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Hardening Off Made Easy

For seasoned and new gardeners alike, the concept of ‘hardening off’ can be shady, but don’t let uncertainty prevent you from digging in and enjoying the process.

Under shelter of the north eave of the house, these Tumbler tomatoes enjoy reflected warmth but avoid direct sun.

Think of hardening off like training wheels for your baby vegetables and tender annuals. Follow a few simple and intuitive hardening off guidelines, and you will be fine. Remember, you are a better gardener than you know.

Technically and generally, hardening off refers to the incremental transition of non-native, greenhouse or indoor grown seedlings from a controlled environment to an uncontrolled environment (indoors to outdoors, in most cases).

Zone denial trees and other perennials may require hardening off also, on occasion. Potted lemon trees, for example.

I keep four factors in mind, when I transition plants — those I start from seed indoors, and those I buy as seedling plants from nurseries — to my garden. These factors are:

  • Sunlight

  • Temperature (soil and air)

  • Humidity

  • Wind

Even the best grow lights cannot compete with the perfection and intensity of natural sunlight, so we must take care to not shock plants into poor performance, or non-performance. The diagnosis for both of these conditions, caused by too much or too little of one or more of the four factors, is known as ‘transplant shock’.

It is important to note that if you are purchasing seedlings or plants from a garden centre where the plant pots or multi-packs are outside on tall nursery racks, exposed to full strength sun, wind and rain, there is a chance that they are already hardened off. There is a chance too however, that they aren’t.

Tender Tom Thumb lettuce transplanted far below the top of this fabric bag planter, sheltered from wind and late frost.

No doubt you have seen banks of wilty seedling packs offered at firesale prices from a parking lot ‘pop-up nursery’ at a big box store. Use caution when purchasing plants from such pop-ups, and be sure to ask a knowledgeable staffer about the age and storage of the plants.

Next time you are at a full-time garden centre, or a general store with a long history of offering seasonal plants for sale, take note of the area used to store and display seedlings. Typically the area is under a roof that offers semi-shade or diffused sunlight, and most often the area is sheltered from wind. Always, the plants are watered regularly to ensure that roots do not stress, and leaves and other soft tissues do not suffer cell collapse from a lack of moisture.

Six-packs especially require consistent observation when hardening off, as the small volume of soil in each cell, and the proximity of roots to hot plastic sidewalls can accelerate shock and/or stunt plant growth.

Plastic totes are easy to lift up to counter level at dusk; tomatoes enjoy warmth held by the glass patio cover.

My golden rule when starting the hardening off process is to mimic nature to as great a degree as possible (permaculture principal #1 - observe and interact), allowing as little as three days for tender greens planted into shade, and as much as two weeks for heat loving tomatoes and peppers. And, like learning to ride a bike with training wheels, I start slowly and work my way up to ‘hands-free’.

There is no one exact way to harden plants off, there cannot be because we all live in different homes, in different neighborhoods, in different growing zones, with different ‘last frost dates’ and different ‘transplant dates’ (also known as ‘seedling planting dates’).

Thankfully, wholesale growers start shipping seedlings to garden centres very near to the ‘transplant’ date’ for each area, so you can be confident that these plants are ready to be either planted directly into the garden (hardy annuals), or hardened off (tender annuals, tender greens, heat-loving plants).

Early started Little Crunch peas and Tiny Tim tomato, safe, dry and warm'ish near the house, next to the barbeque.

Where I live, near the ocean in the mountainous Pacific Northwest, the prospect of cold and damp (and also prevailing wind from the sea) plays as great a role in the hardening off process, as does the amount of sunlight seedlings receive. My tomato seedlings are more vulnerable to root rot and blight than they are to sunburn, so rain cover and air circulation are top of mind for me during the hardening off process. Also, the sun is less intense here than it would be farther south or in a plains or prairie environment, so I can reduce the number of total hardening off days, and increase the number of sunlight hours per day.

In Southern California where the sun is intense and there is little rain, the risk of ‘sun burning’ or ‘heat stroking’ plants is greater that it is here, so it makes sense to expose seedlings to direct sun for a very short time initially — perhaps just 10 or 20 minutes to start — and work your way up slowly to full-time, hands-off exposure.

An empty coldframe acts as a holding area for ground cherries not quite ready for hardening off outside.

A bit of knowledge about where a particular plant originated, will help you understand and anticipate its needs and ideal growing conditions, and mimic them. Tomatoes for example, are native to South America, so it makes sense that they want warm feet, hands and head (roots, shoots and foliage) for as much of the day as possible, but incrementally — not all at once.

Kohlrabi, on the other hand was cultivated over 500 years ago in Northern Europe as a sort of hardy turnip-cabbage, and is perfectly happy transplanted directly into cool soil.

Kohlrabi seedlings are super hardy but still benefit from an overnight trial run in-place, before planting.

I can say definitively what works for me in zone 7B/8A Coastal, and offer general tips and hacks for growers in other zones. Always check your USDA (American) or NRC (Canada) zone map before commencing your hardening off project.

Hardening Off Tips and Hacks

  • Always start seeds and buy plants that are rated for your zone — that is fast-maturing plants for lower number zones, and drought and heat tolerant plants for higher number zones.

  • Wait until all danger of frost has passed, and average overnight temperatures are warm enough to maximize flowering and fruit set — 10C/50F for tomatoes, 12C/55F for peppers.

Several varieties of peppers for friends and family, not ready for hardening off until overnight temps reach 12C/55F.
  • Place seedlings in a clear plastic, high-sided storage bin for easy transport and shelter from wind. This helps to retain and regulate soil temperature as well.

  • Place seedlings under the eaves of the house to keep them dry and/or borrow radiant heat from the mass of the building.

  • Place seedlings under garden furniture (chairs, small tables) to provide shade, moving the furniture from less-sunny to more-sunny spots over time.

Move seedlings in and out from under garden furniture to provide varying degrees of sun and shade.
  • Utilize dappled shade under trees to provide moderate sun exposure over longer periods of time (this works well if you just can’t be home to move plants back and forth).

  • Hang an inexpensive white shower curtain liner or an old sheet from a tree, sun umbrella, clothes line or other structure, to filter light and extend ‘safe’ time outdoors.

  • Place seedlings in a wheelbarrow or on a garden cart, to make moving from shade to part sun, to sun, as needed. Note that a wheelbarrow protects from wind, and a garden cart exposes the bottom of plant containers to cold and wind.

A re-purposed stock pot, modified with castors, makes annual hardening off of an over-wintered olive tree easy.
  • In windy areas, or areas threatened by late frost, plant seedlings in deep containers with high sides, that is don’t put as much soil in the container as you might otherwise. Tender greens benefit from this built-in windbreak. This strategy works well for winter greens also.

  • Keep watering to a minimum, and water at the base of plants only, unless directed otherwise by a plant tag. In lower number zones, wet foliage and roots can lead to fungal diseases, and in high number zones, water droplets can magnify the intensity of the sun.

  • Use shade cloth (40% rating works well) to minimize the intensity of the sun. Hang it vertically, string it up horizontally like a tent fly, or drape it over a greenhouse. Available online at a reasonable cost — buy off-season for greater savings.

40% shade cloth draped over our small greenhouse allows me to harden off tomato seedlings 'in the shade'.
  • Bulk potatoes, beets, carrots and onions often come in net/mesh bags. Cut open so they lay flat, these work really well as mini shade cloths.

  • Companion plant flowers like sweet alyssum, borage, and nasturtiums with tomatoes to attract beneficial insects; planting basil, garlic, and onion alongside help repel predator insects and make menu planning easy.

Mix of vegetables, herbs & companion plants move in and out of the sun and rain before moving to a friend's garden.
  • Companion plants like rosemary, tomatoes, Swiss chard, parsley, cucumbers, and carrots are beneficial to peppers, though it is not recommended to plant beans and brassicas (cabbage, etc) in close proximity.

Suggestions For Hardening Off in Zone 7B/8A (Tomatoes and Peppers)

For plants started from seed, indoors in your home:

  • Wait until all danger of frost has passed, and average overnight temperatures are warm enough — 10C/50F for tomatoes, 12C/55F for peppers.

  • Place seedling pots in a solid bottom plant tray or clear plastic bin.

  • Day one: on a warm (preferably dry) day, place the tray outside in the shade (or out of direct sun), under cover and right next to a wall of your home. They can stay outside for half of the day, before returning to their indoor growing station.

  • Day two: repeat this process, but leave the plants out for a full day before bringing them inside.

  • Day three: repeat this process, but set plants out in dappled shade for a full day before bringing them inside.

  • Day four: repeat this process, but expose the plants to full early or late day sun for several hours (balance of time in dappled shade) for a full day before bringing them inside.

Staked Sungold tomatoes test-driving their environment, before being planted this weekend in three such planters.
  • Day five: assess your plants. If they are happy and seem ready to plant in their forever home planter or garden, then move the individual pots to those sites and leave them in place for one full day and night (morning watering if necessary) to acclimate.

  • Day six: Transplant seedlings into their new homes. Bury 1/2 to 2/3 of tomato plants (not peppers) in the soil, removing side stems as you go. The small hairs along the stem of the tomato will turn into roots — more roots allow for more absorption of soil nutrients, and provide better structure and strength to the plant. See Tomato Tips & Tricks for this and other general tomato growing advice.

* Note that if the weather is unseasonably cold or unseasonably hot, or changes from one to the other very suddenly, the hardening-off period could extend to two weeks, as you change strategy mid-process.

Roma tomatoes, cucamelons, Mexican tarragon, and towering basil, transitioning from indoors before hardening off.

For seedlings purchased from a nursery

  • Wait until all danger of frost has passed, and average overnight temperatures are warm enough — 10C/50F for tomatoes, 12C/55F for peppers.

  • Place seedling pots in a solid bottom plant tray or clear plastic bin.

  • Day one: on a warm (preferably dry) day, place the tray outside in the shade (or out of direct sun), under cover and right next to a wall of your home. Leave outside at night.

  • Day two: move the trays to dappled sun/shade for the day. Leave outside at night.

  • Day three: move the trays to a sunny spot for the day. Leave outside at night.

  • Day four: assess your plants. If they are happy and seem ready to plant in their forever home planter or garden, then move the individual pots to those sites and leave them in place for one full day and night (watering if necessary) to acclimate.

  • Day five: Transplant seedlings into their new homes. See above.

So that’s about it from me for this week. I hope that in sharing my hardening off tips and tricks, I have encouraged you to give it a try yourself, or at least worry less about it.

Happy growing!

To learn about urban permaculture and permaculture principles, watch Modern Farmer Magazine's Urban Permaculture video series in which publisher Frank Giustra and I introduce the basics.

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