If your Washingtonia robusta leaves are turning yellow, there could be several reasons behind this issue. Here are some common factors that can contribute to yellowing leaves on Washingtonia robusta:

1. Natural Aging: It is normal for older leaves to turn yellow and eventually die off. If the yellowing is limited to the lower or inner leaves, it is likely part of the palm’s natural growth cycle, and you can simply prune away the yellowed leaves.
2. Watering Issues: Overwatering or underwatering can both lead to yellowing leaves. Overwatering can cause waterlogged soil, which hampers oxygen uptake by the roots and leads to root rot. Underwatering, on the other hand, can result in leaf dehydration and nutrient deficiencies. Ensure you are watering your Washingtonia robusta appropriately by checking the soil moisture and adjusting your watering routine accordingly.
3. Nutrient Deficiencies: Yellowing leaves can be a sign of nutrient deficiencies, especially nitrogen or magnesium. These nutrients are essential for maintaining healthy foliage. Consider using a balanced palm fertilizer that is specifically formulated for palm trees to address any nutrient deficiencies. Follow the instructions provided on the fertilizer packaging to ensure proper application.
4. Environmental Stress: Washingtonia robusta can experience stress due to extreme weather conditions. Cold temperatures, frost, or intense heat can cause leaves to turn yellow. Protect the palm from extreme temperatures whenever possible by providing shade during hot periods or covering the tree during frost events.
5. Pest or Disease Infestation: Pests or diseases can also lead to yellowing leaves. Common pests such as spider mites or scale insects can cause stress and discoloration. Regularly inspect the palm for signs of pests, such as visible insects, webs, or damaged leaves. If pests are present, treat the palm with appropriate insecticides or seek professional assistance if needed. Additionally, fungal or bacterial infections can also affect leaf health. Look for any signs of disease, such as spots, lesions, or wilting, and take appropriate measures to address the issue.