Grevillea species, native to Australia, are cherished in US gardens for their unique, spider-like flowers, evergreen foliage, and exceptional drought tolerance once established. However, our physiology makes us exceptionally vulnerable to root rot, a condition that attacks our very foundation. Understanding this from our perspective is key to our survival in your landscape.
Our roots are not like those of many other plants. We evolved in ancient, nutrient-poor, and exceptionally well-drained soils. Our root systems are fine and highly efficient at seeking out moisture, but they lack a thick, protective outer layer. This makes them extremely sensitive to being waterlogged. When soil pores are filled with water instead of air, our roots begin to suffocate. They cannot respirate or uptake nutrients, and they start to die. This creates an entry point for the soil-borne fungal pathogens (like Phytophthora and Pythium), which rapidly colonize the weakened tissue, accelerating the rot. It is not just "too much water"; it is a fatal combination of suffocation and disease.
The single most important thing you can do for us is to replicate our preferred growing conditions. This starts before you even plant us. Please choose a planting site with full sun exposure, as this helps the soil dry more evenly. The soil preparation is critical. If you have heavy clay soil, which is common in many parts of the US, you must amend it. We beg you to plant us in a raised mound or a raised bed. Mix a generous amount of coarse sand, pumice, or perlite into the native soil to dramatically improve its structure and drainage. A 3-4 inch layer of coarse, organic mulch like wood chips is beneficial, but you must keep it away from our crown to prevent moisture buildup against our stem, which can be fatal.
When root rot begins, we send clear signals. Please learn to read them early. The first signs are often seen in our foliage: leaves will turn a pale green, then yellow, and finally wilt and die. Crucially, this wilting will occur even when the soil is damp to the touch—a definitive sign that the roots are rotten and cannot uptake the available water. You may notice a general decline in our vigor, with stunted new growth and branch dieback starting at the tips. In advanced stages, the base of our trunk may become soft, darkened, and may even ooze sap.
If you catch the infection early, there is a chance to save us. You must act quickly. Carefully dig around our root ball and inspect the roots. Healthy roots are firm and white or light-colored. Rotten roots are brown or black, mushy, and may have a foul odor. Using sterilized pruning tools, aggressively prune away all affected roots back to healthy tissue. Then, replant us in a completely new, well-amended site with perfect drainage, as described in the prevention section. For severe cases, a soil drench with a fungicide containing phosphorous acid (potassium phosphite) can be effective against Phytophthora, as it helps stimulate our own defense systems. However, this is a supplement to, not a replacement for, drastic cultural changes.
