Hemlady,
To reduce the violets I would cover them with five (or ten) sheets of paper and place a shallow layer of grass clippings over the paper to hide it (or other type of mulch). The paper prevents light from reaching the violets and they will die. The layers of paper allow water and fertilizer through and along with the grass clippings also break down quite quickly and help fertilize. Depending on your climate the paper would need to be replaced more than once a growing season.
Weatherunderground gives Metro Detroit as the closest nearby weather station. If you are in a very different microclimate then the next part will be off.
In a comparison over three recent years that I chose (hopefully randomly) your weather seems more or less like that at Springwood with very slighly less cold in the winter and very slighlty more warmth in the summer. There is nothing obvious in the weather that would explain rebloom at Springwood but not at Melvindale for the same cultivars.
Seeds require substantial resources and resources that the plant puts into making seed and making the pod are not put into growth (or storage for future years). Plants also remove resources from dying flowers to put into growth (or storage) so deadheading flowers too quickly will remove some resources (a little) that the plant could have reused. The less resources a plant has then the less potential for rebloom.
Fetilizing:
Usually the mineral in short supply is nitrogen. Usually that is what makes a plant grow quickly and large. When I fertilize my daylilies for rebloom I use 24-8-16 and other high nitrogen fertilizers. I also use urea which is a form of nitrogen. I mulch (paper plus grass clippings) so that the water does not evaporate too quickly and that also helps maintain an even level of soil moisture. A basic rule for most plant species is that the larger the plant the more bloom - for daylilies that does not just mean the larger the plant the more fans the more scapes but also the larger each fan (and its crown) the more buds on its scape. I did check that many years ago by measuring the width of fans at the soil surface, counting the number of leaves. measuring the height of the scape and counting the number of buds. I did this for two or three cultivars. In each case, the wider fans had more leaves, taller scapes and more buds. These were single fan crowns so the width of the fan at the soil suface was an approximate measure of the size of the crown. Last year I did a check of rebloom and the width of fans at the soil surface for one cultivar. The fans that rebloomed had wider fans (and presumably larger crowns). In this case the smaller fans did not bloom at all, the intermediate fans tended to bloom only once and the larger fans tended to rebloom once. Like most relationships in living organisms these types of rules are general, not exact, so one can always find exceptions to the rule. The important result was that when tested statistically the relationship was significant - that is, strong enough to consider real and not a fluke.
When I bought my first daylilies from a daylily nursery I was told that Munson considered that daylily growth suffered if they were not divided every year. I was told that in Florida increase could be 8 to 1 in a year. I have never checked for growth rates in different size clumps but the larger the clump the more self-competition so dividing clumps when they are larger than a dozen fans might help rebloom.
Apps hybridizes (or hybridized) for long periods of bloom and rebloom. In the Daylily Journal number 46, summer 1991 he stated "I really think nitrogen fertilization is very important. I've heard people say let's not overfeed daylilies; that's baloney. Get in there and put the nitrogen on, and get the increase, make sure the water is available. If your other elements are adequate, nitrogen fertilizers will give you the best rewards for the money invested."
In the same article he indicated that he used all-nitrogen fertilizers, ammonium nitrate and urea (with care as they can burn plants). He stated "I guess I would like to warn all growers that complete fertilizers like 10-10-10, used every year for six or eight years, may get you into trouble. You're going to have too much phosphorus after awhile and that will tie up other elements."
Crooks Henley wrote about fertilizing daylilies in the spring 1999 Daylily Journal. In that article she stated (from scientific research on fertilizers for daylilies) "I calculated that if I applied a 27-3-3 at the rates recommended on the bags, I would be applying about 100 pounds of N per year. Most people think of lawns as needing a lot nitrogen. But the daylilies benefited from two and a half times as much! So much for the old adage that daylilies should not be given very much nitrogen. Perry and Adam concluded: 'This genus of herbaceous perennials obviously responds to high levels of ... nitrogen fertility.'"
It is always possible to overdo fertilizing. All fertilizers have an optimum level. Above that level the plant's growth suffers even if it is not obviously 'burned'. With that qualifier, by giving daylilies that have never rebloomed for me (over more than ten years) fertilizer (high nitrogen - including simply using high nitrogen lawn fertilizer at times), water, mulch, etc., I have pushed the growth of the plants and they have rebloomed (the next year). There will be cultivars that rebloom in Florida that one cannot get to rebloom elsewhere (many locations in Florida have 8000 growing degree days [gdd] per year). Springwood over three years had 3385, 3008, 2489 gdd. Metro Detroit over the same three years had 3580, 3250, 2729 gdd. The same cultivar, treated the same way should rebloom the same way in both locations.
