Elinor Teague

The three sisters garden concept, originated by Native Americans, includes corn, beans and squash grown together for their mutual benefit. (Image: Wikimedia Commons)

Two of the most popular subjects of companion planting on the internet are “Planting a Three Sisters Garden” and “whether planting marigolds as companion plants will repel pest insects and provide control for nematodes.”

The ‘three sisters' trio of plants (beans, corn, squash) is a great example of successful companion planting developed by Native Americans centuries before the arrival of European settlers. The plants are grown together for the benefit of all of them. The corn stalks support the bean vines as they grow; the beans fix nitrogen in the soil; the large squash leaves shade the roots. The timing of seed planting and best varieties for specific climate zones is not always provided by internet sources.

Whether or not marigolds repel pest insects and whether they control for nematodes is not clear from science-based studies, but many gardeners now are firm believers in the practice of planting marigolds as companion plants. There is (some) science-based evidence that green tilling French marigolds (Tagetes patula) into the soil the season before planting does reduce the nematode population somewhat, but not always. Marigolds do produce compounds that may (or may not) repel insects, but which insects and how the compounds work has not been fully studied.

As Master Gardeners we often need to gently correct misinformation gleaned from the internet and redirect home gardeners to research and science-based sources. Over the last decade companion planting has become increasingly popular and researchers are beginning to study the effects of companion planting in home gardens. Companion planting is an effort to encourage natural plant-derived controls for plant diseases, pest insects and weeds, thereby reducing the use of herbicides and pesticides. Some examples of the efficacy of companion planting have been verified by scientific evidence as well as anecdotal observations.

Here are a few of the more commonly recommended companion planting tips that Master Gardeners can offer with confidence:

• The planting of nitrogen-fixing plants, such as peas and beans, and cover crops, such as clovers, raises nitrogen levels in the soil. When the cover crops are tilled into the soil as green mulch, the soil texture and water retention improve.

• Planting host plants like artemisia as habitats and food sources for predatory beneficial insects like lady beetles helps reduce pest insect populations in the garden.

• Planting flower species that bloom in succession and that include all three flower types – umbellifer (dill, Queen Anne's lace), tubular (penstemon, trumpet vine) and open-faced (daisies, coreopsis) – ensures that the garden provides nectar and pollen throughout the growing season as food for pollinators, beneficial insects and hummingbirds.

• Planting taller sun-loving plants next to short ones provides shade for the shorter plants as does planting larger-leafed shorter plants between rows of taller plants. The shade discourages weed growth and helps keeps roots cooler. 

Sources:

Better Together: The New Science of Companion Planting, UC Master Gardeners of San Mateo and San Francisco counties

Beneficial Insect, Insectary Plants, UC Integrated Pest Management Program

Plant a Three Sisters Garden: Corn, Beans and Squash, The Old Farmer's Almanac

Avocado leaves with brown lesions due to the fungi that cause branch canker and dieback. The disease is caused by various fungi in the Botryosphaeriaceae family. (Photo: David Rosen, UC IPM)

Fungal problems on plants are usually short-lived here in the Central Valley. High temperatures and low humidity beginning in May quickly kill off fungal spores. We seldom see common fungal problems like powdery or downy mildews, rust or black spot after May. 

Biological fungicides or biofungicides are gradually replacing a wide range of fungicides including chemically derived broad spectrum products, as well as least-toxic products such as copper and sulfur solutions, horticultural oils, neem oil and jojoba oils. Fungi are plant-specific and biofungicides have a long list of treatable fungi on many different plant species.

Biofungicides are formulated with naturally occurring living microorganisms, beneficial bacteria, that are found on plants or in the soil. Labels on biofungicides for home and garden use may not clearly indicate that the product is a biofungicide. The labels will show bacterial strains as the active ingredient. The two most common are Bacillus subtilis and Bacillus amlolyquefaciens.

The microorganisms are effective at controlling fungal pathogens in several ways: They out-compete fungus for nutrients and space, they trigger resistance in the host plant to the pathogen, they produce antibiotics or toxins that weaken or destroy the pathogen.                         

Biofungicides are organic products and can be as effective or even more effective in treating or controlling most of the common fungal problems in home gardens than can either broad-spectrum chemically derived or the least-toxic fungicides. They are less toxic than copper and sulfur products. 

However, the use of biofungicides is not without problems. They are preventative rather than curative treatments. Great care must be taken to remove all plant debris and even soil from areas in the garden where plants have previously shown fungal problems. New transplants should be examined carefully for signs of fungal infestations and, if in doubt, the potting soil should be drenched with a biofungicide or removed and replaced with fresh, sterilized soil.    

Application timing is easily thrown off by rain showers. I was unimpressed with the results of using a biofungicide last spring on Romano bean plants due to rain showers continuing well into April. Biofungicides, like other fungicides, should not be applied when rain is predicted, temperatures are above 90 degrees or on drought-stressed plants. 

Because biofungicides are composed of living organisms, shelf life is reduced. Two home-and-garden-use biofungicide labels (Serenade and Monterey Complete Disease Control) do not provide use-by dates or storage temperature ranges. The UCONN IPM website does give that information for a range of biofungicides used in greenhouses and agriculture. Maximum use-by date ranges from 18 months to 3 years. Some formulations require refrigeration or have a maximum temperature. Information on whether those same restraints apply to home and garden use products was unavailable.           

The same UCONN IPM article emphasizes the importance of wearing protective clothing and using caution when applying biofungicides. The two labels mentioned above are less emphatic in their recommendations.

Sources:

• CT IPM program, UCONN College of Agriculture, Health and Natural Resources, Biological Fungicides.
• UCIPM, Fungicides, Powdery Mildew on Vegetables
• UCANR, “Pests in the Urban Landscape', Biological Fungicides: Do They Work and Are They Safe?, Steven Swain, 2014, UC IPM Green Bulletin Newsletter.