Overwintering 101: Keeping Tender Plants Safe in Your Greenhouse
When night temperatures slide and frost shows up in the forecast, a greenhouse becomes more than a growing space. It becomes a shelter. With a little planning you can carry favorite herbs, ornamentals, and warm season veggies through winter and meet spring with healthy, ready to grow plants. This guide walks you through what to bring inside, how to prepare plants, and the simple routines that keep them thriving until longer days return.
Know what to bring inside
Not every plant needs winter protection and not every plant is worth the space. Start with a quick sort.
True tender plants that cannot freeze such as basil, rosemary in very cold regions, citrus in pots, figs in pots, geraniums, and peppers kept as perennials
Half hardy plants that dislike a hard freeze such as parsley, cilantro, calendula, chard, and some lettuces
Keep hardy greens in beds if space is tight and prioritize pots that would be lost outdoors
If you are unsure, check your first frost date and the plant’s minimum temperature tolerance. When in doubt, save the varieties that cost the most to replace or that hold sentimental value.
Inspect, clean, and acclimate
A healthy plant going into winter is a healthy plant coming out of winter. Give each plant ten focused minutes before it crosses the greenhouse threshold.
Inspect leaves, stems, and soil surface for aphids, whiteflies, mites, slugs, and fungus gnats
Rinse foliage with a firm spray of water
Remove yellowing leaves and spent flowers
Refresh the top inch of potting mix and add a light layer of compost if the plant will remain in active growth
Quarantine newcomers for a week on a separate bench if you can, then move them in with the rest
Acclimation matters. Plants that have lived outdoors in wind and bright light need a gentler landing. Move them under bright but indirect light for a few days, then slide them into their winter spot.
Right pot, right medium, right drink
Winter success is often the result of small details.
Pot selection
Choose a container that is just large enough for the root ball. Oversized pots hold cold wet media. Terracotta breathes and is great for herbs. Plastic holds moisture longer which can help citrus or peppers in a warm spot.Potting mix
Use a high quality mix with good drainage. Add perlite for air space. For citrus and rosemary, mix in a bit of coarse sand to keep roots drier.Watering
Plants drink less in low light. Water deeply, then allow the top inch to dry on most ornamentals and herbs. Peppers and citrus like even moisture but not soggy feet. When in doubt, check by lifting the pot. A light pot needs water. A heavy pot does not.
Create temperature zones
You do not need one perfect temperature for every plant. Create zones inside the same greenhouse.
Warm zone for peppers, citrus, and basil
Day 65 to 75 degrees. Night 50 to 55 degrees. Place these near the center away from doors.Cool zone for parsley, cilantro, kale starts, and salad greens
Day 55 to 65 degrees. Night 38 to 45 degrees. The edges and lower benches work well.Cold tolerant dormancy zone for figs in pots, geraniums kept resting, and bulbs you are forcing
Day near ambient. Night can dip close to freezing but not below for potted figs. Keep soil barely moist.
Use a simple max and min thermometer in each zone so you know what actually happens overnight.
Trap heat where it matters
A greenhouse gives you the structure. A few simple add ons make it a winter shelter.
Inner row covers
Stretch a light frost cloth over hoops above benches or beds. You will often gain four to eight degrees under that inner tent.Thermal mass
Water barrels painted dark and placed on the north side soak up daytime heat and release it at night. Even a row of five gallon buckets helps.Night routine
Close vents well before sunset so the structure captures late afternoon warmth. Pull inner covers into place as part of your end of day walk through.
Do not neglect airflow
Sealing out drafts at night is smart. Stale air is not. Keep air moving and keep humidity in check to prevent mildew and botrytis.
Crack vents or doors for a short period on bright mid days
Run a small circulation fan on low so warm air does not pool at the ridge while leaves sit cold and damp
Water in the morning so foliage dries by evening
If condensation drips every morning, increase gentle airflow and space plants so leaves do not touch.
Light makes the difference
Short days slow growth, which is fine for many plants. If you want steady harvests or you are overwintering fruiting plants, add a bit of light.
If you use lights, choose full spectrum LEDs on a simple timer for ten to twelve hours total day length
Hang lights high enough to spread the glow and avoid hot spots
Simple heat, smartly used
Many growers carry tender plants through winter with passive measures alone. If you add heat, aim small and strategic.
A safe electric heater paired with a thermostat in the warm zone can hold peppers and citrus above critical thresholds on the coldest nights
A soil warming mat under a nursery flat keeps cuttings and seedlings steady without heating the whole space
Place heaters away from plastic and never leave propane heaters unvented
Think of heat as a backup to good sealing, inner covers, and thermal mass.
Water, feed, and rest
Plants do not follow summer rules in winter.
Water less often but with intention
Feed sparingly. Many plants prefer to rest. A half strength organic fertilizer once a month is plenty for actively growing herbs. Skip feeding for resting figs and geraniums.
Prune lightly. Remove weak or crossing growth and keep canopies airy.
Special notes for popular tender plants
Citrus in pots
Brightest possible spot. Even moisture. Avoid cold drafts near doors. Hand pollinate blossoms with a soft brush if you want winter fruit set.Peppers as perennials
Keep above 50 degrees at night. Prune lightly to an open vase shape. Expect slower winter growth and a burst of vigor in spring.Geraniums
Two paths. Keep in low light and let rest with very light watering, or keep in bright light for modest winter color. Either way, trim leggy growth in late winter.Rosemary
Dry air and good drainage are your friends. Overwatering is the fastest route to failure. Bright light and a slight breeze keep foliage healthy.Tender herbs such as basil
Basil prefers warm nights. If your greenhouse runs cool, keep a small inner cloche over basil at night or plan to enjoy it early in winter and replant in spring.
A weekly routine that works
Consistency beats complexity. Here is a simple cadence.
Sunday
Check max and min temperatures. Adjust inner covers or heater settings if needed.Tuesday
Water and tidy. Remove any yellow leaves. Scan for pests.Thursday
Vent for a short window if the sun allows. Turn fans to a different speed for a few hours to mix air.Friday
Harvest. Clip herbs, pick salad greens, and note which plants need pot rotation for even light.
Keep notes. A tiny notebook on the bench helps you dial in timing for your specific structure and climate.
Troubleshooting at a glance
Droopy leaves in the morning
Likely cold stress or overwatering. Check night lows and soil moisture.White powder on leaves
Powdery mildew thrives in stagnant air. Increase airflow and avoid evening watering.Sticky leaves or tiny clusters of bugs
Aphids. Rinse plants, introduce a gentle insecticidal soap if needed, and check nearby plants so you catch new colonies early.Yellow lower leaves on citrus
Often a sign of wet roots or low light. Ease up on water and move to the brightest location.
Bring it all together
Overwintering tender plants is part science and part simple routine. Choose what to save, clean and acclimate carefully, set up temperature zones, trap heat with inner covers, keep air moving, and water with a lighter hand. With those steps in place, your greenhouse turns cold months into steady months. You will walk in on frosty mornings to find rosemary fragrant, parsley perky, and citrus glossy and green.
If you would like, I can turn this into a printable checklist and a one page bench card that summarizes the watering and temperature targets for your most common plants.
