Photo: Lepidium, Clusters of white flowers in late spring seen amongst blue catmint. 12 inches high.
For a long time I’ve let volunteers grow in my garden to see what they do. Until now, I’ve managed to eradicate anything that I didn’t like or was weedy. Now I have Hoary Cress or Whitetop, a weed that is quite difficult to eradicate by mechanical or biological means. Worse yet its on the very naughty list of noxious weeds. Lets hope this will finally cure me of the – all plants are innocent until proven guilty.
Listed as a Colorado List B Invasive (land owners must remove)
What first appeared in 2016 as a few stalks next to a healthy patch of Lysimichia nummulaira ‘Aurea’ (Creeping Jenny), is now completely overtaking the bed and crowding out the jenny. Years of pulling have only increased its numbers. Here are some pictures of the plants in case you’re tempted to let the pretty white flowers grow in your yard or acreage.
After much review, I may resort to an herbicide for the first time in over 10 years. Because its roots are so extensive its difficult to exhaust. But I intend to sharpen up the hoe, and do my darndest this growing season to weaken it.
Two genus names: Lepidium and Cardaria. Noxious Weeds of Colorado by Colorado Weed Management Association used Lepidium and the common name Hoary Cress. USDA and other classifiy it under Cardaria.
- Brassicaceae – Mustard family
- Flowers April – June
- Seeds: Heart shaped capsules
- Leaves: Alternate, 1/2 – 4 inches long and 1/4-1 1/2 inches wide. Blue-green with small white hairs, oval or oblong. In my case they are arrowhead shaped. The lower leaves form on stalks and the upper leaves are clasping.
- Flowers: Bud formation appears like an italian broccoli – purplish and green tightly clustered head. Flowers are white, 1/4″ wide, with 4 petals in flat-topped clusters at the ends of branches. (4)
All three species of Cardaria are erect perennials up to 2 ft tall. The stems are generally erect and covered with short hairs. The leaves are alternate, gray-green, and variable in shape, some arrowhead shaped. The upper and especially the lower blade surface are covered with short white hairs. The basal leaves are short–stalked, and the upper leaves clasp the stem at their base. Leaves are 0.5 to 4 inches long by 0.1 to 1.5 inches wide. The basal leaves tend to be narrower but longer than stem leaves. All three species reproduce by both seeds and through vegetative means. Numerous small, white, fragrant flowers appear in loose inflorescences in spring to summer. Flowers have four petals 2 to 4 mm long. They produce tiny pods that are heart-shaped to ovate. One plant can produce from 1,200 to 4,800 seeds. Seeds germinate in fall after the first rains. Under field conditions, seeds are short-lived. The three species also reproduce vegetatively, developing new shoots from their extensive system of vertical and horizontal roots. This is the primary method of spread. Within 3 weeks of germination the roots of seedlings can start producing buds. Root fragments also can generate new plants, but regeneration is poor in dry soils. The roots can penetrate deep into the soil, and depths well over 10 ft have been documented in some studies. Roots and rhizomes can account for 75% of the total plant biomass and store considerable amounts of carbohydrates.(1)
Mechanical (pulling, cutting)
Hand–pulling with hoary cress is difficult due its extensive root and rhizome system. Roots can remain alive even when the top–growth has been eliminated for a year.
The WRIC has found that consistent hand hoeing at intervals no longer than 4 weeks for 2 years has been effective. Mowing alone will not control the species as it is very good at storing nutrients when leaves are left. Studies suggest mowing and competitive cropping has been used to control other Cardaria species and may work for hoary cress. (1)
Today I spent about 3 hours hand hoeing a 25 square foot ( 5′ x 5′) patch. One down four to go! This will be repeated at the end of May.
To determine the best chemical to use, you need to know its stage of growth and where its growing.
In my case its in the root zone of my silver maple and interspersed with plants I do not want to harm. So chemicals that act in the soil may be harmful. Non-selective glyphosphate herbicides like Round-Up may harm surrounding plants if spray makes contact but has no soil activity. You also have to catch the plant before it flowers for effective control in some cases. So this is gonna be a toughy!
UC Davis provides good information from field tests of various chemicals. For my purposes the following two are the only options if I want to keep other plants alive.
*** NOTE: The following information is NO subsitute for READING THE LABEL! These are my personal notes I’ve gathered to help inform my decision. The label is the law, and should always be the final source to follow!
- FIRST resort: 2,4-D Growth Regulator, Fall application.
- Timing: Postemergence early in the season before flowering, or to new growth in fall. Control is minimal after the bloom stage.
- Remarks: 2,4-D is broadleaf-selective and safe on grasses. This herbicide will most likely require repeat applications for several years. 2,4-D has little to no soil activity. It is not the most effective treatment, but widely used because of its low cost. 2,4-D is often combined with other active ingredients, e.g. clopyralid or dicamba. Do not apply the ester formulations when outside temperatures exceed 80°F.
- LAST resort: Imazapic (Plateau) Branched chain amino acid inhibitor, Spring/Fall application.
- Timing: Postemergence after blossoms open until plants desiccate. Fall rosettes may also be treated.
- Remarks: Imazapic is safe to apply to most native grasses. Higher rates may suppress seed of some cool-season grasses. (1)
- There is some residual soil penetration but is less with the high pH clay soil I have. (3)
- Cadaria chalepensis draba pubescens – A WEED REPORT from the book Weed Control in Natural Areas in the Western United States , UC Davis Weed Research Information Center, 2013
- Hoary Cress Fact Sheet, Colorado Dept of Agriculture, date unknown
- Imazapic Handbook, Imazapic 7g.7 Weed Control Methods Handbook, The Nature Conservancy, Tu et al. procured from Invasive.org, 2001
- Noxious Weeds of Colorado 14th Edition, Colorado Weed Management Association, 2020
|USDA Results for Scientific Name = lepidium draba