The easiest way to harden transplants is to place them outside in a shaded, protected spot on warm days, bringing them in at night. Each day, increase the amount of sunlight the transplants receive. Don't put tender seedlings outdoors on windy days or when temperatures are below 45° F.
After transplanting to the garden, use a weak fertilizer solution to get transplants growing again and to help avoid transplant shock. Be sure to water plants after hardening them off. Hardy plants can be hardened off when the outside temperature is consistently above 40° F.
Cold frames are a valuable tool for hardening off seedlings and young plants before planting them in the garden. A cold frame works great for sheltering young plants while allowing them a “taste” of life outdoors. The most common type of cold frame used by home hobbyists is a box-style cold frame.
Seedlings, with their tender new leaves, often give up the ghost when temperatures dip to 32-33°F. Tropical plants have differing low-temperature thresholds. Some keel over when temps fall to 40°F; others crumble at 35°F. Other plants are just hardy by nature and can withstand temperatures as low as 18-20°F.
For most plants, start hardening off about a week before the final frost date for your area. Our Garden Planner uses data from your nearest weather station to give an indication of when it's safe to plant outside, providing a helpful guide to work back from.
Although not a difficult process, hardening off involves foresight and planning. You don't want to start the process too early, or you run the risk of cold temperatures damaging your plants. Wait too late, and your plants might still suffer from being in pots too long.
In general, the process of hardening off will take about one week, and sometimes up to two weeks if the weather has an unexpected and dramatic drop in temperatures. If your region has frost, you should time your seed starting in spring so the seedlings can start to go outside after the last frost date.
I did some research and found out that houseplants need to be taken indoors before overnight temperatures dip below 45 degrees. Most tropical plants suffer harm from temperatures below 40 degrees. Well, we have temperatures forecast to dip into the 40s again on Thursday and lower 40s on Friday.
If you're wondering at what temperature threshold you should be bringing your plants, there's a short answer: when nighttime temperatures reach 45 degrees (F), it's time to bring your plants indoors.
Most houseplants are tropical plants that can safely grow in a heated home during the winter. They will never survive outdoors in winter unless you live where the temperature stays at least above 50 degrees.
The easiest way to harden transplants is to place them outside in a shaded, protected spot on warm days, bringing them in at night. Each day, increase the amount of sunlight the transplants receive. Don't put tender seedlings outdoors on windy days or when temperatures are below 45° F.
Much like humans, plants need to slowly increase the amount of sunshine they're exposed to or they'll get burned! If you don't harden your plants, the tender plants will get burned by the sun, the shock of cold, or the wind.
Too cold – Although most winter crops are at least a little bit frost tolerant, they don't grow when it is under 40 F outside. They'll appreciate the cover of your cold frame staying on any time the weather is below 50 F. Too hot – Likewise, cool weather crops don't like it hot.
Once outdoor temperatures remain around 50 degrees Fahrenheit through the night, and if the seedlings are tolerating the hardening process well, you can start to leave them outdoors overnight.
Signs Your Plants May Be Too Cold
The leaves brown and fall off. If your plant is too cold, it may begin to die, and its leaves will brown and fall off. The leaves turn yellow.
"You Must Harden Off Your Seedlings" If you don't harden off your indoor seedlings before planting them outside, they will suffer shock from the brutal sun. Without the opportunity to adjust first, they'll get sunburn and might even die. In some situations this is 100% true.
For many plants, temperatures below 50°F can cause problems. Adjust thermostats to cater to your comfort, but remember your plants need some consideration. Avoid placing plants near cold drafts or heat sources. Keep plants several inches away from exterior windows.
High temperatures over 90 can kill the plant inside the seed. Was the parent plant healthy? Seeds can harbor infection from the parent plant that may prevent sprouting, however, this is not usually the case.
Monitor the Temperature and Moisture
Be sure the temperature in the cold frame does not go much below 50 degrees or above 80 degrees Fahrenheit. While the seedlings are in this warm, sheltered environment, be sure to monitor soil moisture; don't let the plants dry out.
It's important to pay attention to the temperature dropping when the sun goes down. Once temperatures are consistently between 60 and 45 degrees Fahrenheit, it's a good idea to start bringing your plants inside at night. However, you can keep them out during the daylight hours if the temperature stays above 45 degrees.
Some plants can survive outside even when temperatures hit freezing. Others need to be brought indoors when temperatures reach the mid-40s. Be sure you know this before you plant.
Avoid watering when temperatures are below 40°F or when sustained freezing temperatures are expected within 24 hours as this water will freeze and not be available to the plants.
A fan makes the hardening off process easier for them to adapt to outdoor growing conditions. A few tips: Don't place the fan in a location where the air is blowing directly on the seedlings. Place it in the corner of the room to circulate the air and add a gentle breeze to your space.
Hardening off plants
Plants that have been indoors (even those bought early from a local greenhouse) need to be acclimated to the fluctuating temperatures, wind and sunlight of life outdoors. In other words, you need to harden them off.
Try to start the outdoor hardening off process on a day when things are pretty calm out – no extreme wind, rain, frost or things of that nature predicted. Take your seedling trays outside. The morning is preferable, but if all you can do is after work, that is okay too. Start in a shady, protected location.