It is not universally true that weeds are harmful to other plants.

It is entirely subjective what we consider a “weed”. For example, some plants like goldenrod or white snakeroot in North America are both sold in garden centers and planted intentionally, yet are sometimes seen by other gardeners as weeds.

Plants can harm other plants in multiple ways, but they can also help them. Some of the ways they harm them include:

• Directly competing for light, growing taller than and shading out other plants
• Competing for nutrients or water in the root zone
• Attracting insects that eat the other plant
• Allelopathy - producing chemicals that inhibit the growth of other plants

But plants can also help other plants. Some ways include:

• Nitrogen fixation - most legumes and a number of other plants add nitrogen to the soil, which can benefit other plants growing near them.
• Shading or protecting plants that need shade or are otherwise growing in too sunny a location for them
• Cycling water in a way that benefits them (Many maple trees do this by bringing water from deep in the ground, up to the surface during the night so that they and other plants can use them during the day)
• Competing against unwanted plants without harming others. For example, if you grow a black walnut tree, its allelopathy will reduce the growth of many plants, but if you grow it together with plants that co-evolved with it, they will be able to grow well and may benefit from the reduced competition from other plants.
• Attracting beneficial insects, especially when they attract predators that eat insects that eat the other plants. An example would be a native plant that tends to attract specialized aphids that only eat it. Such a plant would indirectly attract predators of aphids like praying mantises or warblers, which would be likely to also eat aphids off other plants. This is particularly true of most native plants, and is one reason I recommend all gardeners to mostly grow locally-native plants.

Just because a plant is considered a “weed” doesn’t mean it is harming other plants. Per esempio, alcune persone considerano il trifoglio un'erbaccia, ma è un fissatore di azoto e la sua presenza porta benefici alla maggior parte delle piante coltivate.

Se si vuole sapere se una particolare pianta sta danneggiando o meno un'altra pianta, si può osservare e fare ricerche. L'osservazione è probabilmente più utile della ricerca... provate a coltivare le piante insieme e poi coltivare la pianta desiderata da sola, e vedete quale fa meglio. Di solito è abbastanza rapidamente evidente (dopo una stagione di crescita o meno) quando una pianta sta danneggiando un'altra, sia attraverso la competizione o qualche altro fattore. E con il tempo, più osserverete queste interazioni, meglio diventerete come giardinieri!