When to Begin Fall Succession Plantings in Idaho Greenhouses

Understanding when to begin fall succession plantings in Idaho greenhouses is a matter of timing, crop selection, and environmental control. This article provides a practical, region-specific guide that walks through climate considerations, crop timelines, greenhouse adjustments, pest and harvest considerations, and clear action steps you can apply whether you manage a small high tunnel or a commercial greenhouse operation in Idaho.

Understanding Idaho’s climate and the greenhouse growing window

Idaho is large and climatically diverse. Elevation and latitude produce growing conditions that range from cool, wet Panhandle summers to hot, dry southern valleys and cold, high-elevation plains. In practical terms, this means the window for successful fall crops differs across the state.

• Northern Idaho (Panhandle, Coeur d’Alene area): USDA zones roughly 4-6. Cool, shorter growing season; first hard frosts often in October.
• Central Idaho and Boise Valley: Zones 6-7 in lower elevations. Longer warm season but cool nights arrive earlier in fall; first frosts often late October to early November.
• Eastern Idaho and higher elevations: Zones 3-5. Short growing season with early fall freezes; plan for earlier plantings and frost protection.

A greenhouse gives you the ability to extend the season by weeks to months. However, the extent depends on insulation, supplemental heat, thermal mass, and ability to control humidity and light. Rather than a single calendar date, plan fall succession based on desired harvest dates, crop days-to-maturity, and the specific climate of your site.

What is succession planting and why it matters in fall

Succession planting is the practice of staggering sowing or transplanting intervals to produce a continuous harvest. In fall, succession planting smooths workload, spreads market supply, and buffers losses from weather or pests. Key principles:

• Work backwards from desired harvest dates using crop-specific days-to-harvest adjusted for cooler fall temperatures.
• Shorten planting intervals for quick crops (e.g., radish, microgreens) and lengthen intervals for slow crops (e.g., winter lettuces, storage roots).
• Use overlapping plantings so different cohorts mature sequentially.

Succession is particularly important in Idaho fall because daylength and temperature decline rapidly. Failing to stagger plantings can leave you with a single short harvest peak followed by a shortage.

Planning your greenhouse schedule: timing and calculations

Start with three pieces of information: desired harvest date, variety days-to-maturity (DTM) under optimal conditions, and expected greenhouse temperature regime in fall. A simple planning formula:

1. Desired harvest date minus adjusted days-to-maturity equals sow/transplant date.
2. Adjust DTM upward by 10-30% to account for cooler fall average temperatures if you lack supplemental heat or supplemental lighting.

Examples of temperature adjustments:

• If daytime greenhouse temperatures will average 65-70degF and nights 45-50degF, add roughly 10% to DTM.
• If you plan minimal supplemental heat and nights dip below 40degF, add 20-30% to DTM or choose more cold-hardy varieties.

Practical calendar approach:

• Determine the end-of-season hard freeze date for your area (historical average first date below 28-32degF). For Boise, that might be late October to early November; for higher elevations, mid-September to October.
• Decide how late you want harvests to continue safely. For hardy greens in a heated greenhouse you can push through winter; for tender crops aim to finish before persistent freezing.

Crop-specific timelines and strategies

Cold-tolerant crops perform best in fall succession plantings. Below are common groups with timelines and practical notes for Idaho greenhouses.

Leafy greens (lettuce, spinach, kale, arugula)

Leafy greens dominate fall successions because of short DTMs and cold tolerance. Typical DTMs at ideal conditions:

• Lettuce (cut-and-come-again): 30-60 days.
• Spinach: 35-50 days.
• Kale: 45-70 days (matures slower but very cold-hardy).
• Arugula: 20-35 days.

Succession guidance:

• Sow lettuce and arugula every 7-14 days for continuous harvests; increase to 14-21 days as temperatures drop.
• Use cut-and-come-again systems: sow denser and harvest outer leaves to extend production.
• Choose slow-bolting varieties and shade or provide consistent cool temps to avoid bolting as daylength shortens.

Brassicas (broccoli, bok choy, cabbage)

Brassicas tolerate cool weather but need longer growing periods.

• Bok choy: 30-45 days for baby, 45-60 for full heads.
• Broccoli: 60-100 days depending on variety.

Succession guidance:

• Start brassicas earlier than greens if you need full heads; stagger plantings by 2-4 weeks depending on size class.
• Consider transplanting seedlings rather than direct seeding to control timing and get earlier harvests.

Root crops (radish, carrot, beet)

Root crops respond well to fall planting but require attention to soil temperatures.

• Radish: 20-30 days.
• Carrot: 60-80 days.
• Beet: 50-70 days.

Succession guidance:

• Sow radishes weekly to fill gaps; carrots and beets every 2-3 weeks to maintain supply.
• Use deeper, well-drained beds and consider insulating soil with row covers or thermal mass to keep soil temps steady.

Herbs and microgreens

• Microgreens: 7-21 days depending on species.
• Basil is tender and not ideal for late fall unless greenhouse is heated; parsley and cilantro handle cool conditions better.

Succession guidance:

• Microgreens are perfect fall cash crops: sow frequently (weekly) to maintain continuous trays.
• Use supplemental lighting for consistent color and growth rate as natural light wanes.

Transplants versus direct seeding

• Direct seeding is efficient for quick-turn crops and roots.
• Transplants give you control for slow-developing crops and those that need a head start before cooler temperatures slow growth.
• Start transplants under heat mats and move outdoors/in greenhouse to acclimate (harden off) a week before final placement.

Practical greenhouse adjustments for fall success

Greenhouse management makes or breaks fall successions. Focus on temperature, light, ventilation, and humidity control.

• Temperature setpoints: Aim for daytime 60-68degF for cool-season crops. Night temperatures can be lower (40-50degF) for hardy crops but maintain at least 45degF when possible for faster growth and pest control.
• Insulation: Use double-layer poly or thermal screens to reduce heat loss. Add thermal mass (barrels of water) to stabilize night temps.
• Supplemental heat: Small heaters targeted for propagation or key production windows can justify higher labor and market returns.
• Light: Add 4-8 hours of supplemental LED lighting for crops needing steady growth when natural daylength drops below 10-12 hours. Use light only when temperature can support growth; lights without warmer temps can lead to inefficient energy use.
• Ventilation and humidity: Cooler temperatures increase disease risk if humidity is high. Use vents, fans, and dehumidification practices to maintain 50-65% relative humidity for most greens.
• Soil and fertility: Growth slows in cooler temps — reduce nitrogen slightly to avoid overly lush plants that are disease-prone. Monitor soil moisture closely; cooler soils hold moisture longer.

Pest, disease and harvest considerations

Fall brings its own pest and disease dynamics. Insects like aphids and thrips can thrive in lower-light, cooler conditions inside greenhouses. Fungal pathogens increase with high humidity.

• Monitor weekly for aphids, whiteflies, and thrips. Use sticky cards, biological controls (lacewings, parasitic wasps), and spot treatments when necessary.
• Sanitation: Remove crop residues immediately after harvests, disinfect trays, and rotate beds to reduce buildup of pathogens.
• Harvest timing: Harvest in early morning when turgor is high. In late-season operations, consider night temperatures: harvesting after a cold night can cause wilting; allow plants to warm slightly before harvest when practical.

Sample schedules by Idaho region

Below are example timelines for starting succession plantings targeted at continuous harvests through October and into November. Adjust DTMs upward if you lack supplemental heat.

• Northern Idaho (hard freeze early October): Start main leafy green successions in mid-July to mid-August to have mature crops through September. For later supply, plan a heated greenhouse or heavy row-cover system.
• Boise Valley (milder fall): Begin staggered sowings from late July through September. For a November supply, start cold-tolerant lettuce and kale in mid-September with a slight DTM extension.
• Eastern Idaho (early fall frost): Start successions earlier — late June through August. Focus on hardy crops and quick-turn microgreens for late fall.

Concrete takeaways and checklist

• Work backwards from your target harvest dates and factor in 10-30% slower growth in fall; adjust based on greenhouse temperature control.
• For leafy greens, sow every 7-14 days early in fall, stretching to 14-21 days as temperatures drop.
• Use transplants for slow brassicas and direct seed for radishes and microgreens; combine both strategies to balance labor.
• Insulate the greenhouse, add thermal mass, and consider supplemental heat if you want consistent harvests into late fall and winter.
• Use supplemental LED lighting when daylength drops below required levels for desired growth rates, but coordinate lighting with temperature management.
• Maintain strict sanitation, monitor pests weekly, and manage humidity to prevent fungal issues.
• Keep a written calendar for each greenhouse zone: record sow dates, expected harvest windows, variety DTM, and actual outcomes to refine future schedules.

Starting the right fall succession planting schedule in your Idaho greenhouse is about planning, flexibility, and adapting to local microclimates. With intentional timing, crop selection, and environmental controls, you can extend profitable production deep into fall and set the stage for winter or early spring crops.