As the days grow shorter and crispness fills the air, we often face the challenge of transitioning from summer’s lush days to autumn’s cooler, more unpredictable months. For those fortunate enough to have a greenhouse, autumn is not a signal to pack away the season’s tools. Instead, it’s an invitation to embark on a season of possibility.
During the fall, your greenhouse becomes a sanctuary for nurturing plants and extending the gardening season. From growing fresh produce to creating a cozy, overwintering home for tender crops and garden beds, this warm, controlled environment is an invaluable resource that can help reduce costs and increase satisfaction. Here are five ways to put your greenhouse to work this fall.
The 8 x 12 cedar greenhouse. View our full line of greenhouses in our shop.
1. Extend the season for your vegetable crops.
One of the most compelling reasons to embrace fall greenhouse gardening is to extend your growing season. As temperatures drop and daylight hours shorten, your greenhouse becomes a haven for plants that would struggle or perish outdoors. Thankfully, many crops thrive in the controlled environment of a greenhouse during the cooler months.
Consider cool-season crops like lettuce, spinach, kale, and radishes, which not only tolerate lower temperatures but prefer them. These crops can provide a bountiful yield well into late autumn, providing they receive the light required to mature. The farther north you live, the earlier you’ll need to plant your fall crops.
Related: Best Crops to Extend Your Growing Season.
Kale is one hardy plant that thrives in the chilly temperatures of a fall greenhouse. Image courtesy of Zoe Schaffer on Unsplash.
2. Protect tender perennials and extend the life of annuals.
Greenhouses offer excellent protection against frost and temperature fluctuations. As the autumn nights get chilly, your greenhouse provides a sheltered environment for sensitive ornamental plants that might not make it through the winter. It also provides a safe space for annuals that might regrow next spring if given the right conditions.
To protect tender perennials and select annual plants, move potted plants into the greenhouse or dig up and repot in-ground plants before the first frost. You can even move planter boxes into the greenhouse for shelter. Pick out dead leaves and clean plants to ensure any diseased or damaged parts are removed before shifting to the greenhouse. This includes checking for bugs and washing off any pests. The plants you choose will depend on your growing zone, but these often include:
- Geranium
- Rosemary
- Lemon verbena
- Coleus
- Peppers
- Eggplant
Moving tender plants to a greenhouse can often help them survive through to the next growing season. Pictured: Snap-n-Grow Greenhouse.
3. Store summer flowering bulbs.
Store spring and summer flowering bulbs
When it comes to your garden, nothing is quite as eagerly anticipated as the colorful blooms of summer bulbs. Gladiolas, crocuses, lilies and more: bulbs offer an array of color that add interest to any space. To ensure a stunning display year after year, use your greenhouse as a storage space during the autumn and winter months. Here’s how to do it effectively.
Identify which bulbs need storage.
Where you live will determine which bulbs are tender and need winter protection and which ones can stay in the ground all year round. The following table outlines growing zones where bulbs do best when lifted from the ground and stored in a greenhouse or insulated building.
| Flower Varieties | Lift in Fall for These Growing Zones |
|---|---|
| Gladiolas, dahlias, ranunculus | Zone 3-7*. Lift and store. |
| Lilies (Lilium family) | Zone 4* and below. Most lilies will survive winter in zone 4 with some mulch, but require lifting below that. |
| Calla lilies | Zones 3-8*. Lift and store. |
| Irises | Zone 5* and below. Some irises are fine left in the ground to zone 6, but others are not as cold hardy. Check your grower’s instructions to be certain. |
| Tuberous begonias | Zone 8* and below. Lift and store. |
Dig and prepare bulbs
Leave bulbs in the ground until they have finished flowering and the stems have fully dried out. You can remove spent blooms sooner than this, but keep those stems in the ground as long as possible for future bulb health. When digging up spent bulbs, be gentle to avoid damage. Shake off excess soil and remove any dead or damaged foliage. You want the bulbs to be clean and healthy before storage.
Cure and dry bulbs
Allow bulbs to cure and dry for a few weeks in a well-ventilated area. This helps the bulbs develop a protective outer layer and reduces the risk of rot. A greenhouse is an excellent location for this step, as it provides the necessary air circulation and protection from rain.
Dahlia bulbs and others may be dug up and stored in the greenhouse for winter protection in some growing zones.
Label bulbs according to variety
Before storing, label the bulbs or containers with the type of bulb and the date of storage. This makes it easy to identify them later, especially if you have multiple varieties.
Choose the right storage containers and bedding
Ensure your containers provide air circulation to prevent mold and mildew. Options include wooden crates or cardboard boxes with ventilation holes. Fill containers with dry peat moss, coconut coir, vermiculite or perlite to help maintain a stable humidity and insulate against temperature fluctuations. Your goal is maintaining a temperature between 40°F to 50°F (4°C to 10°C) at moderate humidity. If temperatures get colder in your region, see the note below.
Arrange your bulbs carefully and store at appropriate temperature
Arrange bulbs in the containers, ensuring they don’t touch each other. This prevents moisture buildup and potential rotting due to contact between bulbs. Place containers in your greenhouse.
Note: If you have in-ground beds in your greenhouse, consider burying bulbs for winter storage instead. Here in zone 7, we find digging bulbs into the ground of our unheated greenhouse and burying under 6-8 inches of soil works well for dahlias, glads, and more. When their new shoots break the soil surface in spring, we dig them back up and plant outside for summer.
4. Propagate perennials
Fall is a good time to propagate plants in your greenhouse, since the controlled environment provides the right conditions for successful rooting and growth. But not all plants are suited for propagating at this time of year.
Fall is a good time to take cuttings for propagating in the greenhouse.
Hardwood vs. softwood
Come fall, most plants have stopped putting out tender, green shoots (softwood) and have developed strong, sturdy branches with a protective bark exterior (hardwood). This hardwood provides strong, reliable material for propagating deciduous shrubs and herbs.
Taking your cuttings
Choose healthy parent plants whose leaves have fallen but before a hard frost freezes the ground. Select nimble branches at least one foot long and up to four feet long with frequent buds. Using clean, sharp pruners, cut the branch into sections 6 to 8 inches long, ensuring each has several leaves attached. Remove any lower leaves to expose a node, which is the area where roots will form.
Rooting and care
Dip the cut end of the stem in a rooting hormone to encourage root development. Be careful not to overdo this step or the cutting won’t root. Plant the cuttings in a well-draining propagation mix in pots, trays or raised beds in your greenhouse where they will receive bright, indirect light.
Maintain consistent humidity levels by misting the cuttings regularly or using a humidity dome. With the greenhouse’s stable temperature and controlled conditions, these cuttings will root and establish themselves faster than if propagated outdoors, giving you a head start on next year’s garden.
Note: you can also root summer herbs in the greenhouse (hello basil!) for growing indoors over the winter. Simply snip off sturdy stems and place in water until roots form. Plant into pots filled with soil medium and let them develop for several weeks. Bring indoors when greenhouse nighttime temperatures consistently drop below 50 F.
5. Start spring seeds.
While fall may not seem like the time to get seedlings growing, some plants benefit from a long growing season. Others enjoy a dormant period in winter’s cold soil, bursting forth earlier in spring thanks to seasonal exposure. Just remember plants that germinate immediately will need enough sunlight to grow to maturity, so check your growing zone and corresponding first frost date for your area.
Here are some common seeds many gardeners can start in a greenhouse during the fall.
- Herbs: Popular culinary herbs like parsley, cilantro, chives, and dill can be started from seeds in the fall. They will establish strong root systems over the winter and provide an early herb harvest in the spring. Beyond culinary herbs, you can also consider starting herbs like lavender or rosemary. These perennial herbs may take longer to establish but can be rewarding additions to your garden.
- Perennial Flowers: Some perennial flowers benefit from a period of cold stratification to break dormancy. Starting seeds of perennials like echinacea (coneflower), rudbeckia (black-eyed Susan), and perennial salvia in the fall allows them to experience the winter chill they need for successful germination.
- Biennials: Biennial flowers and herbs, such as foxglove and parsley, can also be started in the fall. These plants typically require a period of vernalization (cold exposure) to trigger flowering in their second year, which makes fall seeding ideal for getting a jumpstart on their growth cycle.
- Native Plants: If you’re interested in cultivating native plants, fall is an excellent time to start seeds for local wildflowers or native grasses. These plants are often well-adapted to the climate and soil conditions in your region.
The Riverstone MONT is a perfect starter greenhouse for fall gardening.
Other autumn greenhouse tips
- If you added shading to your greenhouse roof in summertime, now is the time to remove it. Your plants will need all the sunlight they can get during this season of shorter days.
- If your greenhouse is unheated and you’re storing tender perennials, consider insulating pots and beds with bubble wrap or thermal blankets to conserve heat during cold nights. You can also use rain barrels filled with water in many growing zones as a heat sink.
- Use a hygrometer to measure humidity in the greenhouse. If humidity levels are consistently high, provide proper ventilation by opening vents or windows during the day to reduce moisture buildup, which can lead to mold and disease issues. A solar-powered ventilation system can help by automating this task.
- Regularly clean the interior and exterior of the greenhouse to remove dust, debris, and algae, which can block sunlight and hinder plant growth.
Related: Greenhouse Cleaning and Maintenance for Beginners
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Frequently asked questions
What can you grow in a greenhouse in autumn?
In autumn, you can grow a variety of cool-season crops including leafy greens (lettuce, spinach, and kale); root vegetables (like carrots, beets, and radishes); and herbs like cilantro and chives. You can even extend the growing season for tomatoes and peppers if the temperature remains controlled and you have enough sunlight.
How can I prevent mold in my greenhouse?
Ensure proper ventilation by opening vents and using fans to promote air circulation. Mold often develops in humid and stagnant environments, so it’s important to control humidity levels. Avoid overwatering plants and remove any dead plant material or decaying organic matter that can harbor mold spores.
How does a greenhouse help a gardener grow plants during the cool season?
A greenhouse traps heat from the sun, creating a warmer and more stable environment for plants even as outdoor temperatures drop. This extended warmth allows gardeners to grow cool-season crops that might not survive in outdoor conditions. The controlled environment also shields plants from frost, wind and heavy rains.
How long does a greenhouse extend the growing season?
A greenhouse can extend the growing season by several months, depending on the location and setup. In most temperate climates, a greenhouse allows you to start planting earlier in spring and continue growing well into late autumn or even winter. By maintaining a controlled environment, it’s possible to grow some crops year-round, especially if you supplement with heating or lighting during the coldest months.
How can I increase humidity in a greenhouse?
While too much humidity is more common in greenhouse environments, some growers may encounter the opposite problem. To increase humidity, mist plants regularly or place trays of water around the greenhouse for increased evaporation. Planting densely can also help, as the transpiration from plant leaves contributes to moisture in the air. Additionally, minimizing ventilation on cooler days will help retain moisture inside the greenhouse.
