Macadamia (Beaumont)

Comfrey is probably the most useful plant you can put under a macadamia. Its deep taproot pulls up calcium and potassium from below the tree's root zone, and when you chop it down and leave it as mulch, those nutrients release right where the surface feeder roots can use them. Clover does similar work at a smaller scale — fixing atmospheric nitrogen into the soil, which cuts down on how often you need to side-dress with fertilizer. Nasturtiums and marigolds draw aphids and thrips toward themselves rather than the tree; plant a ring of them 3 to 4 feet out from the drip line where you can actually see and manage the damage.

Keep eucalyptus and black walnut off the same property if you can help it. Eucalyptus releases volatile terpenes and drops allelopathic leaf litter that suppresses root activity in neighboring plants — macadamia roots can spread 25 feet out, so proximity matters more than it looks on a map. Black walnut produces juglone through its roots and decomposing hulls, and while macadamia's specific sensitivity hasn't been as well-documented as, say, apple or tomato, there's no upside to the experiment. Pine trees create a different problem: heavy needle drop acidifies soil past the 5.0 floor that Beaumont tolerates, and that pH shift makes phosphorus and calcium harder to absorb.