Growth Stages, GDD, and Photoperiod
This chapter introduces several terms that confuse beginners very quickly:
- growth stage
- phenology
- GDD
- thermal time
- photoperiod
- photothermal day
They are closely related, but they are not the same thing.
Growth stages
A crop does not behave the same way from planting to harvest.
It passes through recognizable developmental phases such as:
- sowing or planting
- emergence
- vegetative growth
- flowering
- seed fill
- maturity
These phases are called growth stages or phenological stages.
The exact names vary by crop and model family, but the basic idea is always the same: stage affects what the plant can do and how the model treats it.
Phenology
Phenology is the study or simulation of when these developmental stages occur.
In crop models, phenology is critical because it controls:
- how long the crop remains vegetative
- when it begins reproduction
- when partitioning rules change
- when final harvest components can accumulate
If phenology is wrong, many later variables can also be wrong even if the rest of the model is reasonable.
What is GDD?
GDD means Growing Degree Days.
It is a simple way to express how much useful warmth a crop experienced.
The broad idea is:
- crops develop faster on warmer days
- but only within a biologically meaningful range
A simple textbook version looks like:
GDD = mean daily temperature - base temperature
with rules to prevent unrealistic values when temperatures are too low or too high.
This is only the basic concept. Real crop models may use more detailed thermal response functions than this one-line formula.
Why GDD matters
GDD helps explain why calendar days alone are not enough.
Two crops planted on the same date can accumulate very different development if one season is cool and the other is warm.
That affects:
- emergence timing
- flowering timing
- maturity timing
Thermal time versus GDD
People sometimes use these terms loosely, but thermal time is the broader
concept.
GDD is one common way of expressing thermal time.
In practice, when modelers say:
- "the crop needs more thermal time to flower"
they often mean the model needs more accumulated temperature-driven progress before stage transition.
Photoperiod
Photoperiod means daylength.
Some crops are sensitive to how long the day is, not only how warm it is.
For those crops, development may slow or accelerate depending on whether the day is longer or shorter than the crop prefers.
This matters strongly in hemp because flowering behavior is often tied to daylength response.
Photothermal day
Some model formulations combine temperature and photoperiod into a single idea,
often called a photothermal day or something similar.
The exact equation depends on the model family, but the concept is:
- development is not driven by temperature alone
- temperature and daylength together shape progress toward flowering or later stages
This is especially important when comparing:
- different planting dates
- different latitudes
- different cultivars with different photoperiod sensitivity
Why stage variables matter in outputs
When you see variables related to stages in DSSAT output, they are often trying to summarize this development logic.
Even if you are mainly interested in biomass, stage-related outputs help answer:
- Did the model flower too early?
- Did it remain vegetative too long?
- Did reproductive development begin at the right time?
Those questions are often the key to understanding yield or biomass mismatches.
A practical way to think about calibration
For a beginner, a useful rule is:
- first fix timing
- then fix size
- then fix partitioning
In other words:
- get stage transitions roughly right
- get total growth roughly right
- get organ-specific biomass roughly right
Trying to fit stem or seed biomass while flowering timing is still wrong often leads to misleading parameter changes.
Common beginner confusion
"The model has the right harvest date, so phenology must be fine"
Not necessarily.
It may still have the wrong:
- emergence date
- flowering date
- duration of vegetative growth
- duration of reproductive growth
"More GDD always means better growth"
Not necessarily.
Too much heat, stress, or an unfavorable daylength response can still reduce performance even if thermal accumulation is high.
"Photoperiod only matters at flowering"
Often flowering is the most visible effect, but photoperiod sensitivity can influence the whole developmental pathway around the transition to reproduction.
Beginner takeaway
If you remember only one thing from this chapter, let it be this:
calendar date is not enough.
Crop models care deeply about how temperature and daylength accumulate through time, because that is how they decide when the crop changes from one stage to the next.