4 years ago I started a compost pile in my back lot under a canopy of mature sycamore and walnut trees. The area gets 2-4 hours of sun late in the day during the peak of summer. This is the only area in my yard which is available for composting --- a sunnier location is not an option. Neither is cutting down or limbing up any of the trees. Each subsequent year I started a new pile. This spring I started using the compost from the original 4-year-old pile. About a week after I spread some of the compost in one of my flower beds, it was a mass of little weed seedlings.
My questions: 1) Do I need to wait another year or 2 before using the compost because the shade slows down the composting process that much? or 2) Does the shade prevent the compost piles from getting hot enough to kill all the seeds and thus rendering all of my compost useless?
Shade has nothing to do with it. Hot composting happens when you add products high in nitrogen such as fruit scraps, manure, or even lawn fertilizer (about 1 cup per 6" height).
If you have a compost pile which has gradually decomposed over 4 years, it was likely not a hot composting (but cold composting) as hot composting usually is finished in less than 3 months, not couting the extra few months of curing.
It could very well happen that weed seeds ended up in the pile from the last 4 years of being stored.
Compost is not useless because it contain weed seeds. Usually, weeds at that young age can easily be hand weeded. I do recommend, however, that you do not compost weeds if you have those problems. You can also cover your compost piles to limit the amount of weed seeds flying in.
I would suggest that you check out http://www.mastercomposter.com. This website has a lot of good information on composting. Composting in the shade should not be a problem as the heat generated in a compost pile is caused by the action of the bacteria feeding on the carbon/nitrogen in your pile. The above website will explain the process completely.
YardenMan...yep...sounds like you are "cold composting", which is just fine! No worries there! Although a hot compost (if it gets hot enough) will kill some weed seeds unless you monitor it and turn it to keep it working up heat you will never kill all the weeds. Best thing to do is not put anything in there that has weed seeds to begin with.
As for your location of the compost piles/bins...shade doesn't matter. If you are putting in the proper ratio of ingredients it'll cook!
Another note...you mentioned it was under some sycamore and walnut trees. It would be best to cover your piles so that the walnut leaves do not fall into it. They are well-known to have an inhibitor in them (juglone) that will halt/inhibit growth of certain plants. I'd suggest you not add them to your compost pile unless you have an active HOT pile (the heat will de-toxify the juglone in the walnut leaves).
I just read some interesting things about wormwood (artemesia), in terms of its chemical properties. If you soak it and make a tea from the plant, you basically have an organic bug repellent.
I had recently cut a huge amount back and when they didn't root easily (and I had way more than I really knew what to do with), I decided to just compost the cuttings. I worked the pieces/parts into the soil in my dark corner of death, which I intend to abandon for a season while the soil amends itself.
Now I'm concerned that the chemicals in the plant are not good for the soil in general: the main concern being wormwood's insect repellent properties. Will this keep beneficial worms and critters from getting in there and doing their work to break down all this organic matter? Should I go back in there and dig up and remove all the wormwood from this composted area?