Plants stop manufacturing green chlorophyll pigment in their leaves before shedding them. This makes them change color as underlying yellow carotene pigments remain. While deciduous trees lose all their leaves in the fall after an often spectacular show of yellow and red leaves, any atypical or unseasonal yellowing of the leaves on an outdoor plant must be investigated immediately. Early treatment of the underlying problem will prevent further damage and may even save the plant.
Many evergreen outdoor plants develop and then shed yellow leaves during phases of dormancy. This can be either in the coldest months of winter or during the hottest and driest period of summer. Often it is the oldest leaves on a plant that turn yellow and drop. Natural leaf yellowing in these circumstances in nothing to worry about.
Pale and yellowing leaves all over an outdoor plant often indicate a nutrient imbalance or deficiency called chlorosis. Plants grown in nutrient-poor soil struggle to absorb essential minerals such as iron and manganese. This is especially common in highly alkali and acidic soils or in plants that are grown in unsuitable soils. Individual plants suffering from chlorosis can be treated with a mineral-rich foliar spray or an all-purpose liquid fertilizer. If chlorosis problems are more widespread, buy a home pH testing kit from a gardening center or send a soil sample off to be analyzed by a laboratory to detect mineral deficiencies. Increase the pH of acidic soils by treating them with lime and apply sulphur or acidic organic matter such as peat to lower the pH of alkali soils.
Many plants develop yellow leaves if they receive too much water or are grown in waterlogged soils. Check the soil your plants are growing in for signs of pooled water and reduce watering if it is constantly wet through. Succulents and plants that grow naturally in light, sandy soils are especially vulnerable to over-watering if grown in clay soils or planted in damp areas of the garden.
If a plant is not receiving enough water its natural reaction is to reduce transpiration and water loss by shedding leaves. Outdoor plants such as ficus are very vulnerable to water stress and react to under watering by shedding the majority of their leaves after they turn yellow. Water any plant that has yellowing leaves and is growing in dry soil. A circular moat and raised wall several inches high around the main stem of the plant will allow water to soak into the soil rather than run off.
Other Environmental Stress
Plants with bleached-out yellow sections on their leaves may be growing in an area with too much direct sunshine. Equally, sun-loving plants grown in the shade may develop yellow leaves due to a lack of light. Plants grown outside their optimal temperature range may also develop yellow leaves. Check the optimal growing conditions of the plant in question online or at a gardening center.
Yellowing leaves on an outdoor plant can be a sign of disease, especially if they do not drop off the plant. Check affected plants for signs of gossamer-thin spider webs, which indicates spider mite infestation. Circular yellow spots on plant leaves indicate a fungal disease while irregular yellow patches can be due to bacterial and viral disease. Cornell University has a detailed photographic key of the effects of disease on tomato plant leaves that shows the effects of many diseases. If you suspect disease, take a sample in a sealed plastic bag to your gardening center for a diagnosis.