Answer: Growth rates will vary with the climate each year and cultural conditions (conditions and vigor at planting, light, moisture, fertility, wind, soil type, etc.) since plants respond to these in addition to their "pre-programmed" size range. For example, it would be unrealistic to expect a plant growing in poor soil in a windy location in an area repeatedly hit by drought to grow as steadily as a plant in good soil with ample moisture and perfect conditions. This is why mature sizes are usually listed as a range as well.
Typically, according to Michael Dirr's well respected "Manual of Woody Landscape Plants" a moderate growth rate would be between one and two feet per year. Often, I have noticed that younger plants will grow faster than older plants, too.
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