When I worked at the Plant Place in Carrollton during my first two years at UWG (then SUWG), I learned quite a bit about plants throughout the season. I learned early on to encourage people to purchase their seeds for spring and fall because the seed racks would be returned in August for a credit for the unsold seeds. I learned which fertilizers to encourage depending on people's willingness to do work (organic was not as big a trend as it is now). I also learned to leave certain "weed patches" on the property alone, especially Mr. Trawick's pet kudzu. While the boys that worked at the store maintained the property and I worked mainly with the customers, it was made clear to me early on to NEVER touch the patch of kudzu that grew on the corner of the chainlink fence in the back. Mr. Trawick was the only person allowed to prune the plant. Why you ask??? As Mr. Trawick put it, the kudzu was his insurance policy: "When that vine leafs out, I order all my annuals because there won't be anymore frost." Mr. Trawick was practicing phenology.
Phenology is the study of timing of recurring biological phenomena and their relationship to the weather. Sound complicated? Basically, phenology is how farmers farmed before almanacs were published or affordable. They watched nature for signs. I've found these clues to be helpful. The only downfall is if you either don't know what some of the signal plants are or where to find them. Our plot of land in the city is much to small to plant all of the signal plants, but we've been able to find several of the signal plants around town. Phenology is a fairly easily googled term. I've yet to find a good book with any kind of an extensive list in it.
Below you'll find several of the tips I've collected (and, hopefully transcribed accurately) from various websites so far:
Plant peas When forsythia & daffodils bloom, When maple trees flower
Plant potatoes When 1st dandelion blooms/When shadbush flowers
Plant beets/onions/spinach When daffodils are in bloom
Plant carrots/broccoli/cauliflower/leaf veggies When lilac is in first leaf
Plant beans/cukes/squash When lilac is in full bloom/Petals drop from apple trees
Plant tomatoes When lily-of-the-valley are in full bloom/when peonies flower
Transplant eggplant/melons/peppers When irises bloom
Plant corn When apple blossoms start to fall
Plant kidney beans When elm leaves are the size of a penny
Seed fall cabbage/broccoli When catalpas and mock oranges (english dogwoods) are in bloom
Plant cool season flowers (like pansies, snapdragons) When aspen and chokecherry trees leaf out
Additionally, phenology can help you to determine how bad a particular pest might be one season or when they will show up at all. For instance, root maggots will be fewer when dogwood petals drop. Aphids appear soon after black locust blooms. Japanese beetles will arrive when morning glory vines start to climb. Squash vine borer eggs are laid when chicory flowers- moths will lay their eggs for about a two week period during this time. When gnats swarm. rain and warmer weather are on the way. Wasps building nests in exposed places indicate a dry season. When hornets build nest near the ground, a harsh winter is on the way.
I would post links- but there are dozens out there. You'll just have to search to find one that suits you!