When is it time to harvest my mint, and how far should I cut it down? What is the best way for me to dry it?
You can harvest mint any time it is in leaf. You can cut it back to a few inches above the soil. Just leave at least a few leaves on the remaining stem. This article will help you with preserving it:
https://www.gardeningknowhow.com/edible/herbs/hgen/how-to-dry-herbs-various-methods.htm
I harvest mint as I need it. I trim it fairly short before it flowers & goes to seed. It's hard to get rid of if you decide you don't want it anymore. I keep it in a large pot because the roots send runners everywhere & can get out of control quickly.It grows all year. Just cover when there is danger of frost. I usually use it fresh,but if I do dry it I put it in a paper bag and put it in a freezer, inside the trunk of a car, a dehydrater, a low temp convection oven,a regular oven at lowest temp or natural sun.
Some of the leaves on my mint plant are showing signs that something is happening to the middle of the leaves. The leaves are getting super paper thin in the middles and slightly see through and sheer. I don't see any bugs or any sign of a powdery mildew. What could this be?
It sounds like fungal leaf spot. If possible, trim away infected leaves and make sure the leaves stay dry. Normally this issue will clear up on its own if you do this. Keep it away from other plants though as it can spread.
My mint in the container has been growing very well until today when I noticed that the leaves were wilting. Is there anything I can do to bring them back to their perky self?
It is either under watered, or has been over watered and has root rot. Check the soil and the drainage. This article willhelp you determine if it is root rot:
https://www.gardeningknowhow.com/plant-problems/disease/treating-root-rot-gardening-tips-for-housplants.htm
Will a fertilizer that kills weeds kill mint or lemon balm? I just planted some mint and lemon balm. I have fertilizer, but it says that it kills weeds. I know that both those plants behave like a weed, but will that fertilizer kill them?
Herbs like things a little tough. I would not fertilize them at all but that is just my opinion. I leave all mine to struggle a bit for the strongest flavor when used in cooking.
Is mint a perennial plant, or do you have to buy plants each year?
Yes, mint is a perennial. It can also be invasive so if you're considering adding mint to the garden, you should plant it in containers. Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/herbs/mint/how-to-grow-mint-plants-in-your-garden.htm
In warmer zones, at least, they are perennial and will take over the world.
I'm finding out about how to get rid of a terrible invasion of mint plants. I thought it was a little funny that you said there were a few methods that didn't require harmful chemicals, then you said to try Clorox bleach! Isn't that a harmful chemical?
The article you bring up states: "Many people have had luck using Clorox bleach to kill mint." This does not mean you should grab the bleach and start pouring it on. Actually, in a sense, you are right--bleach can be harmful. However, it would take many gallons to make any significant change to the targeted area. Also, since bleach is an effective bacteria killer, it will kill off your soil bacteria, rendering your soil sterile. For this reason, it should not be used for weed control, though many people still prefer to use it. One thing to consider here if you MUST use this type of weed killer is the fact that non-chlorine bleach is much easier on the environment than regular chlorine bleach.
I went for a walk and found a mint plant. I cut a small cutting and I put it in a glass of water so it could grow roots, but now it is starting to die. Please tell me what to do to revive the mint before it is to late.
Make sure that you are changing the water regularly. Dirty water is what most often kills plants rooting in water.
Also, give it a few days to adjust. it may need a little time to adjust to not having roots. If it is a very long cutting, you may want to consider trimming it down. Mint cutting without roots have a hard time supporting more than 6-7 leaves. If you cutting has more than that, you may want to make your cutting smaller.
Increasing the humidity around the cutting will also help. Placing a larger jar or glass over the cutting can do this or creating a tent of plastic as well. Just make sure that the cutting is out of direct light and that it does not touch the covering.