How Low Tunnels Can Simplify Hardening Off for Healthy Transplants
Hardening off prepares plants for outdoor life. This process is often ignored because it’s labor-intensive, time-consuming, and tedious. However it creates healthy and stable seedlings that experience less stress, transplant shock, and disease while increasing yields.
One simple way to streamline the hardening-off process, especially if you’re short on time or outdoor space, is to use a low tunnel. Low tunnels gradually expose seedlings to outdoor weather patterns and provide protection from swings in spring and fall climates. Here’s how they can simplify hardening off and set your transplants up for success.
What Does it Mean to Harden Off Transplants?
Hardening off is gradually moving seedlings from indoor to outdoor conditions before they’re transplanted.
Why Hardening Off Is Necessary
The hardening-off process is a crucial step when starting plants from seed. When seeds start indoors in a cozy, warm environment, they must slowly acclimate to wind, rain, fluctuating temperatures, and direct sun before transplanting. Without this transition, they’re more likely to suffer from transplant shock, which can stunt growth, invite disease, and lower yields.
Gradually exposing them to outdoor conditions helps build resilience and leads to stronger, more productive plants.
Bootstrap Tip: Start the process on a cloudy or overcast day with no precipitation to avoid harsh sun exposure. If sun exposure cannot be avoided, consider draping shade cloth over them on the first day.
Traditional Hardening Off Methods
Hardening off usually involves moving trays from indoors to a designated outdoor area over the course of one to two weeks. The process begins with a few hours under shade cloth, then transitions to partial days in direct sun or light rain, eventually building up to full days and overnight stays outdoors.
While this method works, it can be time-consuming and physically demanding. Moving trays multiple times a day isn’t always practical, especially if your schedule is full or your growing space is limited. Inconsistent hardening off can lead to stressed plants and reduced transplant success.
These traditional methods are effective, but there’s a more efficient way to manage the process. Keep reading to see how low tunnels can help.
What Are Low Tunnels?
Low tunnels are temporary, moveable structures like mini hoop houses or greenhouses. They consist of hoops, coverings, and anchoring systems that vary in strength and durability based on your region and needs. These structures are typically two to four feet tall and three to six feet wide.
Overview of Low Tunnels
Think of low tunnels as scaled-down versions of hoop houses or caterpillar tunnels. To achieve the highest level of durability, we recommend using 1/2" or 3/4" metal tubing (EMT) for the hoops. Pair them with professional-grade greenhouse plastic or frost blankets for ultimate protection. Use a hoop bender to make bending the EMT a cinch.
The key benefits of low tunnels are:
-
Extending the season in the spring and fall.
-
Protect crops from extreme temperature fluctuations, frost, harsh wind, snow, and pests.
-
DIY low tunnel kits are easy to install and use.
-
Plants have improved root development and increased growth when planted into warmer soil.
-
Proper ventilation reduces fungal disease and damping off in the spring.
-
Inexpensive compared to high tunnels and hoop houses. Perfect for beginner gardeners.
-
Scalable for growers of all sizes and experience levels.
Low Tunnels vs. Other Hardening Off Methods
When it comes to hardening off seedlings, low tunnels stand out for their scalability, quick setup, and lightweight portability. Other options, such as cold frames, outdoor benches, or unheated greenhouses, can also be effective—but they’re often bulkier, more expensive, or harder to move and manage.
How Low Tunnels Simplify the Hardening-Off Process
Let’s discuss why we recommend low tunnels to simplify the hardening-off process.
Smoother Transition for Plants
With low tunnels, you can adjust protection as needed. Change or layer coverings based on the weather. In early spring, for example, you might pair insect netting with frost blankets. Snap clamps make it easy to remove the frost blanket on mild days while keeping the netting in place to prevent pest damage.
For sunny days, add shade cloth to reduce the risk of sunburn and wilting. Shade cloth helps regulate temperature, encourage airflow, and improve ventilation. When seedlings are allowed to harden off slowly under protective conditions, they develop stronger stems and become more resilient to wind, sun, and rain once fully transplanted.
Time and Labor Savings
Avoid tedious daily tray movement, which risks dropping, bumping, stem breaks, and foliar damage, and leave them in a low tunnel. Use the extra time to efficiently manage your greenhouses, start more seeds, and grow larger batches of successful seedlings without fear of damage.
Bootstrap tip: Our easy-to-carry 72-cell air-pruning trays allow you to move trays two at a time. Read Air Pruning Basics: How It Works and Why It Matters to learn why we love them!
Better Plant Health and Less Rebound Time
Hardened-off plants are more resilient, experience less disease, and have an increased tolerance to cold. A 2021 study from the Ghana Journal of Science Technology and Development, Effects of Hardening off on Growth, Yield and Quality of some Pepper (Capsicum annuum) Cultivars, concluded that hardened-off peppers displayed increased stem diameter, number of branches, leaf area, and fruit production.
Add a low-speed fan to indoor seedlings to encourage sturdier stems, foliage, and healthier plants. The fan mimics a gentle wind and is the first step to hardening off, reducing transplant stress and shock.
Tips for Success with Low Tunnel Hardening-Off
If you’re new to using a low tunnel or the hardening-off process, we have tips to help keep your plants healthy and happy.
Time Your Hardening-Off Process to the Crop
Each crop requires a different timeframe for hardening off. Quick-maturing crops like summer squash and zucchini can transition to a low tunnel about three weeks after germination when they have true leaves and have grown substantially in their cell trays or pots. While crops like tomatoes and peppers need more time before they are ready to move outside.
Ensure temperatures are above 60°F (15°C). After the seedlings have remained outdoors overnight a few times and show no signs of stress, keep them in the low tunnel for about five to seven days before transplant.
Hardening off is the process of acclimating plants to outdoor weather so they do not experience stress when you transplant them. If seedlings experienced stress, bring them back indoors for a few days to recover and start the process over, allowing them ample time to adjust.
Use your region’s last frost date as a guide for starting seeds, hardening off plants, and transplanting. Find your last frost date using the USDA Plant Hardiness Zone Map. These are not hard and fast dates but are anticipated based on years of tracking frost. Note that elevation in your area may alter your probability of frost.
Here is a general overview of the timeline:
Consider the type of crop and your region when deciding when to start hardening off plants. If you’re in Zone 3 and see growers in Georgia transplanting peppers, don’t rush yours. They’ll perform better if you wait until the risk of frost has passed before introducing them to cold temperatures and soil.
Use frost blankets for protection as needed in the early spring. Our article When to Transplant Seedlings: Up-Potting & Timing Tips discusses the importance of growing healthy seedlings and when to up-pot and transplant.
Plants should start hardening off one to two weeks before transplanting. Ease them into the outdoors by:
-
Pulling back covers
-
Leaving the tunnel open overnight
-
Limiting cold protection and water
-
Eventually, removing covers altogether
Plants are ready to transplant when they do not show signs of stress and have spent several full days and nights outdoors. For the best garden results, select crops and varieties that will thrive in your region.
Managing the Low Tunnel
Add a thermometer inside the low tunnel and the soil to track daily temperatures. Note spikes, dips, and plant health and behavior. This data will be helpful in future seasons. For example, bump next season’s sowing date back a week or so for plants that have experienced stress. Start a sowing schedule or calendar to keep track.
Remember to adjust the low tunnel coverings when the weather fluctuates. Pull the tunnel sides up during temperature spikes to avoid plants getting too hot. The small space will heat up quickly, becoming dangerous if you’re not paying attention. The same goes for colder temperatures—add layers of frost blankets as needed to adjust the level of protection.
Keeping Plants Happy
-
Monitor seedlings for signs of stress: drooping, discolored leaves, flopping, and stunted growth.
-
If plants show signs of stress, add a layer of frost protection, ventilate more, irrigate less, or pull them out into a heated area. Take swift action to ensure plants don’t perish during stressful situations.
-
Water plants less during the hardening-off process.
-
Check soil moisture daily and avoid letting the soil completely dry. Overwatering at this stage could lead to damping off or fungal disease.
Scaling for Larger Operations
Once you become comfortable using low tunnels, extend your protection by adding more. Connect them by adding stabilizing bracing or positioning them in the same bed as others to increase the protected soil. Use low tunnels inside hoop houses or high tunnels to overwinter strawberries, bulbing onions, and hardy greens.
The University of New Hampshire and UMass Extension conclude in Research Summary - Using Low Tunnels for Overwintering Crops that low tunnels offer excellent overwintering capabilities for many tender annuals. They protect against pests, weed pressure, and diseases. Once you have low tunnels, you’ll find their use extends much further than hardening off plants.
FAQs About Hardening Off Transplants with Low Tunnels
How long should transplants stay in a low tunnel before planting out?
Keep plants in the low tunnel for at least a week before transplanting them or until they have spent several days and nights without signs of stress.
Can I harden off all types of plants using low tunnels?
Yes, but use caution when moving cold-sensitive plants like tomatoes, cucumbers, eggplant, and basil. These plants will not tolerate a frost, so if temperatures dip below 50°F (10°C), bring them inside.
What’s the best material for low tunnel covers?
Using 6 mil plastic for winter coverings is recommended, especially in areas with heavy snow loads and strong winds. Heavy-duty frost blankets may suffice in areas without snow or hard frosts. In the summer, use insect netting to protect against pests.
How do I prevent overheating in my low tunnel on warm days?
Ventilation is essential on warm days, so track the weather forecast. Remove the lowest clamps and slide the cover up during the hottest part of the day, then lower it back in the evening. You can also temporarily open the ends to create a cross breeze. As the weather warms up in spring and summer, switch from 6 mil plastic to a lighter material for covering.
Is it possible to build a low tunnel without specialized tools?
Building most low tunnels will require a hoop bender.
If you use a DIY kit, you only need basic tools like a measuring tape, miter saw, and crescent wrench to build a low tunnel. Our kits require you to buy EMT locally and offer the option to add a hoop bender.
How do low tunnels hold up in high winds or heavy rain?
Attach low tunnels to raised beds with clamps, fasteners, sandbags, bricks, landscape fabric pins, or rebar clips.
Can I reuse my low tunnel setup for other parts of the growing season?
Low tunnels are easily transported to other parts of the garden. They can protect in-ground garden plots or wooden or metal raised beds. With proper care and maintenance, low tunnels are reusable each year.