Howell Mountain AVA, Napa Valley / Zinfandel Grape

Total GDD: 3072 (Nov 15)

Day 246 / 246 (100%)

Current Season

Total GDD: 3072 (Nov 15)

Method: 50°F Sine

7-Day Daily Avg: 5.1

7-Day Projection: 3108 by Nov 22

Season: Mar 15, 25 - Nov 15, 2025

Key Freeze Dates (32°F)

Last Freeze:
10%: 02-24
50%: 03-16
90%: 03-29

First Freeze:
10%: 11-11
50%: 11-30
90%: 12-16

Growing Degree Days (GDD) track heat accumulation to predict when plants reach growth stages, pests emerge, and diseases become active.

Thresholds mark specific GDD values when events typically occur, with crossing dates showing when your season reaches each milestone.

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Thresholds & Notes

Budbreak (EL-4) 100 °D

Apr 9, 2025 | Apr 9, 2024 | Apr 22, 2023 | Mar 27, 2022

Growth Stage

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Apply lime sulfur if delayed dormant spray missed. Check frost protection equipment.

Shoot Emergence (EL-9) 200 °D

Apr 21, 2025 | Apr 21, 2024 | Apr 28, 2023 | Apr 8, 2022

Growth Stage

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Begin suckering passes. First powdery mildew spray if pressure expected.

12-14 Leaves (EL-19) 400 °D

May 15, 2025 | May 13, 2024 | May 21, 2023 | May 14, 2022

Growth Stage

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Shoot thin to 16-20/vine. Apply sulfur + Rally for PM prevention.

Flowering (EL-23) 700 °D

Jun 7, 2025 | Jun 5, 2024 | Jun 21, 2023 | Jun 4, 2022

Growth Stage

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No sprays during bloom. Check soil moisture at 2 feet. Target 65-75% field capacity.

Fruit Set (EL-27) 900 °D

Jun 21, 2025 | Jun 16, 2024 | Jul 3, 2023 | Jun 16, 2022

Growth Stage

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Assess crop load. Target 3-4 tons/acre. Begin weekly petiole sampling.

Bunch Closure (EL-33) 1400 °D

Jul 19, 2025 | Jul 8, 2024 | Jul 27, 2023 | Jul 14, 2022

Growth Stage

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Last botrytis spray if needed. Install bird netting. Check drip emitters.

Veraison (EL-35) 1900 °D

Aug 17, 2025 | Jul 30, 2024 | Aug 21, 2023 | Aug 8, 2022

Growth Stage

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Begin weekly berry sampling (100 berries/block). Reduce irrigation 50%.

Pre-Harvest Zinfandel 2400 °D

Sep 11, 2025 | Aug 25, 2024 | Sep 20, 2023 | Aug 31, 2022

Growth Stage

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Monitor for uneven ripening. Schedule pick crew 7 days advance. Test pH daily.

Harvest - Merlot 2800 °D

Oct 6, 2025 | Sep 13, 2024 | Oct 21, 2023 | Sep 17, 2022

Growth Stage

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Target TA 6-7 g/L. Reserve bins day prior. Confirm winery slot.

Harvest - Cabernet Sauvignon 3200 °D

Oct 5, 2024 | Oct 11, 2022

Growth Stage

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Seed lignification complete. Schedule night pick if temps >85°F. Post-harvest irrigation