Is it OK to use water from the dehumidifier bucket for my houseplants?

The owner’s manual for a dehumidifier says that the water is not safe for drinking, but the manual should, perhaps, include that it is a very good source of water for our houseplants. Rainwater and water from melted snow are usually our best water sources for all of our plants. The water that seems to magically appear in the bucket of the dehumidifier comes from the air—just like the rain and the snow. Sometimes it even goes through a filter before raining into the bucket. Tap water can be very hard, very soft or highly chlorinated, and when used on houseplants can cause a salt buildup that shows up as a white crust on the soil or sides of the pot. Too much salt can change the pH of the soil. Ideally, water should have a neutral pH, be room temperature and, if from the tap, be allowed to sit overnight so that chlorine can dissipate. If you are concerned that the dehumidifier water has accumulated mildew, mold or dirt while sitting in the collection bucket, clean the bucket occasionally with a 1:1 water and white vinegar solution to prevent contaminants from being transferred to your potting soil.

I have self-seeding plants in my garden that I pull out before they go to seed and spread around too abundantly. If I pull them when they are in bloom and put them on the compost pile, will they still go to seed even though the roots are exposed and the plant is dying? I don’t want to end up inadvertently spreading the seeds through the beds when I use the finished compost.

First of all, if you pull these plants when they are still in bloom, they haven’t finished the process of making the seeds. If you miss a few and don’t remove them till after the bloom has long faded, then the seeds have formed. If you maintain a hot compost pile, one that reaches a measurable 140 to 160 degrees, the heat will not only kill many pathogenic organisms, but also self-seeding plants. A hot pile works best if it is made up all at one time and then allowed to compost completely without further additions of material. You don’t say what kind of plants these are, but you might consider deadheading to “trick” the plant into blooming again. Then you can enjoy more blossom time and get the jump on the seed-making process before it begins.

As organic matter (compost, mulch, etc.) is added to the garden, how do the beds not gradually grow higher? Does the organic matter decompose to literally nothing? Or do the particles just become so small that the beds’ height is relatively unaffected?

There are two actions at work. First is that the decomposition process continues after the compost has been added to the beds. A very small percentage of the matter, called humus, is left at the end of the decomposition process. As decades go by with more and more organic matter added to the beds, the soil level would increase.

The second action at work is that plants are being grown in the beds and they consume the humus along with the other elements that comprise the soil. If more organic matter is added to the beds than is consumed by the plants, then very slowly over time the beds will gradually grow higher.