Hardening off seedlings

Hardening off seedlings is a necessary step in preparing your new seedlings for the giant leap in to the wilds of your garden. For those of you new to the seed planting process and its steps, ‘Hardening off” is the name or term give to the steps that “harden” or toughen up seedlings, ensuring they survive their permanent move to their new outdoor conditions.


Tiny seedlings

Tiny seedlings


Hardening off  dates  depend on individual plants and checking the corresponding seed packages will give the best temperature and approximate date for planting them in the garden or outside planters.   Combining this information with the predicted spring temperatures for your region, will allow you to pick the best actual planting day. With this date as the final goal, hardening off seedlings should begin 7- 10 days in advance.

From the warm, moist environment under grow lights or on sunny window ledges, the seedlings need to be gradually exposed to the differences in ultraviolet levels and temperature. In addition there is the physical impact of wind, rain and even garden creatures that can affect their ability to firmly root in the garden. In addition to the changes the tiny plants will face, the structure of the stems and leaves needs to be toughened up.


Plants natural Defense Layer

Specifically the outer waxy layer of the seedlings’ stems and leaves, known as the cuticle, needs to thicken up to prevent seedling death. This protective substance is formed by the outer epidermal layers of the plant to keep the moisture in the plants as well as to minimize sudden changes in temperature. Basically its role is to provide a good barrier between the plant and the environment around it.

Part of Hardening off seedlings then is moving them gradually from the pampered state they live in under our care, to ensure they adjust well to a new harsher environment. According to resources, there are a few things that can be done inside to help the tiny plants begin to form thicker cuticle layers.


Cuticle thickening

Indoor plants in ideal conditions tend to have thinner outer layers and longer, thinner cell structures, both of which make them highly susceptible to breaking off in the wind, drying out quickly and wilting even due to temperature fluctuations.

Allowing the seedlings to dry out between watering is a simple method to force the cells to shorten up and form a thicker cuticle in response to  evaporation  and as the plant tries to prevent any further moisture loss.. In addition a fan or even gentle manual stimulation by hand of the stems and leaves also encourages more compact plants and thicker cuticle formation.





Hardening off Seedlings Schedule

As mentioned, hardening off seedlings can be timed to optimal outside temperatures based on information from seed packages, or simply following the weather patterns for your area and the generally accepted date for the last frost. In south-western Ontario Canada, the Victoria Day long weekend (approximately 15-20th of May), is considered the safe planting time for other than fall hardy plants such as the annual pansies and perennials. Seedlings of annual and most vegetables need to be adjusted for 7 – 10 days starting after May 1st


Hardening off Aids

1. Sun rooms or enclosed porches

2. Greenhouses

3. Cloches

4. Cold frames


Sun rooms or enclosed porches

Hardening off seedlings does not require additional equipment, but there are various versions of plant and seedling protectors, depending on whether you move them outside directly or place them closer to outdoors by moving them to a protected environment such as a glass encased back porch or sun room   If these areas are used, then moving the seedlings earlier is fine as wind, rain, pests, temperatures and light levels are still more regulated than directly transferring to outdoors.



Greenhouses, of course, are a dream come true to many gardeners and the ultimate in starting and protecting seedlings.  Hardening off here can be done by opening the glass during the day and closing it at night to keep the heat in. Of course if the greenhouse itself has additional heating, this should be turned back over a few days and then turned off completely. If none of the glass (or plastic) panels can be opened, then hardening off seedlings must be done by gradually moving them outside in the same manner from inside the house or the porch.

Mini greenhouses with plastic protection are currently available at big box and other garden stores in a price range the average consumer can afford. These come in many sizes, with role up doors, windows and other types of vents. Smaller units even have wheels so they can be used inside or in a porch and be rolled out for gradual seedling adjustment and rolled back in the evening.


Portable greenhouse in sunporch

Portable greenhouse in sun porch



Cloches or bell jars originally made of glass have been used for hundreds of years, by the Italians, French (Cloche is French for Bell), Dutch and the English.  They speeded up plant growth, kept moisture in, as well as wind and bugs out. Their use in North America had decreased over the later part of the 20th century but has been gaining in popularity again.  Traditional glass cloches had no vent openings and could steam the seedlings if not removed early in the day.

With the invention of various types of plastic and the Do-It-Yourself movement, many other variations are currently available at local garden centres or by mail order. The internet provides directions for several homemade versions including plastic milk bottle cloches where the bottle top can be removed to prevent heat build-up. When using cloches of any type, then hardening off seedlings from inside still needs to be done in stages before permanently planting them, to ensure lighting changes promote good growth with no steamed leaves.


Victorian cloches in a walled garden

Victorian cloches in a walled garden


Cold frames

Cold frames are transparent lidded structures that are placed low to the ground, to protect plants against the elements. Although kits are available  generally most are home building projects with    wooden frames have a hinged old glass window or sheeted plastic top that can easily be propped open for ventilation during the day and closed at night.  Due to their size in a home garden, cold frames can be  easily moved to ensure optimal light and  generally constructed with a sloped lid also ensures good light and rain runoff.


a Typical Coldframe

a Typical Coldframe


Basically these cold frames are used as unheated green houses, although heating cable can be used in them.  The micro-climate provided her promotes good see germination and hardening off. In addition they are used to extend extending the growing season by leaving them in place with the top open all summer and then closing it in the evenings once the cooler weather returns. Whether from indoors or greenhouses can be done by moving them in to a cold frame before final in ground planting.


Hardening off Points

– Allow 7- 10 day adjustment period

– begin moving seedling s outside in the early morning

–  put them in a sheltered shady spot like under a tree for approx. 3 hours the first day

–  bring them back in at night

–  increase their outside exposure by 1- 2 hours a day

–  after 2-4 days move them to early morning sun with afternoon shade and in at night still

–  after 7 days they give all day sun and stay out when night-time temperature over 15°C

–  when 7-10 days have passed ,transplant seedlings in to the ground on a cloudy day

–  water well


Additional Notes:  Cold frames or Cloche use

–   if seedlings  are being relocated to the  them begin with the same approximate schedule

–   3-4 hours of open exposure in the shady area for a day or two for light adjustment

–   open lids or remove bell during the day and close/cover in the evenings

–   after 7-10 days seedlings can be planted into the ground and frame /cloche can be removed entirely

–  for veggie crop extension cold frame lid can be left open all summer and used as above in the fall


seedling in ground

Hardened off seedling happy planted in the  ground








Seed planting in Spring and caring for sprouts

 Seed Planting in Spring

Seed planting in spring and caring for the sprouting seedlings are always popular pastimes and certainly April is one month that has Northern Hemisphere gardeners chomping at the bit.

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Whether you had carefully scrutinized seed catalogues for your purchases or chosen them from the countless seed displays in local stores, you will have noticed the huge variety of seeds available!  Perhaps you collected seeds collected from last season’s garden, just as those in the Southern Hemisphere may be doing now as their gardens go dormant as colder weather approaches.

While many seedlings require different germination needs as previously written on this site, no matter  whether gardeners are working in large gardening centres or toiling over their grow lights and window sills, they are all hopeful those conditions have been met.


 Despite the complex process a seed goes through before the first sprout appears, from large global gardens such as Viceroy’s Palace Garden in India and Tsarskoe Selo Russia, through to massive garden shows like Canada Blooms to our small but priceless gardens, each carefully planted seed carries with it the hope and perspiration of many hands.

Now that seeds are under the lights, or on a sunny window ledge the tricky part begins. Keeping a few simple points in mind should ensure the sprouts you watch daily, will grow tall and healthy awaiting their new home in your garden or planters.


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While the information on temperature, light and moisture levels seem pretty straightforward, to novice gardens especially, these both require careful monitoring. In addition, fertilizing is important in the right strength, to help ensure healthy hearty plants come from your seed planting in spring.



If possible, providing a source of gentle warming from beneath is a valuable tool, enabling most seeds to germinate faster as it mimics the warm soil of your garden or planter. Certainly the top of refrigerators were once the site of many seed tray and seedlings, the newer model appliances give off very little heat. However, if they allow good sunlight, this would still be a good location for both new seeds and struggling seedlings.

The temperature requirements of most seeds are on the packages, but generally whether purchased or collected; seeds and seedlings both have minimum temperatures for good growth. Generally this is approximately 65 º F or 18 º C but above 68 º or 20 º is better. In addition, the optimal temperature is one if many pieces of information on the seed packages.

Heated seed tray starting kits are a big help as the try comes with a heating pad designed to warm the seeds from the bottom at an even temperature. Unfortunately on the one I purchased the actual temperature was not listed but the seeds sprout much faster than the comparable one in the small trays beside it.


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If you live in an older home heated by radiators, covered in a non combustible material, trays could be placed there for warmth but monitor the area regularly so they are not baked instead.

There are several problems that can arise if the temperature needs of the sprouting seedlings are not met. The first problem with lower temperatures is the metabolism of the seed/seedling is slower. In addition, the cooler temperatures combined with generally high moisture levels are good breeding grounds for fungus which may result in the disease or condition know horticultural as damping off.


Seeds generally require a good source of light and in fact, some require so much to germinate that the packages suggest little to no coverage of the seeds. In addition, some plants do not grow well if planted early indoors and transplanted. I have had little success transplanting sunflowers and in fact, the package instructions say just to plant in the garden after danger of frost is past. While the root systems may not support the transfer, more often than not the plant’s metabolism requires a high level of natural sunlight, hence straggly plants even when under grow lights.

If you do not have a sunny, no draft location for your seed trays, pots, pellets or whatever you planted in to, perhaps grow lights are for you.  There are many locations from larger big box stores and most most large garden centres  that sell tiered, fluorescent lit plant stands. Another good source to check out are garage sales, or on line used items sites as new tiered light stands are generally quite costly.


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With the availability of new natural daylight frequency bulbs, often even a desk lamp or shop lighting arrangement can be used for seed planting in spring and  to give seedlings the light boost they need until they can be transitioned to outside areas. Of course there are many home made lighting set ups people have come up with, including adding foil around a light source to keep stray light at a minimum. In addition the increased warmth from the lights also helps with maintaining a good even temperature in the growing area.

To ensure good even lighting, start off with the grow lights approximately 6 inches or 15 cm from the seedlings and raise them as the sprouts grow. If the lights are too high above the greenery, the plants will grow taller and spindly as they try for more lights. In addition the lights should be on 16-18 hours a day to ensure the growth of sturdy  not tall thin plants.  Good, adequate lighting  promotes good leaf formation and strong stems,  and helps prevent  damping off.

Moisture levels

Water levels as any gardener know as crucial to plants, but with seedlings it is a bit tricky. Certainly the peat pellets, coir pots, mini greenhouse, coffee cups and pots that have been carefully planted are being watered regularly, but with the extra light and greenhouse like conditions watering may be a challenge. Of course it is highly recommended to either water via mat watering system or use a spray bottle to keep them seeds moist without disturbing them.


Mini Greenhouses on self-wicking water mat

Mini Greenhouses on self-wicking water mat


If corms or tuberous plants such as begonias or dahlias are started now to get a jump on the outside growing season, watering with a small container, gently on to the media is best to ensure thorough watering.

Begonia corms    sprouting

Begonia corms


Possibly the easiest thing for garden novices especially is not drowning the seeds they plant in the spring. While the soil are growing media needs to be moist, try to ensure a consistent source of water by thinking of the seedlings as larger more mature plants. Most plant lovers know they do not need to water a plant every day, but when the soil surface is generally dry. Of course while there are always plants with more exacting water and lighting requirements, this is generally a good run of thumb that should be applied to the younger plant as well.

If there are tray covers on holding in the moisture, check the surface of the media used for possible mould formation. This is also possible when contaminated medium or dirty pots have been re-used. Provided the sprouts are up and growing fine, consider opening any vent on the lid or shifting it to allow more ventilation while maintaining a moist atmosphere over all. If you have rigged up plastic to create a greenhouse effect or are indeed using a greenhouse, remember circulation is vital, so create an opening and consider the use of a fan for good air movement.

Remember not all seeds even of the same plant will sprout at the same time so do not leave the lid on waiting for them all to catch up, so take the lid or dome off when a good percentage have sprouted and mist the rest. While moisture is required, at this point especially, good circulation is a must.


striving for better air flow

striving for better air flow



One element often overlooked with seedlings is when and if they need fertilizer. Remember nutrients are important but the seed embryo itself contains all the food the new sprouts will need until they have developed true leaves, not the first ones formed. In fact, according to several expert sources, even mild fertilizer can kill the seedlings by over accelerating their metabolism and burning them.

If the growing medium you used contains no fertilizer, begin weekly feedings with a ¼ strength dilution of fertilizer, whether synthetic or organic. Keep in mind for most plants higher amounts of Phosphorus for good root and plant formation. In addition, if Coir pots or media are used, that coconut fibre product can compete with the new plants of Nitrogen so a more balanced N-P-K (Nitrogen-Phosphorus-Potassium) ratio is best. After a week or two ½ strength can be used.

Full recommended dosing is not advisable until the plant is larger and ready to be acclimatized in to a cold frame, or shady area on the way to its permanent home. Another option at that point is mixing a granular organic or synthetic slow release fertilizer in to the soil when the seedlings are moved.

Additional Suggestions

There are countless sites, books, garden experts and even gardening forums that offer a variety of information and support and a few are listed below:




Final notes:

While to some, planting seeds in the spring is not something they are interested in, others find it very rewarding. To those of you who cannot spare the time but love to garden, remember most garden centres carry basic perennials, annuals, vegetables etc. at a reasonable cost so you too can have a garden of your own without breaking the bank… so whether it be seeds, sprouts, veggies, ferns, flora or fauna…let your garden inspire you…and happy Spring wherever you are!

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