Do I cut the entire stem and others still bloom?
You can cut down the flower stem as far as you like, but make sure to leave the foliage as the plant needs this to store energy for next year's flower. They often do not rebloom on the same stalk; however, cutting the old flowers stalks will sometimes encourage new flower stalks to grow.
I have some canna lilies that my friend gave me. This is their first year and the taller ones are starting to yellow at the edges of their leaves. Is it their time to die back? It is late August in Georgia. The smaller ones need water, like every three days, or they wilt. But I never water the taller ones. They get a little from the sprinklers.
Actually, this is common behavior for cannas. They will normally dieback around summer, especially when it's been hot, but may return in fall go dormant again over winter and return in spring.
Why are my canna lily leaves turning brown?
Canna lilies often develop brown edges when conditions are less humid than they are used to. Another reason may be salt burn. Salts in the water and in fertilizer can build up over time, accumulating along leaf edgesand causing leaf drying browning. It's important to water deeply and slowly. At least once a month, water deeply enough to "leach" or push salts well below the root zone. Also, if they are receiving too much direct sunlight, foliage can turn brown. Finally, canna leaves may turn yellow/brown as it enters dormancy.
Seed gathering seems to come natural to me. . . . so I have collected seeds from my canna lilies. Is it worth the effort to germinate them in spring? Or should I just drill a hole through them and consider them 'beads'? All humor aside, it appears they would have to be soaked and nicked prior to planting.
You can plant the seeds but it will take a while for the plants to grow or bloom using this method. It is easier to simply divide the plant rhizomes.
If a canna lily is planted outside in the ground, will it survive the cold winter? Can canna lilies be left in the ground year round in northeast South Dakota?
No, canna needs to be dug up and stored for the winter. They cannot survive the cold where you are, even if mulched. Here is information on that: https://www.gardeningknowhow.com/ornamental/bulbs/canna/storing-canna-bulbs.htm
I live in Florida and something is eating the leaves on my canna lilies. I can't ever see anything when I look, but something's eating them. What can I put on them to kill whatever it is? Thank you.
It could be slugs, which feed at night and would explain why you don't see them. This article will help: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm
It may also be rodents, which are also nocturnal. This article will help you with that:
https://www.gardeningknowhow.com/plant-problems/pests/animals/garden-mouse-control.htm
I have just purchased a dozen dwarf Canna plants. Do I have to necessarily grow them in pots? I am scared that in summer, pots may dry out. How tall will they grow if planted on the ground?
In the proper conditions (warm, tropical-like weather), they can be successfully grown outdoors. This article should help: https://www.gardeningknowhow.com/ornamental/bulbs/canna/canna-lily-care.htm