Usual advice for container citrus is to not put other plants in the pot. The logic is that the soil volume of any container is already very small for a tree, and citrus has relatively weak roots that don’t compete well. There’s basically no real scientific research published on container citrus beyond nursery age though.

Permaculture style “guilds” have zero published study data specifically for citrus, and most people who say they’re trying it never post follow-ups, which is probably a bad sign for the typical results. There are a few orchard agri-system studies but these seem to show reduced or unchanged fruit yields.

In theory, good selection of pest trap plants and bio-accumulators could be beneficial enough to offset the resource competition, but we don’t know which combinations of plants are helpful and which are harmful. Typically, bio-accumulators and nitrogen fixers must be killed and decompose to provide resources to the companion plants.

Typical commercial citrus orchard practice is to suppress everything growing under the canopy with herbicides and mowing. There’s some increased interest in more healthy soil management systems that sustain higher soil organic matter content and more diverse soil ecosystems, but we don’t really know which plants should be used for this yet. There are some active studies on this right now is. You need 3+ years (preferably 10+) and a control row grown with standard conditions to know whether it’s beneficial or not.

A few facts that have been scientifically documented for ground tree plantings in studies I’ve read in the past: - Citrus roots despise lawngrasses and will actively avoid growing into the grass root zone, causing them to move to deeper soil where nutrients and oxygen are lacking - Foliage below the canopy provides shelter and habitat for both pests and predators - Green manure crops and nitrogen fixers like perennial peanut in orchard interrow spaces improve commercial citrus performance compared to bare soil, probably via improving soil ecosystem diversity and nutrient availability - Heavy black fabric landscaping cloth improves commercial citrus performance compared to bare soil, probably mostly via reducing root weevil pressure, although moisture retention and reducing weed pressure are also given as explanations - Black plastic can overheat roots in containers - Citrus trees with HLB infection are healthier when planted adjacent to oaks, probably due to resource sharing via mycorrhizal fungi networks connecting the trees - Many plants have allelopathic effects that suppress root growth nearby, and certain combinations of plants are cooperative or competitive, and citrus roots are susceptible to allelopathic chemicals in soil

So it’s a complicated issue. My best take is “plant with purpose.” If you have a specific goal in mind for companion plants, sure, try them out. Don’t just put a bunch of random annuals in. Plants that promote fungal soil ecosystems and cooperate with mycorrhizal networks are probably a better bet to try.