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.
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.
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
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.
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.
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.
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