6.1 Principles of companion planting and plant interactions

Plant interactions are the foundation of thriving ecosystems. Companion planting and symbiotic relationships boost growth and resilience. Understanding how plants compete, facilitate, and communicate chemically helps us design more productive and sustainable gardens.

Diversity is key in planting systems. Intercropping, biodiversity, and strategic pest management through plant combinations create balanced ecosystems. These practices reduce the need for external inputs and increase overall productivity and resilience.

Beneficial Plant Interactions

Companion Planting and Symbiotic Relationships

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• Companion planting involves strategically placing plants together that benefit each other
  • Planting nitrogen-fixing legumes (peas, beans) near heavy feeders (tomatoes, corn) provides nutrients
  • Aromatic herbs (basil, mint) can repel pests from vulnerable crops (cabbage, broccoli)
• Symbiosis occurs when two organisms form a close, long-term interaction that benefits both
  • Mycorrhizal fungi form symbiotic relationships with plant roots, increasing nutrient and water uptake for the plant while receiving sugars from the plant
• Mutualism is a type of symbiosis where both organisms directly benefit from the relationship
  • Pollinators (bees, butterflies) feed on nectar and pollen from flowers while also pollinating the plants, enabling reproduction

Nurse Plants and Facilitation

• Nurse plants are established plants that create favorable microenvironments for young seedlings
  • Larger trees provide shade and wind protection for understory plants in forests
  • Hardy pioneer species (lupines, alders) can stabilize soil and add nutrients, facilitating the growth of other plants
• Facilitation occurs when one plant species enhances the growth, survival, or reproduction of another species
  • Leguminous cover crops (clover, vetch) add nitrogen to the soil, facilitating the growth of subsequent crops
  • Companion plants can attract beneficial insects (ladybugs, lacewings) that prey on pests, facilitating the growth of the main crop

Plant Competition and Allelopathy

Competition for Resources

• Plant competition occurs when two or more plants vie for limited resources such as light, water, and nutrients
  • Tall, fast-growing plants (sunflowers, corn) can shade out and outcompete shorter, slower-growing plants
  • Deep-rooted plants (trees, shrubs) can compete with shallow-rooted plants (grasses, herbs) for water and nutrients
• Competition can be minimized by selecting plants with complementary growth habits and resource needs
  • Combining shallow-rooted and deep-rooted plants (lettuce and tomatoes) can reduce competition for water and nutrients
  • Planting shade-tolerant species (ferns, hostas) under taller plants (trees, shrubs) can maximize use of vertical space

Allelopathy and Nutrient Cycling

• Allelopathy occurs when one plant species releases chemical compounds that inhibit the growth of other plants
  • Walnut trees produce juglone, a compound that can inhibit the growth of many plants (tomatoes, potatoes) beneath their canopy
  • Some invasive species (garlic mustard, tree of heaven) use allelopathy to outcompete native plants
• Nutrient cycling is the process by which nutrients move through an ecosystem, from soil to plants to animals and back to soil
  • Plants take up nutrients from the soil and incorporate them into their tissues
  • When plants die or shed leaves, the nutrients are returned to the soil through decomposition
  • Planting a diversity of species with different nutrient needs can help optimize nutrient cycling and reduce the need for external inputs

Diversity in Planting Systems

Intercropping and Biodiversity

• Intercropping involves growing two or more crops together in the same space
  • Planting compatible crops (corn, beans, squash) together can maximize use of space and resources
  • Intercropping can increase overall productivity and yield compared to monocultures
• Biodiversity refers to the variety of life forms within an ecosystem, including plants, animals, and microorganisms
  • Increasing plant diversity can support a wider range of beneficial insects, birds, and other wildlife
  • Diverse ecosystems are generally more resilient to stresses such as pests, diseases, and climate extremes

Pest Management through Diversity

• Increasing plant diversity can help manage pests by creating a more balanced ecosystem
  • Planting a variety of crops can reduce the spread of pest populations and limit damage to any one crop
  • Incorporating plants that attract beneficial insects (yarrow, dill) can help control pest populations naturally
• Diversity in planting systems can also reduce the need for chemical pesticides
  • Companion planting with pest-repellent plants (marigolds, nasturtiums) can deter pests from main crops
  • Rotating crops between different plant families (brassicas, legumes) can break pest and disease cycles