Oleacea – Fraxinus sp.
Ash Trees

By: H. Wayne Shew, Ph.D.

NAB Certified Pollen Counter

Alabama Allergy & Asthma Center Collection Station – Birmingham, Alabama

If you have been reading my recent blogs, you are aware that I am writing a series on species of plants that are principally wind pollinated and which produce pollen grains that act as allergens. This blog will focus on one of the genera that belongs to the olive family of plants, a group of plants which are known to produce pollen that is often a significant cause of hay fever and asthma. This blog will deal with the genus Fraxinus, a previous one dealt with Olea europaea and a subsequent one will consider the genus Ligustrum, privet.

Oleaceae (olive family) is a family of plants native to temperate and tropical regions of the northern hemisphere. There are 25 genera and some 700 species in this family. The best-known members of this family are olive, ash, lilac, privet, Jasminum and Forsythia. The members of this family are trees, shrubs, and climbers, some of which are important economically. The olive (Olea europaea) is important for the fruit it produces which is the source of olive oil. The ashes (Fraxinus) are valuable as a source of lumber and because of the toughness of the wood is often used for manufacture of tool handles and baseball bats. Ash is also an excellent source of firewood. The shrubs Forsythia, lilac, privet, fringe tree, and jasmine are valued for use as ornamentals in gardens and landscaping.

Ash trees are popular as shade trees because of their deliquescent growth habit and thick foliage. White ash for example, may grow to 90 feet in height and have a leaf canopy that stretches up to 70 feet wide. Green ash trees are typically shorter, some 60 feet in height, but also have broad leaf canopies and therefore are well suited for use in residential settings and as street trees. The green ash was widely used as a replacement street tree for the American elm that was devastated by Dutch elm disease.

Ash trees are found throughout the eastern and southeastern areas of the United States. The trees have compound leaves (each leaf has 5-9 leaflets) and an opposite leaf arrangement. Not many trees show opposite branching (in our area the most frequent are maples, dogwoods, and ashes) which makes this tree easy to identify to genus. The trees are found in areas of full sunlight and well-drained soil throughout their range. Some species get quite large and are resistant to many types of insects and diseases; however, there are growing concerns that the emerald ash borer which was recently introduced from Asia may do major damage to the ash trees of North America. Most ash trees bear either male or female flowers on a single tree. The flowers are greenish yellow to greenish purple and appear in the early spring prior to full leaf development. Male flowers are borne in clusters on second year branches and produce enormous amounts of pollen. Female flowers are borne in panicles and after fertilization the flowers will produce winged fruits (samaras) later in the spring.

The green ash is a very common species and can grow in a wide variety of soil types and conditions. In fact, it is popular as a tree for use in urban areas because of its resistance to air pollution and salt. This species, F. pennsylvanica is also known as red ash, swamp ash, and water ash. The bark appears gray and is smooth when the tree is young but becomes thick and rough as the tree gets older.

Some interesting facts and popular uses of ashes include:

  1. The oil of the tree, which is similar in its chemical composition to olive oil, has been used medicinally to alleviate stomach ailments.
  2. There are reports of the use of ash bark in treating warts in England.
  3. The wood can be used to smoke meat.
  4. The wood is widely used in the construction of baseball bats, tool handles, hockey sticks, and canoe paddles.
  5. Ash is a hard, durable wood that is excellent for furniture and boat construction.
  6. Early aviators used ash in the construction of airplanes.
  7. The wood was used in the construction of carriages in the 19th century and one British company still uses ash in the manufacture of sports car frames.

The pollen grains produced by ashes are prolate (somewhat elongate with a polar diameter that is greater than the equatorial diameter) to spheroidal in shape and have 3-6 colpi. The wall of the pollen grain (the exine) is described as reticulate (net-like) and the grains are 18-28 microns in size. Fraxinus pollen acts as an allergen for many individuals.

Green ash pollen – five colpi

Pollen-polar view left; equatorial view right

Green ash

Green ash leaves