Similar Climate of Origin
  Lack of Natural Enemies
  Prolific Reproduction
  Seed Dispersal
  Fast, Early Growth
  Novel Weapons

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Fast, Early Growth

Fast, early growth at the beginning of the growing season is advantageous in places like the Great Basin where most of the yearly precipitation comes during the winter. Cold temperatures prevent seeds from germinating and perennial plants from growing. With warmer weather, growth is initiated and the race for water and nutrients begins. Soil moisture becomes a critical factor for survival as the temperature rises, especially with scarce spring rains.

Many weedy invaders in the Great Basin are able to germinate and/or initiate growth at lower temperatures than native plants. This gives the weeds an added advantage, a lead if you will, in the race to grow before soil moisture is depleted. Also, growth is often more rapid. This allows the invasive weeds to mature early, increasing the likelihood of reproductive success.

Image of early spring growth

Annual weeds
Annual weeds, including Russian thistle, yellow starthistle, medusahead, and cheatgrass germinate at low fall and spring temperatures. Seedling growth is rapid and sustainable at low temperatures. They become established quickly, capturing much of the soil moisture, severely limiting the chances of native seedling survival through the hot, dry summer months.

Close-up of a knapweed flowerhead

Perennial weeds
Perennial invasive weeds with fast, early growth are tough contenders for soil moisture and nutrients. The ability of squarrose knapweed to persist on very dry sites with shallow soils is attributed to capturing early spring moisture. Leafy spurge seedlings can appear early, even when temperatures are still freezing. A unique metabolism in Russian knapweed allows growth at cool temperatures. These and other tactics of invasive weeds can tip the delicate balance of resource competition in their favor.

Images:
-Early spring growth: Southwest Regional Gap Analysis Project. Field site database. (http://earth.gis.usu.edu/swgap/trainingsites.html, 26 September 2006).
-Russian knapweed: Eric Coombs, Oregon Department of Agriculture, (http://www.forestryimages.org, 26 September 2006).

Text references:
-University of Nevada Reno. Cooperative Extension. Fact sheet 4-38. Available:(http://www.unce.unr.edu/publications/FS04/FS0438.pdf 27 September 2006).
-Young, J.A., and R.A. Evans. 1978. Population dynamics after wildfires in sagebrush grasslands. Journal of Range Management. Vol. 31 No. 4 pp. 283-289.
-Zouhar, Kris 2001. Acroptilon repens. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: (http://www.fs.fed.us/database/feis/, 26 September 2006).