Ask Mr. Smarty Plants

You certainly chose the right time of year to plant wildflowers in your part of the world; unfortunately, the other issues you mentioned about removing weeds, especially thistles, are going to take about a year to bring about. If you have already sowed the seeds, hopefully they were not too expensive and you can consider it an experiment. 

To begin, we want to suggest you read a couple of our How-To Articles: Meadow Gardening  and A Guide to Native Plant Landscaping.  In the second article, pay particular attention to the section on "Soil Preparation" as it addresses your questions about pre-planting work that needs to be done. 

Next, let's talk about the seeds. If you purchased them locally or ordered them from a supplier that provides seeds specific to an area, that's good. Unfortunately, there are a lot of offers for a "wildflower wonderland" that are composed of seeds that are no longer viable, not native to where they will be planted, and possibly will produce undesirable plants that turn out to be invasive weeds where you live. There are probably seed suppliers in your area that sell the kind of seeds you need, but if not, you might try some online suppliers, like Wildseed Farms. This links you to a page where you can select your part of the country, select mixes, with the names of the plants included, and order online. It is important to plant wildflowers (or any other plant) that are native to your area, as they will already be adapted to your climate and conditions, and will need less water, fertilizer and maintenance. In response to your question about putting down cardboard with a thin layer of soil, and throwing seeds on top; again, this is a matter of experiment. If the wildflower seeds can sprout there, so can weed seeds, but it's worth a try.

Now, on to your more specific questions about ridding your site of thistles.  In our Native Plant Database, we found Malacothrix sonchoides (sowthistle desertdandelion). If you follow that plant link, you will find our page on it, with some pictures; however, when we checked on that in the USDA Plant Profile, it did not show that particular plant growing in Missouri.  There are so many common names for every plant, we thought we'd look a little further and see if we could find the specific plants you are dealing with.

Sow thistle: This website on Sonchus arvensis from the Minnesota Department of Natural Resources probably refers to the plant you have.  It points out that this is a perennial, and that it can reproduce both by seed and vegetatively, which means that a new plant can re-grow from just a little piece of root. Talk about double whammy! They make suggestions for getting rid of it, but the bottom line is it needs to be pulled out as soon as it sprouts, and/or cut down before it goes to seed. The problem is that the seeds can lie in the soil for years and resprout, and birds, wind and animals will bring more seeds from now on. Be patient, just keep pulling it out. And remember, get root and all, because it can resprout from a leftover root piece.

Canadian thistle: Who knows how this plant got its name? It is not a North American native, but an invasive weed native to Europe and Asia. From Oregon State University, we found this site on Cirsium arvense, which indicates that it is, under state weed control laws in at least 16 states including Missouri, a noxious weed. Some controls are suggested, including plowing the area several times a season and some chemical controls. The Lady Bird Johnson Wildflower Center recommends neither for nor against the use of herbicides, but urges that any application be done carefully and according to instructions on the product. 

Now, on the bright side, let us suggest some wildflowers native to Missouri that you should be able to enjoy, after you have rid your area of the villains. We are going to go to our Recommended Species section, click on Missouri on the map, and NARROW YOUR SEARCH by specifying "herbs" (herbaceous plants) under the dropdown menu for Habit. You can do the same thing, specifying also the amount of sunlight available, soil moisture, even color and time of bloom. If you would like to have some local sources, go to our Native Plant Suppliers site, type your town and state in the "Enter Search Location" box and you will get a list of native plant nurseries, seed suppliers and landscape consultants in your general area.

Asclepias tuberosa (butterfly milkweed) - perennial, sun to part shade, blooms orange, yellow May to September

Baptisia alba (white wild indigo) - perennial, part shade, blooms white June and July

Campanulastrum americanum (American bellflower) - annual, part shade, blooms blue, purple June to August

Coreopsis lanceolata (lanceleaf tickseed) - perennial, sun to shade, blooms yellow April to June

Echinacea purpurea (eastern purple coneflower) - perennial, sun to part shade, blooms purple April to September

Geranium maculatum (spotted geranium) -perennial, sun to part shade, pink, April to July

Lobelia siphilitica (great blue lobelia) - perennial, sun to shade, blooms blue July to October

Monarda fistulosa (wild bergamot) - perennial, sun, part shade, blooms white, pink, purple May to September

Penstemon digitalis (talus slope penstemon) - perennial, sun to shade, blooms white June and July

Ratibida columnifera (upright prairie coneflower) - perennial, sun, blooms orange, yellow, brown from May to October

Symphyotrichum ericoides (white heath aster) - perennial, sun to part shade, blooms white, pink, yellow, blue August to October

Viola pedata (birdfoot violet) - perennial, sun to shade, blooms blue, purple March to June

Asclepias tuberosa

Baptisia alba

Campanulastrum americanum

Coreopsis lanceolata

Echinacea purpurea

Geranium maculatum

Lobelia siphilitica

Monarda fistulosa

Penstemon digitalis

Ratibida columnifera

Symphyotrichum ericoides

Viola pedata