, or imported with a plant from a friend or the local nursery. Whatever the source, they are part of every garden.

The weed seed bank might be called simply the seed bank, because it includes wildflowers and other garden-worthy plants as well as weeds. Abandoned gardens eventually sprout their hidden wealth of weeds and wildflowers.

Weed seeds can remain in the soil, ready for germination, for several years. Gardeners are wise to use mulch to discourage the germination of weeds and help realize “low maintenance” gardening.

Dormant seeds can be amazingly long-lived under the right conditions. Earlier this year, Russian scientists reported their discovery in Siberia of seeds that a squirrel had buried in the Early Pleistocene era, about 31,800 years ago. The seeds soon were frozen in permafrost and didn’t thaw until retrieved by the research team. With great care, scientist Svetlana Yashina cared for the seeds, which germinated and produced a flowering plant and a new generation of seeds.

The plant is the Narrow-leaved Campion (Silene stenophylla), once known by mammoths and wooly rhinos. An evolved form of this plant grows today in Arctic regions. The genus Silene includes many species, including several wildflowers of Europe and elsewhere in the Northern Hemisphere.

The story of bringing this prehistoric plant into blossom raises the possibility that more plants could be recovered from frozen seeds in Siberia, the Arctic and the Yukon, and the intriguing prospect for the gardeners to grow prehistoric weeds and other plants. Examples of other specimens include the Sago Cycad (Cycas revoluta) and the Maidenhair Tree (Ginkgo biloba). Both of these plants, which are distant relatives, have fossil histories from more than 250 million years ago, and are available today as young plants.

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A second example of interest in these lowly plants involves harvesting them for the dinner table. Dandelions and purslanes are only a beginning. A new cookbook, Foraged Flavor: Finding Fabulous Ingredients in Your Backyard or Farmer’s Market, by Tama Matsuoka Wong, with chef Eddy Leroux, describes many culinary and nutritional benefits of several common weeds. Ms. Wong’s website, Meadows and More, offers more details and an extensive weed identification section.

More

For more about the prehistoric Narrow-leaved Campion (Silene stenophylla), including pictures, browse to the article in Discover magazine.

Another interesting ancient plant is the Amborella trichopoda (no common name), which has been called “the most primitive living flowering plant.” It may be the earliest of the angiosperms: flowering plants that emerged about 130,000,000 years ago. A useful article about this plant is available on Wikipedia. This Arboretum at the University of California, Santa Cruz is the only place in the United States that is growing this plant for botanical study. For a 1999 article on the Arboretum’s work with this plant, click here.

For more information on the book, Foraged Flavor, browse to the New York Times article.

Ms. Wong’s website, Meadows and More, has a wealth of information on culinary uses of common weeds, and an extensive series of weed photographs, with expert identifications.

For identification of weeds that are common in California, visit the Weed Photo Gallery on the University of California’s Integrated Pest Management website.