Hardening off plants grown indoors

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Hardening off plants grown indoors is important, especially for seedlings that are transplanted outdoors to grow into mature plants. This protects your plants from pests, diseases and general hardships of cold weather and early frost.

One technique we use at the farm, and have used in our greenhouses and our open garden, is called “cold hardening” or “frost hardening”. This was developed many years ago by farmers, and originally just meant waiting for cold weather, when the temperature would drop below the freezing point of water. A cold winter was an easy way to help young plants get established in the spring. The farmer in this situation would just need to be patient and wait for the temperature to drop to the point where it would cause the frost to sublimate or drop to the ground.

When the farmers came indoors, they would remove all the frost and clear off any snow from the plants and start watering them. They would move the plants from the colder outside areas into a heated greenhouse, where the plants would dry off and get ready to grow again. They would then give the plants a good hard water and mulch to give them a chance to establish themselves for the rest of the winter.

The main problem with this practice is that a lot of growers do not realize that they have the opportunity to do this hardening for plants they have in their own greenhouses, rather than having to wait for the snow to start and fall. It was never meant to be a long process, just a way to reduce the damage to the young plants during the first hardening down period. With the recent advances in grow lights, which can keep a plant almost inside year round, it is possible to harden plants in their indoor grow light right up until the first frost of the year.

A second problem is that our local farmers are not willing to give us a lot of water in winter for our plants. The water is needed for the soil to warm up, but in the winter they just do not have the extra water. The amount of water needed for the plants is actually much less than you would think. At this point in the winter, any good commercial grow light will provide about 3 times the amount of light needed to grow as long as it is over the plants and not to the side. This means that a 4 foot light on a south facing wall of a heated greenhouse should only need 12 hours a day, and even more in the summer. If you water the plants enough, you will get them almost ready to grow in the greenhouse any time of year.

I would like to add here that just because you are a good grower of plants does not mean that you have a good climate in the winter. The ability to grow in the winter or in the cold really comes down to how much water you have available to them. The plants that are grown in commercial greenhouses, or as greenhouse crops have very easy access to water, and in the winter it is pumped into the greenhouse. These greenhouses are also not as cold as a traditional greenhouse and the greenhouse plants get a lot of heat from the greenhouse heaters. We keep many different plants year around in our greenhouse, and they keep them growing in the winter with access to our tap water, and even a few plants in our outside cold frame even through the first frost of the year. We keep all of our plants watered through the winter, and have plenty of warm days through the winter.

We just use a simple watering system that can be seen in the diagram on the next page. The water is from our water source, and is heated by a heater, and then a heat exchanger sends the heat to our plants. The water then flows into a large holding tank, and then to the greenhouse through gravity. In the winter the plants are not getting more than a few inches of water, but I think they could survive a lot more than that. We have always had a good winter growing a lot of plants, and have even had winter killing freeze-outs without causing any stress to the plants.

In the summer the water comes from a drip emitter, and the tap water is also flowed to our outdoor water system. As this is where we have the most water use, we have to use our drip irrigation system to get it to the greenhouse. This means there are no more pumps and no more electric, but it also means it is more flexible as to how many plants we can grow. You also don't have to worry as much about your garden hose running out or needing to go to a pump to re-fill it in the summer.


In early spring, when the soil is still frozen, the seeds can be planted into little cups in the greenhouse. We typically start about 7-10 seeds in every cup, and by the time the seeds sprout, the seedlings are transplanted into larger pots. As the weather warms up we can go from seedlings to plants. While the greenhouse gets to about 40 degrees, plants can't keep growing because the roots can't tolerate cold weather. It is important that they are moved from the greenhouse into the root cellar as soon as possible to avoid any freeze damage. Once the soil temperature gets into the 40s, the greenhouse is closed and any plants not moved are moved outside. It is worth checking the temperature of the soil in the root cellar once it gets up to about 40 degrees. This is because if it does get above 40 in the greenhouse, the roots inside the greenhouse soil will begin to freeze. I have never had a problem with the roots growing inside the greenhouse soil and I would say I think there are fewer problems if the soil is about 5 degrees colder. If it is cool outside or warmer, you may be able to just turn off the heat and keep the heat vents open. The last thing you don't want to do is close the vents and forget about it. We've had some extreme weather over the past two years, and I have lost a couple of seedlings because the vents were too hot. By the time the seeds or the transplants had germinated and were ready to be moved, the roots had been exposed to too much heat.


I use sand as the base of the compost because it is easy to get, it is biodegradable and it is cheaper than using a peat/fir/wood combination. It is important to keep the sand dry. Once it starts getting wet, the compost can begin to rot. The first thing you will notice when it gets wet is that it will smell and you will also notice the worms not be able to move. I also like to keep the pots out in the sun to dry the compost as much as possible. Once they are dry I place them in the root cellar to prevent it from getting wet again.


To build a layered compost I use four bags, one layer each of compost, hay, straw, straw and hay.

First I mix the compost and water to help with the decomposition of the material. Then I pour it into the base of the pot. I don't put it in and then water it in the same way as a normal compost. I want the compost to be somewhat dry before I add the layer of hay. This way the layers are more homogeneous. I then water the mix and let it stand overnight. Then the next day I take the compost out and water it again, followed by a second layer of hay. Then I add the final layer of straw, then water it and let it sit. Once the compost is full I put it in the root cellar. It is important to wait until the compost is dry before placing it in the root cellar. During a heavy rain the compost will continue

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