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How to Harden Off Greenhouse-Grown Plants

You may have heard the term “hardening off” regarding seedlings you start indoors, but many folks don’t realize it’s also important for mature plants grown inside a greenhouse. To harden off a plant means to slowly acclimate it to a new environment so that they don’t go into shock and suffer damage or even die. When plants are raised indoors in early spring and then moved outside for planting, they have to adapt to direct sun, cooler temperatures, lower humidity, and increased air movement and winds–let alone different pest and disease pressures–all of which are now out of your control and highly unpredictable.  


If a plant is properly hardened off, the cell walls and leaf structure literally thicken. More energy is put toward root development and reduction of excess water in the plant, which means it will be stronger, more rigid, and less susceptible to freeze damage. As plants harden off, they will grow less quickly than indoors but that slower growth will create a firmer and less floppy plant that can handle the stresses of outside. If a plant is not hardened off, wind, sun, or cold can lead to significant wilting, browning leaf edges, or even death. (Note that even with thorough hardening off, frost-prone plants can still get frost damage.)


If you are following most guidelines for hardening plants off, especially if you’ve started your own seedlings and are adapting them to outdoor conditions, it is a pretty time intensive process and ideally done over two weeks. While this is certainly a good rule of thumb, we tend to simplify it a bit when moving crops outside for sale as well as at our own homes. Generally, adapting seedlings to the outdoors is a much more sensitive process than bringing home well-rooted more mature plants from a greenhouse and acclimating them to your new environment. 


For less cold-sensitive crops, such as many perennials or cool-season annuals and veggies, we use this process throughout early spring, but for the more frost-tender crops (Boston Ferns, most annual bloomers, tropicals, tomatoes and warm-season veggies, etc), we tend to wait to harden off until risk of frost is unlikely. At the greenhouse, we often bring warm weather crops out in mid April rather than the recommended mid May due to our merchandising needs, but may still have to cover or even move the more sensitive crops back indoors in the case of freezing nights. If you want to be extra safe at home and avoid having to move plants back and forth, the general rule of thumb is to wait until temperatures are consistently in the low 50’s. In our area, the tradition is to wait till after Mother’s Day, but with warming temperatures over the last few decades (the US zone map has relabeled Asheville area as a zone 7 rather than a zone 6), we rarely have to wait that late to be safe from hard frosts. 


When we bring plants home to plant outdoors, we try to follow the below protocol:

  • Our absolute best piece of advice for hardening plants off–as well as just a great rule of thumb for when to plant–is to wait for a string of overcast and lightly rainy days if possible. This will both protect plants from sun scald and help with watering them in but also encourages healthy root growth as they harden off.  (Our go-to method when moving large numbers of plants outdoors from a heated greenhouse at Painters.)

  • Try clustering the pots of plants together so that they can protect one another from wind gusts and being knocked over. Plus, this makes it easier to cover them with shade cloth and/or frost cloth as needed.

  • Group the pots in an area with protection from high winds as well as protection from full sun. The edge of a covered porch, the entrance to a garage, or a spot on the south side of a building that stays warmer but still has some wind and direct sun protection is best. If you don’t have a spot that protects from full sun, drape netting that offers 10-20% shade above the plants for the first 4-5 days. 

  • Wind can be just as damaging and drying as the sun, so we recommend watering the plants deeply (or dipping the pots into a bucket of water to saturate them) prior to taking them out to harden off. Over the next week or two, you can gradually reduce the amount of water you give the plants–just be aware that while adapting to outdoor conditions, they are likely to dry out faster… if you see them starting to wilt, check to see if they’re dry (wind or sun damage can also cause wilting, so only water them if they are actually drying out).  

  • To be extra careful, bring the plants back inside for nights of the first week if predicted to be lower than 40 deg. Then the following week, cover plants with frost cloth or fleece each night. If you do manage a week of overcast and wetter weather, you may not have extreme cold, so leaving them outside with some additional frost protection for 40 or below should be adequate.   

  • After 1.5 to 2 weeks, you should be able to plant your hardened off plants wherever you plan to keep them (assuming you’ve chosen spots suited to their needs). Just watch for late frosts and cover if needed.  


*NOTE: When protecting plants with heavy material or frost cloth, you should prop the cloth up above foliage and blooms with stakes/supports–if thicker cloth or frost blanket lays on the plants, they can still burn from the cold where it touches the foliage. 


So what plants at Painters are safe to plant outside right now and how do you know if something needs hardened off?  


A general rule of thumb is that if you see something outside on the retail tables at Painters, it’s safe to plant it outside at home. We do often cover crops on the outside tables even if hardened off when the temps are predicted to be 34 or lower, as customers aren’t as likely to want a frost-burned plant (even if the roots and majority of growth are healthy, it doesn’t look so good until the new growth overtakes any damaged). 


If you see crops on tall carts but not on tables, they are likely frost-tender items that want fresh air and sun, but we aren’t ready to unload them permanently, instead wheeling them back inside until chilly nights are done. 


Our shrubs and trees are all container-grown and kept outside throughout the year, so they are safe to plant at any time (ideally before the heat of summer as they are much harder to keep well-watered and happy when planted at that time). 


Perennials from our cold frame (the large, low greenhouse sans tables, adjacent to the parking lot) are able to handle light freezing temps, but do need some protection against hard frosts and lower temps until acclimated.


When purchasing items from inside our main greenhouse (the tall, heated greenhouse with tables), you may want to double-check with staff regarding when to plant outside. It is safe to assume that anything purchased from the main house has not been hardened off. We have perennial ferns and shade perennials that may be ready to go outside right now and will just need to be hardened off per the above rules (knowing that a hard frost can still cause leaf and bloom damage but plants will be OK once hardened), but we also have tender tropicals, hanging ferns, and succulents that prefer not to be left outside until nighttime temperatures stay above 55 degrees. Signs should tell you if it’s a perennial or annual and have a listed growing zone, but if in doubt, ask a Painters team member!  


In general, the warmer the environment the plant is coming from, the longer the hardening off process should be–plants in our cold frame have had moderate protection from cold and therefore mild hardening off already, whereas plants from our heated greenhouses have not been exposed to temperatures below 55 and will need a longer introduction and more protection as they adapt. 


Don’t want to bother with any of this? Then wait until mid-May to plant everything! The only risk is that you may miss out on many of our popular items, as some crops sell out by late April or early May at Painters.


The bottom line for us is that we want you to have success with anything you bring home from Painters. A lack of proper adaptation to its new environment (including proper planting and watering), can lead to poor plant health or even failure. We are here to help, and if you can’t find a staff member available during your shopping, feel free to email us with questions!



 

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