Phyllochron Leaf Stage & Leaf Number from Heat Units
Stages wheat
A crop puts out a new leaf for every fixed accumulation of heat units, so leaf number = accumulated GDD ÷ phyllochron— enter both to read the leaf stage that guides top-dressing and sprays.
Estimate leaf stage
Next: expect the plant to be at roughly the 7.5-leaf stage; about 40 °D more thermal time will unfurl the next leaf.
The phyllochron is fairly stable for a crop/variety but lengthens under stress, short days, or low light. Use leaf stage (not calendar days) to time herbicide and N applications.
Phyllochron — key facts
- Leaf number
- accumulated GDD ÷ phyllochron
- Phyllochron
- °C-days between leaves
- Cereal range
- ≈ 80–110 °C-days/leaf
- GDD
- daily (mean − base) °C, ≥ 0
- Next leaf
- GDD to the next whole leaf
- Reads as
- Haun-style decimal stage
- Use it for
- timing top-dress & sprays
- Privacy
- Runs in your browser; nothing uploaded
Read the crop's stage by heat, not by the calendar
Crops develop on accumulated heat, not on dates. A new leaf appears for every fixed parcel of growing degree-days — the phyllochron — so a warm spell pushes leaves out faster and a cold one stretches the gap. Divide the °C-days the crop has banked since emergence by the phyllochron and you get its leaf number, a stable measure of where it really is that travels across seasons and sowing dates far better than a count of calendar days.
This tool gives the leaf number, the accumulated GDD, the phyllochron used and the degree-days to the next leaf as soon as you enter them. Use the leaf stage to time top-dressing nitrogen, growth regulators and label-specified sprays, then watch the next-leaf figure to plan the following pass. Pair it with the Growing Degree Days, Germination Time by Temperature and Crop Calendar tools for a full development timeline.
Time top-dressing
Apply nitrogen at the right leaf stage.
Hit spray windows
Match label leaf-stage timing for sprays.
Track by heat
Follow development in °C-days, not dates.
Plan the next pass
See the GDD until the next leaf appears.
Frequently Asked Questions
What is the phyllochron?+
The phyllochron is the amount of thermal time — measured in growing degree-days (°C-days) — between the appearance of successive leaves on a crop's main stem. A crop puts out a new leaf for every fixed accumulation of heat units, so the phyllochron is the heat 'cost' of one leaf. Typical cereal phyllochrons run around 80–110 °C-days per leaf.
How is leaf number calculated?+
Leaf number = accumulated growing degree-days ÷ phyllochron. For example if the crop has accumulated 540 °C-days since emergence and the phyllochron is 90 °C-days per leaf, it is at roughly leaf 6 (540 ÷ 90). The whole-number part is the fully emerged leaf; the fraction shows progress toward the next one.
What are growing degree-days?+
Growing degree-days (GDD) are accumulated daily heat above a base temperature: each day adds (mean temperature − base) °C-days, with negatives counted as zero. They measure development by heat rather than calendar days, which is why a warm spell speeds leaf appearance and a cold one slows it. See the GDD calculator to build the accumulation.
Why use thermal time instead of calendar days?+
Because crops develop on heat, not dates. Two crops sown a week apart can reach the same leaf stage together if it turns warm, while a cold spring stretches the calendar between leaves. Counting in °C-days per leaf gives a stable, transferable measure of where the crop really is in its development.
Why does leaf stage matter for management?+
Leaf stage guides the timing of top-dressing nitrogen, growth regulators, herbicides and fungicides, which are often label-specified by leaf number or growth stage. Hitting the right leaf stage means the input lands when the crop can use it and when the label allows — getting more response and staying compliant.
What is the next-leaf figure?+
It is the additional growing degree-days the crop must accumulate before the next leaf appears — the gap from the current fractional leaf number up to the next whole leaf, times the phyllochron. Combined with the forecast daily GDD, it tells you roughly how many days until the next leaf shows.
How do I find the phyllochron for my crop?+
Use a published value for the crop and variety (cereals often 80–110 °C-days per leaf), or estimate your own by dividing the GDD between two known leaf stages by the number of leaves between them. Daylength and sowing date shift it, so a locally measured phyllochron is most accurate.
Does this relate to the Haun stage?+
Yes — the Haun scale expresses leaf development as a decimal leaf number (e.g. 5.4 = five leaves plus 40% of the sixth), which is exactly what the phyllochron approach produces. The leaf number this tool gives can be read directly as a Haun-style stage for the main stem.
Are the figures precise?+
They are good development estimates. Actual leaf appearance shifts with variety, base temperature choice, daylength, stress and how accurately you measure GDD. Re-check against the crop in the field, update the phyllochron if it drifts, and treat the result as steering for timing rather than an exact prediction.