Hardening Off Your Plants

Picture a beautiful 15°C day in September…you might need a toque, definitely a sweater or jacket. Now picture a beautiful 15°C day in March…it’s shorts weather baby! But watch out because those legs might get burned. The reason the same temperature feels different in March is because we Ontarians are acclimatized to colder temps, but our legs just aren’t used to seeing the sun yet. 

Your plants are kinda like that too! Plants that are used to being indoors or in a greenhouse will need your help getting used to or reacquainted with the great outdoors before they can survive the elements. It’s an easy process that your plants will thank you for. 

What does hardening off mean? 

The term hardening off refers to the process of gradually preparing a plant to survive (and thrive) in outdoor conditions that are colder, windier and brighter than inside a greenhouse. The time it takes for this process will vary a little bit based on the situation, but usually takes 7 to 14 days.

What plants need to be hardened off?

  • Greenhouse grown early spring bloomers like pansies, violas and primulas that aren’t hardened off by the greenhouse before you buy them.
  • Forced spring flowers like ranunculus, daffodils, tulips, hyacinth and hydrangeas
  • Cute little seedlings
  • Houseplants that are going back outside for the summer

How to harden off frost-tolerant spring bloomers

Spring bloomers that are naturally cold tolerant can be transitioned outside pretty much as soon as the temperature is reliably above 5°C. Once they’re hardened off, they can usually tolerate a light frost (we’re not talking about days on end of -5°C though). Plants considered frost tolerant spring blooms include pansies, violas, campanula and forced bulbs/flowers like ranunculus, daffodils, tulips, hyacinth, crocus, and muscari. 

  • Find a shady sheltered spot outside for your brand new plants (if you don’t have a sheltered spot, you can use an open cardboard box for the first few days!)
  • Leave the plants outside for a few hours the first day, increasing the time outside and exposure to sun (start with a couple hours of morning sun ideally) and wind every day.
  • Move plants to an indoor spot like a garage, shed or inside when the forecast calls for temperatures below 5°C.
  • If a deep freeze is expected, just keep your plants inside until you can go back to the hardening off process. 
  • Ensure you keep plants watered during the process! Plants growing indoors often don’t need as much water because the wind and sun don’t dry them out as quickly
  • Keep this process up for about a week to 10 days, and then you can plant them in the garden as long as soil is workable. If a deep freeze is coming after planting out, cover with a cardboard box, inverted pot, or row cover just in case.

How to harden off seedlings

Seedlings and young greenhouse purchased veggies will take upwards of 14 days to harden off, so a good time to start the process is about 2 weeks before the last frost date, or 2 weeks before you’re hoping to plant them into their containers or garden beds. If you have a large number of seedlings to harden off, you can place them all in a wagon or wheelbarrow to assist with moving them around every day. 

  • Find a shady sheltered spot outside for your seedlings that are out of direct sun, and extreme wind. Keep in mind that some critters and insects also love seedlings, so you may want to ensure they’re out of reach of those as well. 
  • Leave the plants outside for an hour the first day, increasing the time outside and exposure to sun (from shade, to dappled shade, to morning sun, etc.) and wind every day.
  • Move seedlings back inside once their time outside for the day is up. If weather cooperates, they should be able to stay outside overnight within the first week.  
  • Ensure you keep seedlings watered during the process!
  • Don’t put seedlings outside if the temperature will be less than 10°C, or if it’s a particularly windy day. Just pause the process and keep them cozy inside, ready to pick it back up when the weather improves. 
  • After a couple of weeks, transplant your seedlings and water well. If possible, choose a cloudy day to transplant. 
  • If a frost is expected after transplanting, ensure to cover your tender plants with a cardboard box, inverted pot, or row cover to protect it. 

How to harden off houseplants

In the spring, there are a few ways you can help your plants thrive which include possible transplanting, and updates to watering and fertilizing schedules! For more information on the winter to spring transition, check out our article about helping houseplants through seasonal changes

House plants are like indoor cats, but sometimes when the weather is nice, they might benefit from catching some (likely indirect) sun and feeling a breeze through their leaves, regardless of what type of plant you have. Moving your houseplants outside is technically more of an acclimation than a hardening off, and it’s done later in the year like May or June when temps are balmy. It can take anywhere from 7 to 14 days. Here’s how: 

  • It’s pretty safe to start the transition when temps are consistently above 15°C or so. 
  • Place your plants in a nice shady location for a couple hours the first day, slowly increasing the time it’s left outside. 
  • Pay attention to watering, as you may need to increase if the plant is in a location that does not get rained on, or decrease, if rain can reach your plant.
  • Most indoor plants that can tolerate full sun should still be eased into it starting about 5 days into the acclimation period (like those white legs noted above, the leaves need to build a little tolerance so they don’t get scorched). 
  • If temperatures are set to dip, remember to bring those plants back inside!

And now you know hardening off plants 101

Overall, hardening off your plants is a simple and straightforward task – although it does take a little planning and attention – but it’s one that will ensure your planty investment is starting off on the strongest foot possible. 

As always, if you have any questions, we have so many awesome Daymakers that just love talking plants and would happily chat about this or any other garden topic you might need some info on. Or you can hit us up on social media!