Thanks for sending in this lawn challenge!
First off, weeds in lawns can be indicators since healthy grass can compete with many weeds in its (the grass’s) preferred conditions. And the extension recommends organic practices in home lawns since many weed & pest killers can be both ineffective, as you found with Roundup, costly, and have unintended side effects. https://extension.umd.edu/resource/organic-lawn-care/
Sun exposure or Shady moist conditions? 

Wild violets could indicate shady, moist or other conditions that impede the grass, rather than the violets competing with the grass. For example, wild violets can thrive in areas too shady for grass, and/or areas under trees, although shade might not be a factor in your lawn. 

Creeping charlie can indicate poor drainage and/or shade.

Cutting lawn short? Keeping the grass blades short keeps the roots short too, leaving the grass less resistant to weed competition. 

Grass type, there are many grasses, maybe another grass/blend of grsses would do better on your site.

Soil, wild violets are natives that can flourish on a wide range of soils, but grasses have different needs. Dropping off a soil sample to be tested could determine the makeup of your soil and how to amend it.

Meanwhile, overseeding with clover, which used to be part of grass seed mixes, can fill in any gaps created by pulling out Creeping Charlie, while gradually making more nitrogen available in a slow, steady and safe manner.

We would really like to help you with your lawn! Could you send some photos since we are remote? Photos that could help us see where it is sunny or shady? Where the wild violets and Creeping Charlie are growing? Is your neighbor’s lawn shady? 
And would you like information on submitting a soil sample?

Looking forward to helping with your lawn!

KatieMacDC, Extension volunteer team

Thank you for this very informative email. I have creeping Charlie in both shady and full sun areas , the latter has sections of weak grass (even though fertilized in the Spring) that with the rains became yellow and balding. I dug out almost all of the many wild violets which was quite a job and many but not all were in the shadier areas.  I would be interested in recommendations for other types of grass seed; we have used tall fescue to date.  I will take photos this weekend.  Where would I take a soil sample?  

UDC’s Soil Testing Lab is in Bldg 42, Rm 110. 
website: https://www.udc.edu/causes/eqtl/

Suggest emailing or calling ahead to confirm hours during summer break. Regular hours were M-Th, 9-3pm for dropping off samples.

Bldg. 42 is very close to Van Ness Metro, ask the lab for directions.

Campus map: https://www.udc.edu/about/campus-map

Here is a video on taking a soil sample: 

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However, your areas of concern are the areas where the grass is weak & Creeping Charlie thrives, so suggest taking your sample(s) there. Don’t include plants or mulch in your sample & better 1 1/2 cups than 1/2 cup. A plastic bag is OK if you don’t have paper bags the size he had in the video. But do take the sample when you are going to deliver it rather than sitting around. Write your concerns about grass failing to thrive in the memo on the form.

There is a fee for testing service, but you only need the most basic test which costs the least. 

Meanwhile, the abundance of Creeping Charlie instead of grass in sunny areas could be an indicator of compacted soils since CC is said to be able to outcompete grass on compacted soils. A garden fork is useful for loosening soil,  push straight down into soil and pulled back slightly so the tines leave holes. However, plants are better at loosening soil! For example, top dress areas that had Creeping Charlie with compost and seed with clover, for example Durana that the University of Georgia co-developed to handle both heat and cold, per bag directions and  keep moist enough for the clover to establish. Over time, clover can improve soil.

Per Geoff Rinehart, a lecturer in UMD’s Turf department, most lawns should be kept at 3” or so in the summer in our area to handle the heat. https://www.wusa9.com/article/tech/science/environment/lawn-care-survival-heat-wave/65-f7a1e48d-bfc7-4816-a715-0a0e6f94d96f
After seeing this video last year, I walked around our neighborhood during a dry hot spell and all the lawns mowed by various mow & blow services were cut much shorter than Rinehart’s recommendations & scalped on the hills. Most of their lawns were brown, and most had bare patches. In addition, the mow & blows on our block mowed on Thursdays, even if wet, rather than when the grass needed cutting.
The one exception was mowed by a landscape garden service, was cut at over 3” and was lush. To be fair, the homeowner did water with a sprinkler twice a week.  

For more lawn tips: https://extension.umd.edu/resources/yard-garden/lawns/lawn-care-and-maintenance/

Will look for the photos!

KatieMacDC Extension volunteer team