The unsafe temperature hardening off threshold sits at below 45°F (7°C) for most seedlings you will grow in your garden. Warm season crops like peppers and tomatoes need it even warmer than that to stay safe. Once temps drop below these limits, your plants risk damage that can set them back for weeks or kill them outright.
I learned about cold damage the hard way during my second year of growing vegetables at home. A late spring cold snap dropped temps to 42°F (6°C) while my pepper seedlings sat outside on my porch. I thought they would be fine since there was no frost that night. Those peppers turned purple at the leaf edges and stopped growing for almost a month afterward.
In my experience, different plants have very different cold limits that you need to know about. My basil seedlings show cold damage at anything below 55°F (13°C) which surprised me at first. Tomatoes can handle temps down to around 45°F (7°C) for short periods but they hate it. Peppers and squash need it above 50°F (10°C) or they suffer for weeks after.
The minimum temperature seedlings outdoors can handle varies by plant type quite a bit. Hardy brassicas like cabbage and broccoli can take temps down to 35°F (2°C) without much trouble at all. Tomatoes need temps above 45°F (7°C) to stay healthy during hardening sessions outside. Cucumbers and melons require at least 50°F (10°C) while basil needs 55°F (13°C) or higher.
Research from Michigan State shows that cold damage seedling hardening causes can last the whole season. Melons and cucumbers suffer lasting stunts when temps drop below 50°F (10°C) even once. Chilled cauliflower may fail to form heads months later. What seems like a minor chill can ruin your whole harvest before summer even starts.
Your seedlings show clear signs when they get too cold during hardening outside. Leaves may turn purple or bronze along the edges and veins of the plant. Growth stops or slows way down even after you bring them back inside to warmth. Some plants wilt and never perk back up no matter how much care you give them.
Put a thermometer right at seedling height to track real temps your plants face each day. Ground level temps run several degrees colder than weather reports show for your area. Check the forecast before every session and bring plants inside if temps will drop at night.
Always bring your seedlings back indoors before temps drop lowest outside at night. Set a phone alarm for late afternoon so you never forget to gather up your plants before dark. One cold night can undo all the progress your seedlings made during hardening so far. The extra work of moving plants keeps them safe from damage that cannot be fixed later on.
Read the full article: The Complete Guide to Hardening Off Seedlings
