European pear and comfrey work well togehter. The pear shades (ie inhibits) the comfrey while the confrey keeps grass from growing under the tree. They also share a similar fungal-bacterial balance in the soil as well as pollinators.
One nettle plant planted with your herbs will increase the oil content of the herbs.
Plant aromatic plants (onions, garlic, mint) to confuse the chemical signals pests use to locate their prey. Planted around the edge of your garden you can create a barrier for you plants.
Do not deadhead, leave the plants for winter feed and habitats of overwintering spiders and insects. Polyculture works when species occupy different niches (different forms, sizes, root patterns, light needs, water requirements, etc).
Learn your sites limiting factors.
Give Native Species Preference
Invasive species decrease habitat diversity. Invasive exotic plants can cause the extinction of native species. The exotics do not support a healthy ecosystem because they have no native insects (or other animals) that use them as a food source.
Use exotics only when a native does not fit the conditions or needs of an area. Consider the site and its context.
In a native region climate is a critical factor.
These are notes I made about gardening. See more gardening notes