Cover Crops & Soil Enhancement

Spring is here and plants in the Edible Demo Garden are beginning to bloom. Not all the blooming plants in the garden are edible crops – some are there for their flowers. The flowers add to the beauty of the garden, but they don’t just look pretty – they have important jobs to do. They attract pollinators, repel pests, provide a habitat for beneficial insects, and contribute to the biodiversity of the garden. It turns out that many flowering plants can be good companions for the edibles.

 

A variety of flowers are grown in the Edible Demo Garden. Some are perennials such as yarrow and lavender. Others are annuals that are grown from seed in the greenhouse and planted each year. These include zinnias, marigolds, and calendula. Then there are the flowering plants that self-seed and return on their own like the California poppies and the sunflowers. Sunflowers have been so successful in the Edible Demo Garden, that they sometimes show up where they are not wanted.

 

Favorite flowers for attracting pollinators

It’s easy to tell which flowers are popular with bees – just head for the buzzing sound. The California lilac (Ceanothus), planted in the garden several years ago, blooms early, ensuring that there will be bees around when the spring vegetable crops start blooming. Although it’s also a culinary herb, rosemary (Rosmarinus officinalis), is another bee magnet. It turns out that many of the culinary herbs and herbs for tea produce flowers that attract pollinators. Mexican tarragon (Tagetes lucida), Anise hyssop (Agastache foeniculum), Coneflower (Echinacea)and Chamomile (Matricaria recutita) all do double duty in the Edible Demo Garden. Nectar producing flowering plants, like the Cleveland sage (Salvia clevelandii) and California Fuschia (Epilobium canum) also attract hummingbirds and butterflies to help in the pollinating work.

 

Flowering plants that discourage pests

There is increasing scientific-based evidence that certain plants are effective in managing pests in an edible garden. They can confuse pests, luring them away from their favorite target and disrupting their feeding and egg-laying behaviors. Studies have suggested that plants with strong odors can discourage pests by masking the scent of the desired host plant. Marigolds (Tagetes) have long been a pest-repelling favorite of gardeners, but the supporting science is scant. However, there is research showing their effectiveness against parasitic nematodes and some flies affecting vegetable crops. In addition to their ability to attract pollinators, Nasturtiums (Tropaeolum majus), have been found to reduce squash bug damage when grown with zucchini. Their edible flowers make them especially welcome in the garden.

 

The importance of diversity in the garden

Mixing in some flowering plants with the edibles encourages beneficial insects that can keep pests in check. Members of the Asteraceae family, which includes yarrow, sunflowers, zinnias, calendula, and asters, support beneficial insects that dine on aphids. Flowering herbs provide nectar for parasitic wasps which help control caterpillar pests. Low -growing flowering plants such as thyme and oregano provide a sheltered habitat for ground-dwelling beneficials.

 

The right flowers in an edible garden are not just ornamental, they balance the garden. They help in creating a habitat that supports plant growth and production and reduces the need for pesticides and other pest-management strategies.

 

Click here for more information about plants that attract pollinators.

The soil in the Edible Demo Garden gets a lot of attention. Compost and organic amendments are added regularly. Moisture levels are monitored, and rainwater is supplemented with drip irrigation and hand watering. Mulch and cover crops are used to protect the soil and tilling is kept at a minimum. The goal of these efforts is to maintain the structure and composition of the soil so that the growing plants have access to the air, water, and nourishment they need.

However, soil can look and feel healthy, and still be lacking nutrients essential for optimum plant growth. Plants suffering from malnutrition look unhealthy and show symptoms of deficiency. Growth may be stunted, leaves may be chlorotic (yellowed) or distorted, and shoots appear short and thin. Too little or too much of any one nutrient can cause problems. The best way to check for a lack of nutrients in the soil is to test a sample.

 

How are the nutrient components of soil tested?

Inexpensive do-it-yourself soil test kits are readily available in garden centers, and they can be a convenient way to determine soil pH and levels of the primary macronutrients: nitrogen, phosphorus, and potassium. More comprehensive tests which cover secondary macronutrients and micronutrients, can be performed by commercial soil testing laboratories.

Any soil test should start with a carefully collected soil sample. Use a stainless-steel trowel or shovel to obtain a sample at least six to eight inches below the soil surface. Avoid wet soil, if possible, the sample needs to be dry for testing. Taking samples from several places in a garden bed and mixing them can be useful in measuring overall chemical properties. However, samples from different garden areas (vegetable, perennial, lawn, etc.) should be kept separate. Soil testing laboratories generally provide specific instructions on sample collection.

 

Why does soil pH matter?

Nutritional elements in the soil need to be in a soluble chemical form to be absorbed by plant roots. There must be sufficient moisture in the soil to allow the roots to take up and transport the nutrients. The temperature of the soil also needs to fall within a certain range for uptake to occur. Finally, the solubility of the nutritional elements and the activity of the microorganisms responsible for the chemical transformation of nutrients are directly influenced by soil pH.  An essential nutrient can be present in the soil, but not in a form the plant can use. Each nutrient has an ideal pH range for plant availability. At pH levels that are too acidic or alkaline, some nutrients can become insoluble and not accessible to plant roots. Phosphorus, calcium, and magnesium are elements that become less accessible as soil increases in acidity, while the availability of iron, zinc, and manganese is reduced in more alkaline soils. The ideal pH for most plants is between 5.5 and 7.5.

 

What nutrients are commonly lacking in soil?

The nutrient most commonly lacking in garden soils is nitrogen. Most plants absorb nitrogen in the form of ammonium or nitrate. These forms readily dissolve in water and leach away. Excessive irrigation and heavy rains can lead to nitrogen deficiency in the soil. Phosphorus, potassium, calcium, magnesium, and sulfur are the other macronutrients that may be deficient, depending on drainage and pH levels. A lack of iron, zinc, or manganese, all considered micronutrients, results in interveinal chlorosis, a condition in which the leaf veins remain green while the rest of the leaf turns yellow. The uptake of these micronutrients is also affected by pH.

 

For more information on assessing soil health, click here.

For hands-on practice in soil testing come to the workshop co-sponsored by the Edible Demo Garden and the Edibles Guild. Click here to register.

In December EDG volunteers planted winter garden edibles including carrots, fennel, cabbage, kale, and – artichokes. While artichokes may not be on every gardener’s cool season vegetable list, they are fun to grow, and December is the right time to plant them for an early spring crop. Since almost all artichokes sold commercially are grown in California, it was designated the official state vegetable in 2013.

 

What is an artichoke?

Artichokes are large plants in the sunflower family (Asteraceae). The edible parts are the large flower buds that form on the tops of tall stalks. The buds are typically green, but some varieties are violet-tinged. Buds that are not harvested for cooking and eating will open into attractive purple thistlelike flowers.

Although some may consider artichokes exotic and too much trouble to eat, they have been enjoyed since Roman times. Italian immigrants brought artichokes to Monterrey County where they grew well in the Mediterranean like climate. When artichokes first became popular in the US there was a problem meeting the demand and there are stories of a Mafia attempt to corner the market. Fortunately, we can now have all the artichokes we want to eat when they are in season.

 

What artichoke varieties grow well in Marin?

Artichoke plants need exposure to temperatures below 50 degrees to form buds, but they are easily damaged by frost and freezing temperatures. The plants also suffer in hot dry summers. Artichoke varieties differ in their tolerance for cold and heat but they like the coastal Bay Area climate. The standard variety is ‘Green Globe’, the variety most often found in the supermarket. The two varieties planted in the Edible Demo Garden are ‘Imperial Star’ and ‘Colorado Star’. These varieties mature faster and require fewer chill hours than ‘Green Globe’.

 

Are Artichokes Annuals or Perennials?

In California artichokes are commonly grown as short-lived perennials producing both spring and fall crops. After the spring harvest they are cut back to the ground and kept dry to encourage summer dormancy after which they are regrown in the fall for another crop. However, early maturing varieties like ‘Imperial Star’ and ‘Colorado Star’ also work well as annuals when planted in late fall. In the Edible Demo Garden, the two new varieties replaced older plants that had aged past their prime and suffered some gopher damage.

 

How to Grow and Harvest Artichokes

The artichokes planted in the Edible Demo Garden were grown from seed in the Indian Valley Organic Farm and Garden greenhouse. Seedlings are ready to plant in 8 to 12 weeks. The plants require a sunny location with fertile, well-drained soil. They need consistent watering and monthly doses of high nitrogen fertilizer. They also require some space since the plants are typically 3-4 feet wide and tall.

The buds on the artichoke plant are ready to harvest just as the lowest bracts begin to open. Once a bud opens more fully, the tenderness and flavor diminish.  Harvesting the top bud stimulates the lower buds to develop. Buds should be harvested by cutting the stem about 2 inches below the bud.

For more information on growing artichokes, click here.

After the fall harvest was over, it was time to clean up and replenish the soil in the Edible Demo Garden. Healthy soil is fundamental to the success of the next season’s crops. Even in beds that are temporarily fallow, the soil needs to be nurtured and protected. In November, volunteers were busy with three major soil-boosting activities - spreading compost, adding mulch, and planting cover crops.

Why add compost?

The value of compost cannot be overestimated. Most edible crops are heavy feeders and leave the soil depleted of nitrogen and other essential nutrients after they are harvested. Adding a layer of compost to the soil is an earth-friendly way to help restore those nutrients and support good soil structure. Since compost is decomposing organic matter, it needs time to make the nutrients available to plants. In the Edible Demo Garden, compost is applied to all the garden beds prior to planting the next season’s crop.

What is the purpose of mulch?

Mulch is any material placed on the soil to protect it by conserving moisture, moderating temperature, and preventing erosion. Mulch is also used to suppress weeds. Mulch material can be organic or inorganic. Only recycled organic mulch is used in the Edible Demo Garden. Unlike compost, mulch is not intended to amend the soil and the materials used are generally coarser and break down slowly.

In late fall, Edible Demo Garden volunteers take apart the straw bales used to grow warm season vegetables. The used straw becomes recycled mulch. Initially, the straw was considered suitable as compost material, however, subsequent testing found it to be low in nitrogen. Although much nitrogen was added to the bales during the growing season, most of it was either taken up by the plants or washed out by watering and rain.

Why plant cover crops?

Cover crops, sometimes referred to as “green manure”, are another excellent way to protect and improve soil. Cover crops are divided into two main categories: legumes, such as clover and fava beans, and non-legumes, like rye and buckwheat. Legumes have the added benefit of fixing nitrogen by taking it from the air and converting it into a form to be used by plants. Cover crops are turned over when they start to flower and allowed to decompose prior to spring planting.

For several years Edible Demo Garden volunteers have been cultivating a large garden area known as “the back 40”. Planting a cover crop of fava beans in the fall is one of the strategies for improving the poor soil in that area. Before the fava beans could be planted in November, some light tilling of the soil was necessary to improve the penetration and decomposition of the four wheelbarrows full of compost spread by the volunteers. Prior to planting, the bean seeds were soaked in an inoculant containing rhizobacteria to maximize their nitrogen fixation effects.

Paying careful attention to spacing and depth requirements, 150 inoculated fava bean seeds were planted in the back 40 by the volunteers. Some watering will be necessary until the rains begin, but a good crop of green manure should be available to nourish the soil in the spring.

For more information on building healthy soil, click here.