Smallest and Largest Plant Pollinators
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
Angiosperms (flowering plants) and gymnosperms (mostly conifers) produce pollen. The pollen grain is the male gametophyte of plants and is analogous to sperm cells in animals; that is, it is needed for fertilization of the egg cell found in the female gametophyte of these plants. An obvious question is which plant species produce the most pollen? Do large plants produce more than small ones? Do trees produce more than weeds? The following are some generalities that can be made regarding pollen production by plants, but they are just that, generalities.
Trees do not necessarily produce the most pollen just because they are larger than herbaceous plants. Trees that produce large flowers, or even small flowers, but which form pollen that is large and “sticky” and which are insect pollinated will produce less pollen than trees that are wind pollinated or weeds and grasses that are wind pollinated. Trees like birch, elm, alder, ash, oak, mulberry, maple, and hickory are wind pollinated and produce large amounts of pollen that can cause significant problems for people each spring who are allergic to these tree pollens. Additionally, conifers such as cedar, juniper, cypress, and sequoia which are also wind pollinated, produce large amounts of pollen that trigger allergy symptoms in numerous individuals. These plants may start producing pollen in the winter and continue this production into the early spring. Therefore, the first generality that we can make is that wind pollinated plants will produce much more pollen than insect or animal pollinated plants.
Some plants produce perfect flowers (male and female structures in the same flower) while other plants are monoecious (male and female flowers are separate but on the same plant) or dioecious (plants are either male or female regarding the flowers they produce). A grove of male mulberry trees for example would produce a greater quantity of pollen than a mixed grove of male and female mulberry trees. It is not necessarily true that dioecious plants will always produce more pollen than monecious plants and if one considers the grasses you could even say that dioecious plants don’t always produce more pollen than plants having perfect flowers. However, the usual case is that plants with perfect flowers are less likely to be wind pollinated. Thus, a second generality that we can draw is that dioecious and monecious plants are more likely to produce greater amounts of pollen than plants having perfect flowers.
A third generality that we can make about pollen production is that plants that have the largest populations and which are widely distributed will produce greater amounts of pollen than species that are limited in distribution or who remain at low population levels. In general, plants we consider to be weeds are more widely distributed and reach higher population levels than plants we consider useful or aesthetically pleasing. Of course, the definition of a weed is any plant growing where you don’t want it; which could be roses or daffodils if they are in the middle of your vegetable garden. When we think of weeds that are significant pollen producers we typically think of plants such as plantain, ragweed, sagebrush, or lamb’s quarters. The other group of plants that are widespread and are present in large populations in some areas are the grasses, plant family Poaceae. Grasses have perfect flowers but are wind pollinated. They produce pollen grains that are difficult to distinguish from one another; all grass species form pollen grains which are monoporate and have an exine that looks very similar from one species to the next. Grasses such as Johnson grass, Bermuda grass, Kentucky blue grass, fescue, perennial rye grass, Paspalum species, etc. all produce large amounts of pollen that get into the air and become part of the aerospora.
Therefore, the best answer to our question regarding which plants produce the most pollen is, it varies. Method of pollination, types of flowers produced, and population size and distribution all affect the total amount of pollen produced by a given species. It would be safe to say that ragweed produces very large quantities of pollen. This is because ragweed plants are widely distributed, have large population sizes, are dioecious, produce small, nonsticky pollen grains, and are anemophilous.