Hardening Off Seedlings and Plants

20170513A Tomwsulcer, WC

Windowsill-grown plants need a period of hardening off before they can face outdoors conditions. Photo: Tomwsulcer, Wikimedia Commons

You’ve started plenty of seedlings or cuttings indoors or you have houseplants to move outdoors for the summer. No problem: you can do that! Still, it’s important to always acclimatize them to outdoor conditions before placing them out for the season. Or as gardeners say, to harden them off.


Plants raised indoors live under near-ideal conditions: it’s always warm, the light level is pretty stable, plus there is no wind to break their stems or rain to flatten them. Furthermore, most of the sun’s ultraviolet rays, the ones that give us sunburn if we aren’t careful, are filtered out by window glass, nor are they given off by the fluorescent or LED lamps we use to light our plants. As a result, our tender, mollycoddled plants will be in for quite a shock if you suddenly stick them outside in the real world where the conditions are always changing and those nasty ultraviolet rays beat down.

Plants placed outdoors too quickly, especially if you place them in full sun, often suffer from damaged or dried leaves (they are said to have “burned”), their stems can flop or break. In fact, many will be killed outright.

Hardening Off

The purpose of acclimatization or hardening off is to toughen up plants that are transitioning to outdoor life. By gradually exposing them to more intense ultraviolet rays and to wind, rain and temperature changes, you can help them adapt to move. As they cope, they start producing a thicker cuticle on their leaves, stiffening their stems with more lignin, adapting their growing system to changing temperatures, etc.

You can acclimatize most plants fairly readily, in about 7 to 10 days.


Any spot where you can find shade is a good place to start, even if that’s only under garden furniture.

If you intend to plant out at about the date of the last frost in your region, that means you’ll have to start hardening them off before the date of the last frost, when there is still a risk of cold. In years when it’s colder than normal at planting-out time, it may be wise to delay acclimatization … and also planting out.

Under ideal conditions, it will be at least 60°F (15°C) the day you start to harden your plants off. Place them in the shade in a spot somewhat sheltered from wind. If temperatures are expected to dip below 50°F (10°C) at night, it would be wise to bring them back indoors in the evening. If there is to be a beating rain, move them to a shelter. Otherwise, they can stay outside. Repeat that for 2 or 3 days.

Now find a location in part shade in a windier spot, one that receives some direct sunlight, especially in the morning, but shade for the rest of the day. Or sunlight filtered through tree leaves. Normally, the temperature will have warmed a little bit since the beginning of the acclimatization period (that’s just way spring goes: we tend to gain a little heat almost every day). Again, place the plants there and, if possible, leave them out at night too. But by now they’re already tougher and can handle 45°F (8°C) nights. And again, this step can last 2 or 3 days.

The last step is to place the plants out in full sun and wind for yet another 2 or 3 days, leaving them it outside day and night if possible. (Obviously, with plants that will be spending their summer in shade or partial shade, exposure to full sun won’t be necessary.)

When all goes well, your plants will then be ready to face outdoor conditions, no matter where you place or plant them.

Greenhouse Acclimatization


One of my temporary greenhouses: they partially harden off young plants.

If you’re like me, you start a lot of plants indoors where it’s evenly warm, then move them to an unheated greenhouse or cold frame when they start to grow. Such structures are heated by the sun during the day, while cooling off at night yet remaining above freezing.

This is a major step towards acclimatizing your plants to outdoor conditions, since they start to undergo cooler nights than indoors, plus a bit of wind and more ultraviolet rays: that’s because you’ll be opening the door or panels on hotter days. They’ll have begun to experience temperatures closer to those of outdoors and more ultraviolet rays will have reached them than would have been the case inside your house. As a result, they’ll be receiving near-to-outdoor conditions and will be naturally sturdier than windowsil-grownl plants. Even so, it’s still wise to harden them off just a bit more, especially to those nasty ultraviolet rays, by placing them outside in a spot protected from full sunlight for 2 or 3 days before you plant them out.

When Weather Throws You a Loop

Obviously, everything doesn’t always go as planned. If you hear frost is expected, or even near-freezing temperatures, during the hardening-off period, temporarily cancel the experiment and move the plants back indoors, even if that’s only overnight in a garage or tool shed. Even temperatures of 45°F (5°C) early in the hardening-off process can send the plants into a state of shock that will considerably delay their growth, not something you want.

If conditions are such that you have to keep them under cover for more than 48 hours, consider the acclimatization efforts you’ve made so far to have been in vain, as plants “soften up” pretty much as quickly as they harden off. Simply start all over. Sorry, that does sometimes happen.

Generally, however, acclimatization does carry on pretty much as you planned and you’ll soon have some very tough plants perfectly ready to face a summer outdoors!20170513B


A Seed-Starters Glossary


20150414April is the main season for sowing seeds of vegetables, annuals, herbs, etc. indoors. Garden centers are presently full of displays of seed packets of all kinds and there is an even bigger choice when you order by catalog, either by mail or online. But what about the mysterious vocabulary seen on the back of seed packs and in printed and virtual seed catalogs? For many beginners, it’s like a foreign language! To help you, here are a few terms you may encounter and their definitions.

Acclimatization: A vital action taken just before transplanting seedlings outdoors. It simply means to place the seedlings, still in their pots, outdoors in the shade for 2 or 3 days, then in partial shade for 2 or 3 days, then in full sun for 2 or 3 days. The seedlings are now “hardened off” and ready to transplant to their permanent location.

Annual: a plant that completes its life cycle, from germinating to seed production, in one single year, then dies. Ex.: cosmos, marigold, sunflower.

Biennial: a plant that completes its life cycle in two years, usually producing a rosette of leaves the first year and flowers and seed the second. It dies after seed production. Ex.: foxglove, parsley.

Perennial: a herbaceous plant (not woody) that lives more than two years and that blooms more than once. It does not die after flowering.

Botanical Name: see Scientific Name.

Bush-type: see Non-Running.

Chlorosis: when leaves contain insufficient chlorophyll. They are often pale, yellow, or yellow-white. An iron deficiency, or lack of iron, is a common cause of chlorosis. See Nutrient Deficiency.

Cold Treatment (Stratification, Vernalization): subjecting to cold seeds that must go through a cold period before germinating. Usually they are sown in pots and placed in a refrigerator for several weeks before exposing them to heat. An alternative is to sow them outdoors in the fall in a cold climate where they will naturally undergo cold temperatures. Many perennials, shrubs, and trees from temperate climates require a cold treatment.

20150414BCotyledon: a seed’s first leaf, usually simple and often very different appearance from mature leaves. Most seedlings have two cotyledons, but there are seedlings with only one cotyledon, more than 2 cotyledons and without any cotyledon.

Cross-Pollination: The transfer of pollen from the anther of a flower of one plant to the stigma of a flower of another plant of the same species. It is usually carried out by insects, birds or wind.

Cultivar: a plant raised and multiplied by humans, that does not exist in nature. It’s name is typically written between single quotes (‘   ‘). The name derives from “cultivated variety”. Ex.: in Cosmos bipinnatus ‘Sonata White’, ‘Sonata White’ is the cultivar name.

Determinate: said of a tomato plant (and a few other plants) in which each branch ends in a cluster of flowers, which therefore limits its upward growth. Determined tomatoes make fairly small plants and don’t always need staking. They tend to produce all their tomatoes at about the same time.

Indeterminate: said of a tomato plant (and a few other plants) whose flowers appear in the axils of branches and not at the stem tip. Therefore the stem continues to grow in height throughout the growing season. These tomatoes need staking or a large tomato cage. They may produce less fruit at once than a determinate tomato, but usually do so over a long harvest season and often give double or triple the yield of a determinate tomato.

Dioecious: refers to a plant whose male and female flowers are borne on different plants. The asparagus is dioecious.


On this female flower (monoecious), you can see the ovary that will later become the fruit.

Monoecious: refers to a plant that produces separate male flowers and female flowers on the same plant. Often the female flower is easily recognized because it has a small ovary at its base in the shape of the fruit to come. Squash, melons and cucumbers are monoecious.

Perfect Flower (Bisexual Flower, Hermaphroditic Flower): said of a flower that has both male and female organs, thus both a stigma and stamens. This is the most common situation in nature.

Direct Sowing (Direct Seeding): sowing a plant directly outdoors where it is to grow. Beans, marigolds, and corn are often direct sown.

Do not cover: said of a seed that should not be covered with soil at sowing, usually because it is either very fine or requires light to germinate, or both.

GMO: genetically modified organism. Said of a plant into which humans have inserted genetic material from another plant or even an animal without going through pollination. There is, for example, corn containing the genes of Bt genes (a bacteria) and varieties of canola and soybean which with inserted genes that make them resistant to herbicides.

Hardening Off: see Acclimatization.

Heirloom Vegetable (Heirloom Plant): an old variety. Some authorities consider a plant having been introduced more than 50 years ago to be an heirloom variety, others prefer the definition “before the 1940s”. Most heirloom vegetables are produced through open pollination, that is pollination carried out by insects, birds, or wind. Examples.: ‘Brandywine’ tomato, ‘Golden Bantam’ corn, etc.

Hybrid: plant resulting from the crossing of two different breeds, species or genera. F1 hybrids are the most common type of hybrid and are the result of a first generation cross (F1 means “1st filial generation”). Usually, F1 hybrids are more robust than non-hybrid plants, but more expensive, because they have to be manually pollinated in a greenhouse setting. F2 hybrids, less common on the market, are seeds of F1 hybrids, thus the second generation (2nd filial generation). They are cheaper, as they are generally produced by natural pollination, but tend to give less reliable results than F1 hybrids.

Last Frost Date: the date used to calculate when to plant tender plants, referring to the approximate date when you can expect the last spring frost to occur. On seed packs and in seed catalogs, you’re often told to plant or sow outdoors so many weeks (6 weeks, 8 weeks, etc.) before the last frost date. You can ask a local garden club or garden center for the last frost date in your region, then simply count backwards to find the right date for sowing seeds.

Latin Name: see Scientific Name.


In the case of a nutrient deficiency, it’s hard to tell which mineral is missing: in this leaf, the damage could be a lack of iron or of nitrogen. Applying a complete fertilizer will solve both.

Nutrient Deficiency: results from the lack of a vital mineral in the soil (phosphorus, potash, nitrogen, zinc, boron, iron, etc.). It can have various symptoms include discolored or deformed leaves or slow growth. Treatment with a complete fertilizer (containing all the trace elements, such as a seaweed or fish fertilizer) will usually overcome a deficiency.

Organic: various definitions. Organic seeds are harvested from plants that have not been treated with synthetic (that is to say, chemical) pesticides or fertilizers. Organic pesticides are derived from natural sources, not from chemical synthesis.

Running: said of a squash with long creeping stems that require a lot of space in the garden, like a pumpkin. This is the natural state for squash.

Non-Running (bush type): said of a squash that produces a short stem and a rosette, taking up less space in the garden than a running squash. The zucchini is the best known non-running squash.

20150414CPeat Pot: pot made of pressed peat, usually used for seedlings that will not tolerate transplanting. The peat pot allows roots to grow right through its sides and bottom and can therefore be transplanted into the garden without removing it. The roots of the plant will then grow right through the peat pot into the surrounding soil.

Pelleted Seed: seeds covered in a product (usually clay) which facilitates their handling.

Pinching: removal of a plant’s terminal bud (bud at the end of the stem). Pinching stimulates branching, giving a more compact and attractive plant, but may delay flowering. It is traditionally done by “pinching” the top growth between the thumb and forefinger, but can also be done using pruning shears or scissors.

Requires Light to Germinate: said of a seed that germinates only in presence of light, be it sunlight or artificial lighting. These seeds should be sown without covering them with soil and the pot should be placed in a brightly lit spot.

Scarification: action of filing, nicking, or cutting a seed before sowing it. It can also involve soaking it for several hours in warm water. The goal of scarification is to penetrate very hard seeds (morning glories, hibiscus, etc.) and thus accelerate their germination.

Scientific Name (Botanical Name, Latin Name): it consists of two words, the first being the genus name (name shared with related plants, much like a human surname) and the second, the specific name, which determines the plant accurately. For example, Solanum tuberosum is the scientific name of the potato and Solanum melongena, of the eggplant. Both share the same genus name, Solanum, because they are closely related, while the specific name serves to make it clear to which type of Solanum the writer or speaker is referring. The scientific name is usually written in italics when possible.

Self-fertile: refers to a plant whose flowers can self-pollinate, that is to say that its own pollen can ensure seed production. Most plants are self-fertile.

Self-sterile: said of a plant which has to be pollinated by another variety in order to produce seeds. Many fruits (blueberries, apples, pears, plums, cherries, etc.) are self-sterile or partially self-sterile. It their case, it is always best to cultivate at least two cultivars of the same species nearby, as cross-pollination is necessary for them to produce abundant fruits.

Stratification: see Cold Treatment.

Thinning: removing some seedlings or fruits in order to allow room for others to grow better. Usually this is done by cutting the excess plants or fruit stalks the base.

Transplanting: moving a plant from one place to another. In the case of seedlings, this is usually from the pot in which they were sown into a larger pot or into the ground.

Treated seed: seed has been treated with a fungicide to prevent rot in cold or wet soils. This treatment is not considered acceptable to organic gardeners.

Untreated Seed: seeds that have not been treated with fungicides and therefore acceptable in organic gardening.

Vernalization: see Cold Treatment. It can also mean subjecting growing plants to cold to stimulate flowering.