Plants use light as their energy source and are constrained by temperature and weather. This lesson explains how light and climate affect garden plants and introduces the concepts a gardener uses to evaluate them.
Assumed audience
- Reading level: general adult.
- Background: has completed the plant basics lesson and understands that leaves use light for photosynthesis.
- Goal: understand how light, temperature, frost, and climate zones affect what a gardener can grow.
How plants use light
Leaves absorb light and use its energy to convert water and carbon dioxide into sugars. This process — photosynthesis — is the foundation of all plant growth. Without adequate light, a plant cannot produce enough energy to grow, flower, or fruit.
Not all light is equal to a plant. Direct sunlight (unobstructed by buildings, trees, or clouds) provides far more energy per hour than indirect or filtered light. The intensity of sunlight also varies by time of day: morning light is cooler and less intense than afternoon light.
Light categories
Gardening references classify light exposure into three broad categories:
- Full sun: six or more hours of direct sunlight per day. Most fruiting vegetables (tomatoes, peppers, squash) and many flowers require full sun.
- Partial shade: three to six hours of direct sunlight per day, or dappled light through a tree canopy for most of the day. Leafy greens (lettuce, spinach) and some herbs tolerate or prefer partial shade.
- Full shade: fewer than three hours of direct sunlight per day. Few food plants thrive in full shade; some ornamentals and ground covers are adapted to it.
These are generalizations. A seed catalog or growing guide for a specific cultivar will provide more precise requirements.
Temperature and plant growth
Every plant has a temperature range within which it grows:
- Minimum temperature: below this, growth stops. Further below, cells are damaged and the plant may die.
- Optimum temperature: the range in which the plant grows fastest and healthiest.
- Maximum temperature: above this, the plant is stressed. It may stop flowering, drop fruit, or wilt even when watered.
These ranges vary widely by species and cultivar. Cool-season crops (peas, lettuce, broccoli) grow best between 10–20°C (50–68°F) and tolerate light frost. Warm-season crops (tomatoes, peppers, beans) grow best between 20–30°C (68–86°F) and are killed by frost.
What frost is and why it matters
Frost occurs when the air temperature at ground level drops to 0°C (32°F) or below. Water inside plant cells freezes, expands, and ruptures the cell walls. The damage is often fatal to tender plants and to the actively growing parts of hardier ones.
Two frost dates define the boundaries of a location’s growing season:
- Last frost date: the average date in spring after which frost is unlikely. Frost-sensitive plants should not go outdoors before this date.
- First frost date: the average date in autumn when frost is likely to return. The growing season for frost-sensitive plants ends here.
The interval between last frost and first frost is the frost-free growing season. Its length determines which plants can complete their life cycle outdoors in a given location.
Cooperative extension services and regional gardening organizations publish average frost dates for specific areas.
Hardiness zones
The USDA Plant Hardiness Zone Map divides North America into 13 zones based on the average annual minimum winter temperature. Each zone spans a 10°F (5.6°C) range and is subdivided into “a” (colder half) and “b” (warmer half).
A plant’s hardiness zone rating indicates the coldest zone in which it can survive winter outdoors. A perennial rated for zone 5 can survive minimum temperatures of −29°C to −23°C (−20°F to −10°F). An annual does not have a meaningful hardiness rating because it completes its life cycle in one season regardless.
Hardiness zones are useful for choosing perennial plants (fruit trees, berry bushes, perennial herbs) but do not capture everything a gardener needs to know. They say nothing about summer heat, rainfall, humidity, or soil type. They are one piece of the research, not the whole picture.
Microclimates
A microclimate is a small area where conditions differ from the surrounding region. Common causes:
- South-facing walls: absorb sunlight during the day and radiate heat at night, creating a warmer zone. Plants placed against a south-facing wall may survive in a zone colder than their rating.
- Pavement and stone: absorb and hold heat similarly to walls.
- Low-lying areas: cold air is denser than warm air and flows downhill. Depressions and valley floors collect cold air on still nights, creating frost pockets where frost arrives earlier and lingers later than on higher ground nearby.
- Wind exposure: open, exposed sites lose heat faster and dry out more quickly. Sheltered sites (behind a fence, hedge, or building) retain warmth and moisture.
- Bodies of water: large bodies of water moderate nearby temperatures, making adjacent areas warmer in winter and cooler in summer.
A gardener can exploit favorable microclimates (planting tender crops near a south-facing wall) and avoid unfavorable ones (keeping frost-sensitive plants out of low spots).
Seasons and timing
The combination of frost dates, day length, and temperature patterns creates a planting calendar specific to each location. Different plants are started at different times:
- Cool-season crops: planted in early spring (before last frost) or late summer (for fall harvest). They grow in cooler temperatures and can handle light frost.
- Warm-season crops: planted after the last frost date when soil has warmed. They need consistent warmth and are damaged by any frost.
Seed packets and catalogs specify days to maturity: the number of days from planting (or transplanting) to harvest. A gardener compares this number to the length of the local frost-free season to determine whether a crop can be grown outdoors in their area.
Why this matters for later lessons
When researching how to grow a specific plant, a gardener needs to know how much light the plant requires, what temperatures it tolerates, and whether the local growing season is long enough for it. When assessing a site, the gardener must observe light exposure, identify microclimates, and know the local frost dates. This lesson provides the vocabulary and concepts for both tasks.