Pollen Library (Plants That Cause Allergies) - 花粉（能引起過敏的植物）

Pollen allergies affect many. Learn more about what the most common plants that cause pollen allergies and where they are located.

Each year, hundreds of species of plants release their pollen into the air, causing allergic reactions in scores of people in North America. However, a relatively small number of plants are responsible for most of the itching, sneezing, and watery eyes associated with hay fever. In case those afflicted hadn't suffered enough, certain pollens like ragweed can even survive through the winter, playing havoc with some immune systems year-around. All that pollen has created a booming market for antihistamine and decongestant makers, but has left millions of allergy sufferers still begging for relief.

Worst Offenders
Certain plants are worse than others. The following list is the top 10 plant allergens in North America and where they’re located:

ragweed: throughout North America
mountain cedar: Arkansas, Missouri, Oklahoma, and Texas
ryegrass: throughout North America
maple: the seven species of maple can be found throughout North America
elm: species of elm are located throughout most of North America
mulberry: species of Mulberry can be found throughout the United States (although rare in Florida and desert regions of the country)
pecan: Southeastern United States.
oak: several species located throughout North America
pigweed/tumbleweed: everywhere
Arizona cypress: Southwestern United States

Spring Pollen Allergies
Spring is tree allergy season. Certain trees in the South start releasing their pollen as early as January, while others continue their onslaught into summer. Thankfully, only around 100 of the more than 50,000 tree species cause allergies.

Most allergy sufferers are only allergic to one type of tree, although cross-reactions may occur with certain species such as those in the alder, beech, birch, and oak families and between junipers and cedars. Tree pollens are dry and lightweight and therefore can be transported great distances by the wind. Some of the worst tree allergens include:

alder
ash
beech
birch
box elder
cedar
cottonwood
date palm
elm
hickory
juniper
oak
pecan
Phoenix palm
red maple
silver maple
sycamore
walnut
willow

Grass Pollen Allergies
Late spring and summer is when grass allergy season kicks in. There are more than 1,000 species of grass in North America, but only a handful cause serious allergic reactions in humans. Grass allergy sufferers must take extra care when doing yard work—especially when mowing the lawn. If possible, finding someone else to do that particular job or wearing a mask is the best preventative measure.

Grass should be cut short as well or, ideally, replaced with a ground cover such as bunch, dichondra, or Irish moss that doesn't produce much pollen. Grass is easily tracked indoors as well, so vacuuming frequently may also help relieve symptoms. The most common grass allergens include:

Bermuda grass
Johnson grass
Kentucky bluegrass
orchard grass
rye grass
sweet vernal grass
Timothy grass

Weed Pollen Allergies
Late summer and fall is the season for weed allergies, with pollen levels usually peaking in mid-September. Pollen counts for weeds are at their highest in the morning, usually between 5 and 10 a.m. Weed pollens are the most prolific allergens of all. A single ragweed plant, for instance, can produce a billion pollen grains in a single season. Not only that, but wind-carried grains may travel for hundreds of miles. Weeds responsible for the most allergies include:

English plantain
lamb's quarters,
ragweed (which affects nearly one in five Americans)
redroot pigweed
sagebrush
tumbleweed (Russian thistle)

Source: http://www.healthline.com/health/allergies/pollen-library#3