Friends of the Wild Flower Garden

Hoary Alyssum

Six Midwest Invasive Plants You Don't Want in Your Garden

When Summer is here the villains of our gardens will be in full growth. Here are six invasive plants you want to keep out of your garden. Unsurprisingly two are members of the pea family and four of the mustard family - hardy and vigorous plant families - adaptable, fast growing, lots of seeds, and of course - not native. The flowers are so nice looking you are tempted to keep the plants - but if you do - dig them out before they go to seed. Use the links on the plant name to go to a full info sheet with more photos.


Black mustardBlack Mustard - Brassica nigra (L.) W.D.J.Koch

Everything about this plant is large: Stems - to 8 feet high; leaves - to 10 inches long; the inflorescence - up to 2 feet long; yellow flowers - large; seed pods - too numerous to count; seeds - thousands. It has one redeeming virtue - seeds of Black Mustard are used in condiments and for the oil contained in the seeds. The plant is cultivated for that purpose. The plant has an ancient history being recorded by the Greeks and Romans. Later, Parkinson and Culpepper included it in their writings. The seeds themselves do not have an odor. The flowers are pretty also. Seed pods are held tight against the stem.

Fortunately for the gardener, the plant is an annual and must re-seed itself, so if you root it out before seed-set you have a chance of eliminating it.



Yellow RocketYellow Rocket - Barbarea vulgaris R.Br.

Also known as Winter Cress and Cressy-greens Cress. This is a common plant here in Hennepin County, competing for road side space in the shady green ditches with Dames Rocket. Hard to say who’s winning the space battle. This one is a biennial - flowering the second year - so you have a two-year slot of time to eradicate it. The flowers are small but pretty with 4 yellow lobes that flare outward. Like most mustard family species, while you are looking at the nice flowers on top, just below are the developing seed pods of yesterday’s flowers and before long you have a thin, green, branched stalk with linear seed pods sticking out all over on the green stalk.

Interestingly, the genus name, Barbarea, comes from a plant once known as the herb of St. Barbara, Herba sanctae Barbarae. The plant author (R.Br.) is Robert Brown better known as the man for whom ‘Brownian Motion’ in physics is named.



Hoary AlyssumHoary Alyssum - Berteroa incana (L.) DC.

Familiar with Alyssum are you? You know, those cute little white flowered border plants grandma used to plant. Here is the big sister. About 1 to 2+ feet tall on a stiff stem which branches at the top to display a number of round clusters of small white 4-petaled flowers. Those petals are divided by a deep notch so it looks like 8 petals or 4 sets of mouse-ears. The ‘hoary’ part of the name comes from the grayish down that covers the leaves and seed pods.

This one is a perennial with a taproot so work must be done to eliminate it if a patch gets established. To further impress you, if the plant sets early seeds, these can establish flowering plants the same season! The plant has no biological control, so pulling, burning or as a last recourse - glyphosate - will get rid of it. We can thank our European ancestors for bringing this one across the pond. Yank it out - it didn’t make the MN Restricted Weed list for no reason.



Field PennycressField Pennycress - Thlaspi arvense L.

Also known as ‘stinkweed’. This, the last of my four mustard family plants, also has white flowers - tiny white flowers - only 1/8 inch wide, in rounded clusters atop the stems. Like other mustard family members, while you are admiring the nice flowers, the seed pods of yesterday's flowers are forming below - except they are not linear seed tubes - they are flat and round, 2 stuck together, with a wide encircling wing for the wind to catch, and then attached upright to the end a long stalk so they look like a bicyclist giving a right-turn signal.

This plant is an annual, so your chances of eliminating it are good unless your neighbor has some and lets the wind move the seeds over to you. That name ‘Stinkweed’ comes from the odor of crushed foliage - some say it is garlic-like.

This is another import from Eurasia, first found in Minnesota in 1883. Chemicals in the plant will cause tainted milk in cows but they usually avoid it. That tongue-twisting genus name, Thlaspi, comes from a Greek word thalo which means ‘to compress’ referring to the flattened seeds. It is listed as a Restricted Species in Minnesota.



Now to the pea family


Birdsfoot TrefoilBirdsfoot Trefoil - Lotus corniculatus L.

You must have seen this plant somewhere along path edges where you walk - it seems to be everywhere. Lovely yellow pea-type flowers with bright red veins to guide insects to the nectar. Flowers are grouped in a whorl atop a stem rising above the leaves.

When it goes to seed it produces linear brown pods, with a small beak on the tip, that project outward from the flower cluster and look like bird's feet, hence the name. The pods are about 1" long and split in two when ripe. The upper part of the leaf has three leaflets resembling a trefoil.

This plant arrived in North America not by the usually accidental means but was purposely brought in for agricultural forage. It does not cause bloat in cattle and in dry pasture land it grows better than grass. Of course, when it escaped, that ‘growing better than grass in dry places’ virtue became today's curse and thus it takes its place on the Minnesota noxious weed list.



Crown VetchCrown Vetch - Coronilla varia L.

Our final pest is one you have all seen if you drive the freeways in Minnesota. it was for years used by the Minnesota Dept. of Transportation for erosion control on roadside cuts. One can now see how these plants have formed dense mats and obliterated anything in their way as they spread. The seeds have hopped the roadsides and now appear in numerous unintended areas. It is considered to be a serious threat to prairie and dune areas and creates a mono-culture whereever it takes hold. It was originally brought to North America from the Mediterranean area for animal fodder and for erosion control. Now, you need to go to Alaska or Nunavut to not find it.

But it is pretty - those whorls of pinkish-white pea-type flowers - keeping all those bees happy. Unlike Birdsfoot Trefoil, Crown Vetch has stems that can grow out to six feet as it trails along on top of what ever is in it’s way.

The flowers produce linear brown seedpods containing many seeds that remain viable in the ground for up to 15 years. Plus - its a perennial and can spread via the rhizomes in the ground. MNDOT liked it because it in nitrogen producing and stabilizes the soil. Hopefully they have stopped using it now that it is listed by the DNR as an invasive weed.


Well that’s a handful-plus-one of invasives and I have not even mentioned Garlic Mustard, Black Bindweed and Leafy Spurge.


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