The importance of April, May and June in a growing Season

Budburst

🌱 APRIL: Budburst and Early Shoot Growth

Phenological Stage:

• Buds begin to swell and burst open, revealing the young green shoots.

• This is the official start of the growing season.

Ideal Weather Conditions:

• Temperatures: Daytime highs of 10–15°C (50–59°F).

• Low risk of frost, stable temperatures.

• Moderate rainfall: Encourages early vine growth without saturating soils.

Negative Weather Effects:

• Spring Frosts (late March to late April):

  • Occur when night temperatures drop below freezing after budburst.

  • Damage: Destroys young shoots and buds, causing permanent yield loss.

  • Regions at Risk: Burgundy, Loire Valley, northern Italy, Germany.

  • Management: Wind machines, sprinklers, candles (bougies), and helicopters are used for frost protection.

• Excessive Rainfall:

  • Waterlogged soils reduce oxygen availability to roots, slowing growth.

  • Encourages fungal disease: Phomopsis, Powdery Mildew risk increases.

  • Canopy growth becomes too vegetative (too leafy), reducing energy for fruit.

• Dry, sunny spells:

  • Promote uniform budbreak and shoot growth.

  • Essential for early season vigor and development of inflorescences (future grape clusters).

🌼 MAY: Rapid Vegetative Growth & Flowering (Late May in Warm Regions)

Phenological Stage:

• Vines grow rapidly; inflorescences (clusters of tiny pre-flowers) develop.

• In warmer regions (e.g., southern France, California, Spain), flowering begins late May.

• Flowering is self-pollinated and weather-dependent.

Ideal Weather Conditions:

• Temperatures: 20–25°C (68–77°F).

• Dry, calm weather during flowering.

• Low wind speeds to ensure successful self-pollination.

Negative Weather Effects:

• Cold temperatures (<15°C/59°F): Delay flowering or lead to incomplete fertilization.

• Rain during flowering:

  • Results in coulure: When flowers fail to pollinate and fall off (no fruit forms).

  • Leads to millerandage: Some berries fertilize, others don’t, producing uneven clusters (with small, seedless grapes).

  • Reduces yield and creates inconsistent ripeness, complicating harvest decisions.

• Hailstorms:

  • Especially common in late spring in Burgundy, Beaujolais, and parts of Spain.

  • Can destroy large parts of the canopy and flowers in minutes.

• Excessive heat:

  • Can reduce pollen viability and shorten flowering period.

  • Leads to poor fruit set if extreme (rare in May, but possible in regions like inland California or southern Spain).

🍇 JUNE: Fruit Set & Early Berry Development

Phenological Stage:

• Fruit set occurs: fertilized flowers develop into pea-sized berries.

• Vines shift energy from canopy growth to berry formation.

• Cell division phase of berries begins, critical for potential berry size and yield.

Ideal Weather Conditions:

• Warm, dry weather (~25°C / 77°F), low humidity.

• Consistent sunlight promotes strong fruit set and prevents mildew.

Negative Weather Effects:

• Rain and humidity:

  • Lead to poor fruit set (low yields).

  • Increases pressure from downy and powdery mildew, particularly in dense canopies.

• Heatwaves (>35°C / 95°F):

  • Can cause flower abortion or shatter, particularly in delicate varieties (e.g., Grenache, Merlot).

  • Stress vines and limit proper berry development.

• Hail remains a threat: Though slightly less common than May, isolated storms in early June can still devastate.

• Diseases:

  • Downy mildew (Plasmopara viticola) thrives in humid, wet conditions.

  • Powdery mildew can also begin to establish in dense canopies if weather is warm and cloudy.

🌍 Regional Examples:

Burgundy (France):

• April: Spring frost a major concern (e.g., 2021 frost disaster).

• May: Rain during flowering = high coulure risk in Chardonnay and Pinot Noir.

• June: Hail is historically frequent; vineyards use hail nets or insurance.

Napa Valley (USA):

• April-May: Stable weather, but May heat spikes can cause early flowering.

• June: Risk of heat stress, especially in inland valleys.

Tuscany (Italy):

• April: Budburst occurs mid-month. Late frost is a growing issue due to climate change.

• May–June: Often ideal conditions but rainfall variation affects fruit set.

Mosel (Germany):

• Cooler climate = delayed budburst and flowering.

• Rainy Mays pose mildew threats.

• Poor weather in June often causes millerandage in Riesling.

Summary Table: Detailed Effects by Stage

Stage	Timing	Weather Risks	Impact	Ideal Conditions
Budburst	April	Frost, rain	Yield loss, slowed growth	Mild temps, no frost, some sun
Shoot Growth	April-May	Cool temps, hail	Delayed canopy development	Warm, dry, steady growth
Flowering	Late May-June	Rain, wind, cold, hail	Coulure, millerandage	Dry, warm, stable conditions
Fruit Set	June	Heat, rain, humidity	Uneven berries, disease pressure	Warm, dry, disease-free weather