To understand our struggle with humidity, you must first know our origins. Most Grevillea species, like the popular 'Robyn Gordon' or 'Canberra Gem', are native to Australia. Our home is not the tropical, humid coasts but rather regions with well-draining, often sandy or rocky, soils. We thrive in a Mediterranean climate: think warm, dry summers and cool, moist winters. Our root systems are highly specialized to avoid sitting in water, and our foliage is often fine or wiry, adapted to dry air. High humidity is a foreign and challenging concept for us, as it disrupts our very core biological functions.
The most significant threat to us in a humid environment is not the heat but the constant moisture in the air and soil. Humidity creates the perfect breeding ground for fungal pathogens like root rot (Phytophthora) and fungal leaf spots. Our roots, which are adapted to dry out between waterings, suffocate and decay in constantly damp, poorly drained soil. This is a death sentence. Furthermore, moisture sitting on our foliage for extended periods invites mildew and other foliar diseases that can quickly defoliate and weaken us, making us susceptible to other stressors.
Humidity also interferes with our natural cooling process: transpiration. We release water vapor through pores in our leaves (stomata), which pulls water and nutrients up from our roots. In highly humid air, which is already saturated with water vapor, this process slows down dramatically or even stops. This is akin to you trying to sweat in a steam room. The consequences are severe—it can lead to nutrient deficiencies, reduced growth, and overall physiological stress as we cannot effectively cool ourselves or move vital fluids throughout our system.
While not ideal, some of us can survive with intense human intervention designed to mimic our native conditions. The single most critical factor is soil. You must plant us in extremely well-draining soil; a mix of native soil, coarse sand, and fine gravel is best. Mounding the soil into a raised bed or planting on a slope is highly advisable to ensure water flows away from our root crown. Secondly, select a planting location with maximum possible sun exposure and excellent air circulation. This helps our foliage dry quickly after rain or dew, thwarting those fungal foes.
Not all Grevillea are equal in their tolerance. Some species and hybrids are more adaptable than others. You might have better luck with tougher varieties like Grevillea robusta (Silk Oak) or Grevillea 'Long John'. Care must be meticulous. Water us deeply but very infrequently, only when the soil is completely dry several inches down. Absolutely avoid overhead watering which wets the leaves; water at the base instead. Prune us to improve air circulation within our canopy and remove any diseased material immediately to prevent spread.
