Calls are coming in about a noxious weed -- tall whitetop or perennial pepperweed (Lepidium latifolium). People are seeing a white-blooming weed and wondering if it is tall whitetop.
The white-headed plant drawing attention now is actually hoary cress or low whitetop, Cardaria draba. Tall whitetop blooms later. The scientific names tell us these are completely different plants, although they are both in the mustard family. Both are on Nevada's noxious weed list, designated by state law NRS 555.010.
Both are perennials, coming back year after year from roots. They also spread by underground runners (rhizomes) and seed. Both can produce 450 or more new shoots per plant in a single growing season from the rhizomes. They are both heavy seeders. Tall whitetop can produce 6 billion seeds per acre. The seeds spread through wind, water, animals, clothing, vehicles, contaminated soils and contaminated crop seeds.
Tilling or pulling is only effective on young plants in small stands. All lateral and vertical roots need to be removed. Pulling and cultivation must be done regularly -- every 10 to 21 days -- until no more seedlings emerge. The site then needs to be checked regularly, year after year, to ensure seedlings don't return.
In this case, weed killers are the answer. They need to be mixed with a nonionic surfactant to spread and stick. Buy the surfactant at nurseries and horticulture supply stores.
To control tall whitetop, 2,4-D can be applied at bud to early bloom stage in multiple applications each year. Hoary cress should be mowed, and all flower heads should be collected, bagged, sealed, and tossed in the garbage. Then allow the plants to grow again to just before they bloom, and spray with 2,4-D.
Don't mow and immediately spray, as the plant has to be actively growing for the herbicide to work. It can be sprayed while in bloom, but effectiveness is greatly reduced and the flowers will still go to seed even as the plant is dying from the herbicide. The seeds will spread and grow new weeds.
After the weeds are under control, plant the area with more desirable plants such as bunch grasses or the undesirable noxious weeds will return. Be vigilant ever after!
There are weed identification posters and control information at http://www.unce.unr.edu/publications/natural.htm. Or call your local University of Nevada Cooperative Extension and ask for information on hoary cress or tall whitetop. You can also "Ask a Master Gardener" by e-mailing mastergardeners@unce.unr.edu.
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension.