The Best Way to Harden Off Plants for Transplanting

Whether picking up a few plants from your local nursery or starting seedlings indoors, there will come a time when your plants will be ready to be sent out of your home and into the garden. Learning how to harden off plants will ensure they thrive. According to the New York Botanical Garden (NYBG), hardening off refers to a one to two-week period of exposure and acclimation to outdoor weather conditions before transplanting plants to your outdoor patch.

"To harden off a plant you are getting it adjusted—acclimatized—to a new environment," says Eva Reutinger, a horticultural consultant specializing in edible landscaping, floral design, and organic maintenance. "This may be transitioning [a] vegetable seedling from inside to outside, moving an outdoor plant inside, or vice versa. Think of plants like babies. They are sensitive to changes in their environment. They need sun protection, a sweater when cold, and lots of attentive care. Similarly, plants that are hardening off need shade, frost protection, and a careful eye to observe how they are reacting to their new environment." Consider these steps before hardening off your plants to give them their best chance to flourish.

Research the last frost date.

When you're ready to begin the process, the first thing you should do is research the last frost date for your specific region, according to Reutinger. "Seedlings are ready to be transplanted when they have more than their first true leaves (cotyledons), as well as the temperature outside being above 45 degrees," she says. "Plan your hardening off period for after that frost date." Blythe Yost, CEO and co-founder of Tilly, an online landscape design company, adds that beginning the process too soon will put your plants at risk from a late frost. "Absolutely don't move plants outdoors before your final frost date unless you're putting them in a cold frame and have the necessary tools in place to keep that frame above freezing when the sun goes down," she says.

Slowly move an indoor plant outdoors.

Yost shares that plants cultivated indoors will be more tender and delicate because interior climates tend to be more constant. Hardening off a plant gradually introduces it to the outdoor environment, which will include temperature fluctuations along with wind and sunlight. "The most common way to harden off is to move indoor plants out to a protected location for longer and longer periods of time over a seven to 14 day period," she says. "Seedlings can also be transferred to a cold frame before the final frost date and slowly acclimated there instead."

Acclimate your plant.

Once you get your plants outdoors, Reutinger recommends finding a space in your garden that is shaded and wind-protected. Place your seedlings there for a few hours every day, making sure to avoid direct sunlight. Afterward, move them back inside. You'll do this repeatedly, increasing "outside time" as the days pass. "After three days your seedlings can start to receive the softer morning light," Reutinger says. "The afternoon light is much harsher and you will want to avoid it [until] the end of the process."

Yost mentions that the hardening off process is really about gradual exposure to the elements over many days, rather than getting it done all at once. "Hardening off needs to be gradual," she says. "Even though most vegetable seedlings need full sun to thrive, if they are immediately planted in their final location they will likely burn and desiccate—[known as] transplant shock."

Transplant your plant for good.

After you have acclimated your plants or seedlings, they are ready to be transplanted to their final garden location. Reutinger says that you should plan to transplant on a cloudy day, as this will help you avoid evapotranspiration—when water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants, according to the United States Geological Survey.

Watch out for bugs.

"Young seedlings are often sought after by pests," Reutinger says. "Some pests to watch for are slugs, pill bugs (rollie pollies), cabbage loopers, and cutworms. There are many great organic practices for these, but the easiest first step is sprinkling Sluggo ($15.87, amazon.com) an organic white pellet, around your seedlings once transplanted. Keeping a low can of beer in your garden will help attract/drunkenly kill slugs and other bugs. Topdressing your plants with diatomaceous earth will keep away the pill bugs and help deter those cutworms," she says. If all else fails make small aluminum foil collars to place around the base of your plants, she recommends. This will literally create a barrier from those worms.

Care for your transplanted plant.

Reutinger recommends avoiding liquid fertilizers. "Young seedlings will likely get salt burned," she says. "Slow-release fertilizers like osmocote, vermicompost, or light compost are great to mix into your soil a few days before planting."

Above all, don't worry if your first attempt goes awry. As Reutinger says, you can always buy a new plant, so don't be afraid to lean into the learning process. "We all get so attached to plants and baby them like our own, but remember it's a learning process and don't be too hard on yourself if a few die," she says. "The best tip for a first-time veggie gardener is always to buy a combination of seeds and starts. As a good rule of thumb any large seed is easy to sow and grow directly outside (pumpkin, sunflowers, squash, and peas) there are more carbohydrates in those seeds so they have more energy to sprout quickly!"