Types

• Of the ash trees commonly grown in the United States in gardens or parks that produce yellow fall foliage, the green ash (Fraxinus pennsylvanica) may be the most planted. Numerous cultivars of green ash exist, and all produce varying shades of yellow-green, bright yellow or golden yellow fall foliage. Other ash species known to yield predominantly yellow leaf color in autumn include narrowleaf ash (F. angustifolia), blue ash (F. quadrangulata), Oregon ash (F. latifolia), black ash (F. nigra) and Arizona ash (F. velutina).
  
  

Pigmentation

• For yellow tones to appear in fall on ash trees, the predominant pigment that remains, after chlorophyll degrades, must be xanthophyll. Only xanthophyll yields a yellow color. Combinations of brown tannins, beige-white flavonoids and orange carotenoids with xanthophyll create the varying shades of yellow that range from beige- or tan-yellow to golden yellow with hints of orange. Other ash trees may display reddish or purple pigments, called anthocyanins, that will not create obviously yellow or golden colors. The white ash (Fraxinus Americana) is a prime example, with only hints of yellow that are overpowered by purple or red-orange colors.
  
  

Developing the Strongest Color

• Healthy ash trees develop the best yellow fall foliage color. Drought, nutrient-poor soil and stresses from insects or disease can limit the amount of pigments and water already existing in leaves since summer. Shorter days and cooler nights in September and October trigger the degradation of chlorophyll and movement of molecules out of the green ash foliage. Bright, sunny days that are warm, followed by cool, crisp nights that do not reach freezing, develop the boldest displays of yellow in leaves. Warm, wet and cloudy fall weather leads to less intense color, and the trees may, at best, only display chartreuse or yellow-green fall foliage.
  
  

Selection Tip

• Gardeners interested in planting an ash tree with yellow fall foliage should not rely solely on plant label information or other people's testimony about the fall color. Color is subjective. If possible, look at photos of the ash tree species or cultivar to evaluate the yellow foliage quality. Or purchase ash trees when they display their autumn color. This ensures you get an ash tree with the shade of yellow or seasonal timing of the color change that suits your aesthetic.