How to Use the Growing Degree Day Slide Rule

The slide rule consists of two rules, a stationary scale on the top of the slide rule, and a sliding or moveable scale on the bottom.  The stationary scale represents the indicator plants and weed information.  The sliding scale represents the crop plant information.  Both scales are in units of 1 to 800 Growing Degree Days (GDD).

The value of zero (0) on the crop plant (sliding scale) represents the seeding date of the crop plant.  To move the crop plant scale, simply click and drag the scale to the right or left.

Event information for indicator plants and crop plants are viewed by touching the black information circles on the scales with your cursor.

1. Move the zero (0) value (seeding date) of the crop plant scale next to the event (or GDD if known) you observe on the indicator plant scale.  For example, if you are seeding wheat when the dandelions are first flowering, place the zero at 260 GDD.
    
2. Use future indicator plant events on the fixed scale to correlate to future crop development events on the moving scale.  For example, you can expect that just after you see the oaks greening (375 GDD), your wheat crop will be emerging (385 GDD).
    
3. Use future indicator plant events on the fixed scale to correlate to weed emergence events.  For example, when your wheat emerges, you can expect that 30% of the wild oats had also emerged by that time and 20% of lambsquarters (under no-till).  Use this information to decide whether to use a pre-emergent herbicide or wait until more weeds have emerged and use a post-emergent herbicide..

Learn how indicator plants can determine crop development.