Catalpa

The beneficial companions listed for Catalpa are all shade-tolerant understory plants — Hosta, Ferns, Astilbe, Heuchera, Wild Ginger, Impatiens, Caladium, Coral Bells — and that's the whole logic. A mature Catalpa throws dense shade across a 30-40 foot radius, and most common garden plants give up under it. These companions are chosen because they've already adapted to low-light, root-competitive conditions. Hostas and ferns in particular handle the dry shade that develops under a broad canopy once the tree is 10-plus years old, where summer rainfall doesn't penetrate the leaf canopy reliably. Wild Ginger (Asarum canadense) works as a slow-spreading ground cover that fills gaps without staging any real fight for resources at the root zone.

The harmful companions tell a cleaner story. Black Walnut (Juglans nigra) produces juglone, an allelopathic compound that leaches through the soil and can stunt or kill a wide range of plants within its root zone — siting a young Catalpa near one is asking for a slow decline you'll spend two seasons misdiagnosing. Grass lawn directly under the drip line is the more common mistake: dense turf competes hard for water and nutrients, and mowers nicking the bark year after year cause cumulative cambium damage that opens the tree to Phyllosticta and other opportunistic fungi. Roses and tomatoes are on the harmful list mainly because they need 6-8 hours of direct sun — they'll fail on their own under a Catalpa canopy well before they cause the tree any trouble.