Hardening off

 

Hardening off plants

 By Kim Willis These articles are copyrighted and may not be copied or used without the permission of the author.

Every spring many gardeners, even some experienced ones, are going to kill or harm some of their plants.  What will they do?  They will move them outside without giving them time to adapt gradually to outside conditions, called hardening them off.  It may be houseplants, it may be seedlings you started under lights inside, it may even be plants you purchased at the greenhouse, but it was plants that weren’t ready to be shoved outside without some extra care.

 There are several things that can harm plants when they are first taken outside.  One is cold, but the sun and the wind on a beautiful warm day in spring can prove equally harmful.  The shock of changing from sheltered conditions inside to the real world outside can damage or kill most plants. That’s why any move outside is done carefully and gradually. The process is called hardening off and it’s essential for healthy plants.

Cold

Most gardeners know enough not to put plants out when it’s too cold, but sometimes when they see greenhouses selling them, they assume it’s time to put them outside.  Don’t assume, greenhouses don’t mind selling you new plants after a frost kills the first ones you bought.  Know your average last frost date and keep an eye on the weather forecast when purchasing plants in early spring or deciding when to move seedlings or houseplants outside.

Most annuals and houseplants are killed by frost, a freeze is deadly even to some new perennials that haven’t been hardened off.  A frost can occur even above the freezing point when nights are clear and calm and temperatures dip below about 36 degrees.  Even after frost normally doesn’t occur in your area an odd weather pattern can bring a surprise frost in late spring. Gardeners should pay attention to weather forecasts.

When you can’t resist buying that gorgeous hanging basket of begonias or those nice tomato plants early in your growing season, make sure you have a place to move them into when frost threatens.  You can use a garage or shed or even your car but if frost is predicted move tender plants to a protected spot. If you are moving seedlings outside in flats or pots, they too, should be moved inside if frost threatens.  I leave mine on a cart so that I can simply pull it into the barn. (Don’t leave plants in the car after the sun comes up, they may get too hot.)

If you have planted in the ground or large containers and frost threatens you can cover the plants with paper or cloth, not plastic as it can harm plants. If the leaves are touching the plastic during the frost the cold comes right through or if the morning sun warms things up too much before you can get out there and uncover them the plants will cook.  Covering plants when a hard freeze is coming won’t protect them.

Having a small unheated greenhouse is wonderful in the spring.  You can purchase plants early when the selection is good and hold them inside the greenhouse until the weather is settled.  Or you can move seedlings from a heated place to the unheated but protected space.

Most houseplants are tropical and shouldn’t be moved outside until temperatures stay above 40 degrees at night.  A few warm days can be deceptive.  If you are like me it’s a major job moving plants out for the summer and you don’t want to do it more than once.  Wait until the weather is quite warm and settled before giving those houseplants a summer vacation.

If a perennial is winter hardy in your area, it’s ok to plant it as soon as the soil can be worked when its dormant.  But if the plant is in a pot and well leafed out when you get it, and similar plants in the ground outside aren’t leafed out yet, then wait before planting it outside until conditions are warmer.  Putting the leafed-out plant outside too soon may kill the young growth that hasn’t hardened off.  This will stunt the plants growth or may even kill it.

You can put the plant outside in a sheltered place- sun is ok after a day or two of acclimation if that plant likes sun- and move it inside if a frost or freeze is expected. When similar plants in the ground already have leafed out then your potted perennial can be planted.  If all danger of frost has passed it can also be planted.

The sun and wind

Even if the temperature isn’t going to be a problem, moving plants outside into full sunlight and the wind from an indoor location can harm them. Even you might burn if you spent a whole day outside in the sun early in spring before building up a little tan. The sun outside heats the plants leaf surfaces quickly, unlike most indoor lights.  Heat and the stronger light both affect the plants.

Plants change the type of leaves they have to suit the light conditions. Some drop leaves and replace them, others make the leaves thicker, add more chloroplasts, or change the angle at which leaves are presented to the sun. This change takes a few days to a few weeks depending on the plant.

It doesn’t matter if the plants were in very bright conditions inside, like under grow lights or in a southern window.  Even plants known to like full sun, like cacti and some succulents, and those tomatoes and peppers may burn if moved directly into sunlight from inside. Seedlings are very vulnerable. 

Even if plants spent the summer outside the previous year, if they have been inside all winter, they need an adjustment period or hardening off, this year too.

 Plants that sunburn may get reddened areas, or yellow or white areas especially at the top of the plant.  They may look bleached out or like they are blistered. These areas may then turn brown and crisp or they may remain soft and rot.  Leaves may wilt. The wind, even if it’s a light breeze, contributes to problems by quickly drying out plants. Sunburn and wind drying can happen in just a few hours.

Plants may not die from sunburn, but they will be delayed in growth and production as they work to repair the damage.  It can take most of the season just to repair that early damage. They may never look as nice.  And some plants will die, some after just a few hours in the direct sun.

So how do you harden off plants?  Put the plants outside in a shaded location protected from the wind for a few days before gradually moving them into full sun or their preferred lighting, and the wind. Start with an hour or so of full sun and mild breezes, then move them back into shade.  Lengthen the time by a couple hours each day. When they are in sun for 6-8 hours a day they can be planted in the sun.  (This is for plants that like full sun.   Shade loving plants should never be put into full sun.)

If you don’t have a shady area make one with a beach umbrella, a tarp, or putting them under a table. Choosing to move plants outside during rainy or cloudy weather is also a good idea. A cloudy, rainy, mild spell is the perfect time to buy plants, and move plants outside.

Make sure to keep plants moist while hardening them off.  Pots and flats will dry out much more quickly outside than inside.  Having them near a water source is a good idea.

What about plants that came from a greenhouse?  Some of those plants may also suffer, especially from drying out. When I buy annuals and vegetable plants from the greenhouse, I don’t plant them the first day.  I put them in light shade (if they are sun lovers) or shade (if they are shade plants) and keep them well watered for a day or two. That’s usually enough adaptation time for greenhouse plants. 

If you are going to store the plants for any longer time before planting, put them in a shady protected spot first for a day or two, then move them into partial or filtered sun if they are sun loving plants. Plants in small pots and cell packs dry out quickly. Don’t forget to water them frequently.  This could be several times a day if it’s warm.

If you do decide to plant on a sunny day in full sun, you should protect the plants (shelter in place?).  When my grandfather planted tomatoes outside, he made little hats out of newspaper for them.  These were left on for three days.  It worked for him.  You can also use paper, (paper lunch bags work well), row cover, shade cloth, old thin sheets, or other things to cover newly planted plants. You don’t want things that hold too much heat, tear the corners off bags and lift row/bed covers up off plants so air can circulate, or you will cook the plants.

Some houseplants that like that southern window inside may not be able to stand full sun outside, even after hardening off.  My Norfolk pines are in a southern window with grow lights as supplements in the winter.  When they go outside, they go under the high shade of some cedar trees for the summer and they thrive. 

Aloes, cacti and many other sun lovers still need an adaptation period.  Some succulents like Christmas cacti are not found in full sun in the wild, and don’t appreciate it at your house either. A rule of thumb for houseplants is shade first – for at least a week, then partial shade for another.  Then a great many of them still need partial shade or dappled shade or even full shade for the summer outside.  Know what light conditions your plant species prefers in its native environment.

It may seem like a lot of time and work to harden off plants.  Some people don’t do it and for some plants in some instances that may work. But taking just a little time and effort to let the plants adjust to new conditions makes them healthier and happier.