This static map image portrays the average date when lilacs start bloom in Idaho. Information on dates when plants and animals reach various stages in their development is referred to as phenological data. The purple common lilac (Syringa vulgaris L.) was chosen as the indicator of plant development in western regional phenological studies because it is well adapted and widely distributed throughout the Western United States. Approximately 160 observers scattered throughout Idaho observed the dates of lilac bloom for the 10 years of data used as a base for this study (from 1957 to 1966). Without the unselfish dedication of these volunteers this study would not have been possible.
Montana Agricultural Experiment Station Bulletin 607, published in 1966, entitled "Pattern of Plant Development in the Western United States" explains the specifics of data collection and establishment of the lilac network. These 5-year average dates of begin bloom, which were slightly modified to represent averages for the 10-year period, 1957-1966, plus the elevation of each station enabled this map of plant development to be constructed.
The minimum elevation of each 6-mile square in the state was found and taken as representative of that particular area. The corresponding isophanal value from the phenological observations was added to the individual elevation constants, resulting in a begin "bloom-day" number for each square. Lines were drawn connecting squares with equal numbers for each 3-day interval until May 30, and every 5-days thereafter. An underlying topographic map was also used as a guide to assist in drawing the isophanes. The latitude and longitude coordinates of your particular location should enable you to determine the normal date when lilacs begin to bloom for your general area. In addition the location of each county seat is noted by a star.
Annual lilac development has been compared to a meter keeping track of the daily multiplication of solar radiation (calories per square centimeter per day) and mean daily temperatures above 31 degrees Fahrenheit. These "Solar Thermal Units" were reported by Caprio in the September issue of "Horticulture", published by the Massachusetts Horticultural Society, to be approximately as follows:
Phase Solar-Thermal Units
First bud leafed 150,000
95% buds leafed 200,000
Opening of first bloom 380,000
Peak of first bloom 520,000
95% flowers withered 660,000
Additional uses are being made of this "barometer" of plant development. Moving the decimal of Solar Thermal Units 5 places to the left converts these units approximately into seasonal inches of water loss. For example, at the time when lilacs begin to bloom, which requires approximately 380,000 STU, about 3.80 inches of water have evaporated/transpired from fields which are well watered and completely covered with vegetation.
Irrigation scheduling, weed and pest control, and timing of planting to provide an orderly flow of produce at harvest are examples of future uses of annual phenological observations.