This article is part of a series written for HTGSupply by plant biologist Dr. E.R. Myers.
Extending the growing season of plants has many advantages. You can harvest earlier and/or get a greater yield. Starting plants indoors helps plants through the difficult part of their early lifecycle. Plants are at their most vulnerable after they germinate. This is when they are susceptible to fungal infections called damping off*. In order to help reduce damping off you need to have air flow and keep the growing medium moist, but not wet. Furthermore, seedlings have limited resources inside themselves: low energy reserves, low nutrients and a small surface area to collect light. By starting plants indoors you can maximize the needed resources which means maximum plant growth. The most critical and influential time to manipulate a plants growth and help it past potential pitfalls is during the first few weeks of its life. Starting plants indoors allows you to put out young vigorous plants at the very beginning of the growing season. Before plants can withstand the wind, rain and strong sun in the outdoors, tender young plants need a period of gradual adjustment. The simple but crucial process of acclimating seedlings to life in the garden is called hardening off*. But before you can get your plant ready for the outside, you need to germinate the seeds.
A seed consists of an embryo (the baby plant), stored food and a seed coat (the protective outer surface). In many cases seeds will not germinate even though all the necessary environmental conditions for growth are satisfied. This phenomenon is termed seed dormancy*. Seed dormancy is important biologically because it maximizes seedling survival by preventing germination under unfavorable conditions. For example, plants grown in temperate regions (where there is summer and winter) would NOT benefit by germinating in the fall right before winter. There are two types of seed dormancy. The first is seed coat induced where the seed coat keeps water or oxygen etc. from the embryo. Large thick seeds are prone to this. This type of dormancy can be broken by soaking the seeds in water for a couple days, or by scratching or putting a small hole in the seed coat. The second type of dormancy is embryo induced. This means something in the embryo prevents germination, usually enzymes or other molecules inhibit the embryo from growing. Environmental conditions eventually break the inhibition such as cold temperatures breaking down the enzymes, or water or oxygen causing inhibitory molecules to leach out of the seed.
How do you know if your seed needs to have its dormancy broken? Most common vegetables which are often tropical plants don’t need any manipulation to grow. In the tropics the best strategy is often to start growing as soon as the seed hits the ground so that plants can get big and capture light. If you are growing a tropical plant you most likely do not need to worry about breaking seed dormancy. Many temperate plants need a cold period before they will germinate so that cold temperatures bring about changes inside the seed. Simply place the seeds in a plastic bag with moist sand in the bottom of the refrigerator for a few weeks. After the time in the refrigerator put the seeds in warm moist conditions and they should grow. Desert plants usually inhibit their seeds with phenols, which dissolve in water so that after there is a lot of water on the seeds they begin to germinate. This is successful in deserts since water it usually the most limiting factor in a desert compared with temperature in temperate regions, and light in tropical regions. Seeds from different parts of the world all have different strategies of how to survive in the wild.
Whether your seeds need to have their dormancy broken or not, all seeds keep for longer periods in cool-cold temperatures. Keep seeds for long term storage in a refrigerator or freezer. Do not put the seeds on the door, since seeds need a constant cold temperature, and the door warms up and down each time it’s opened. Seeds should be placed in labeled containers, with the date on them so that you can keep track of time. This is also a good place to list specific traits each plant had to reference for breeding (look for future articles on plant breeding) Damping off – Infection that attacks weak germinating plants. This is causes by a fungus that likes wet humid environments, so to kill the fungus add air flow and lower the humidity. *Hardening off – This refers to a period before transplanting plants from indoors to outdoors when they must gradually get used to sunlight, cold night temperatures, wind and rain. *Seed dormancy – is defined as a state in which seeds are prevented from germinating even under environmental conditions normally favorable for germination
Seeds will begin to grow when the dormancy is broken AND/OR when environmental conditions are right. Small seeds like those of orchids and basil should be put directly into the growth medium, I use soil and/or Rockwool. Medium and large seeds can be started with a plastic bag and paper towels. You will use the paper towel as the medium, get it only wet enough so that no water drips from it as you hold it above a sink. Keep the towel moist, but not wet until the seeds begin to germinate. You can put the paper towel in a bowl with plastic over the top or in a Tupperware container with the lid NOT sealed shut. You can also put the seed into a plastic bag; I usually put the seeds on top of the paper towel when I use a plastic bag so that I can see when they germinate, or if gray fungus is growing on them. The nice thing about germinating seeds with a paper towel is that you can check the progress of seeds. The first few days a seed starts germinating, often only the roots grow, by putting seeds in soil you won’t know for days if you have a plant growing in a container or just wet dirt. However you germinate the seeds put the seeds in a warm spot between 70-85 degree F. A lack of cold is more important than high temperatures. Keep in mind that too high a temperature can kill a seed. A good spot in your home would be on top of the VCR or the refrigerator – any continually warm surface would be good. After seeds sprout carefully take them out, making sure not to damage their delicate roots, and put them in pre-moistened soil, Rockwool or starter plugs. Once seedlings germinate you must decide of you are going to do the extended hardening off or the minimal harden off. (See below for details)
I find plastic cups work great as pots for seedling plants indoors. The cups are cheap, disposable and take up little space. The 16-20 oz. size allow for several weeks of growing before becoming root bound. When I use plastic cups I put holes in the bottom of the cup with the plant (and dirt) and put the potted plant/cup into another cup without holes which acts to catch overflow. It is a good idea to put some pebbles or sand in the container cup to allow for good drainage. Select the strongest seedlings i.e. thickest stem, healthy leaves, not necessarily the tallest specimens, and give them the best spot under your artificial lights. If you have limited space in your garden, you will only want to put out the best plants. To get short sturdy plants use lights high in blue light like fluorescents with a Kelvin Temperature Rating of 6500K (a “cool white” fluorescent bulb will work as well) and mild temperatures (less than 85 degrees but more than 70 degrees F). The fluorescent light should be hung within 3” and as low as 1” above the plants for maximum efficiency. After germination, you need to maximize growth and help the plants get ready for life outside. Use an all purpose diluted fertilizer (10-10-10 for example) when you see the first true leaves on the plant. Young plants can benefit from having lights on for 24hrs a day the first couple weeks. Yet another advantage to starting plants indoors is you can give them more hours of light than they’d get outside. Plants will eventually die without a dark cycle (night), but seedlings do not need one for 2-3 weeks, and since light is the energy source for plants, 24 hours of light helps plants grow at their maximum rate. After a couple weeks, using a timer for your lighting system, switch to 18 hours of light and 6 hours of dark. Starting plants indoors maximizes growth but can leave a plant susceptible to the elements outside. You should imitate windy conditions with a circulation fan indoors, plants should jiggle, not be visibly bent over from the fans air movement. Plants exposed to a fan will be thicker stemmed and better able to withstand windy days. Another benefit to using a fan to imitate wind indoors is that airflow will help increase photosynthesis which should lead to faster growth rates. Misting your plants with water will help them have strong leaves that can tolerate rain. The biggest problem plants will face when transplanted outdoors is sunlight. It may seem odd since plants need sunlight to survive but sunlight is the harshest factor to overcome. When you harden off you plants, think about the summer. You shouldn’t go running outside half naked for 12 hours on the first sunny 90 degree day, unless you want sunburn or heatstroke. The same goes for plants. The sun emits multiple types of energy collectively termed radiant energy. White or visible light which is what plants use for photosynthesis and is found in grow lights is only a small percentage of all the sun’s radiant energy. Ultra Violet (UV) radiation and other types of radiation are all part of the suns radiant energy. Just as you need to gradually get used to the sun in the summer so must plants. Plants grown under artificial lights or in a greenhouse under glass must be hardened off. It is a misconception that plants grown in windows or a greenhouse do not need to be hardened off, I have transplanted plants from my greenhouse to my backyard without hardening them off and they nearly died. The plant leaves got brown spots, they did not begin to grow again for weeks and I wasted valuable growing season time. Even if you have put plants outside in the past without hardening off they may not have turned brown but I imagine they did not grow for days to weeks due to stress from not being hardened off. I recommend you try hardening off with some of your plants and compare. I always encourage anyone to test any of my suggestions. True enlightenment comes from within it can not be given.
Start the hardening off process a couple of weeks before you anticipate planting into the garden. Each day you will be moving all the plants outside for increasing lengths of time, allowing them to gradually get used to the sunshine, wind, rain, and other outdoor conditions. Hardening off takes at least a week or more if done properly, the more gradual exposure to sunlight you give your plants before planting, the better. The first day, putting the plants outside in the shade is a good idea. Indirect sunlight still has most of the radiant energy but in lower doses. Once the plant is exposed even to indirect sunlight, genes will turn on that make molecules to protect it from direct sunlight. Similar to how humans make melanin (get a tan) with increased sun exposure. Give the plants more sunlight each day, and make sure they do not dry out.
Minimalist Hardening Off
This hardening off technique starts at germination. It is not recommended if you are using expensive, or limited seeds. If you are going to put your plants out early in the growing season you need to get them ready for cold nights. Immediately after germination you can place seedlings with a moist towel in a plastic bag and put them in the refrigerator. Make sure (with a thermometer) that the temperature does not go below 40 degree F, less than that causes tissue damage in many plants. You can put them in the refrigerator for a week. This will begin to harden off plants for cold. Cold temperatures cause certain plant genes to turn on to make molecules that protect the plant from cold. By exposing your seedlings to cold, it will be easier for the cold protection genes to turn on again once your plants are outside in the wild. This is also a way to slow plant growth, so it will add a few days to the normal growth schedule. After the cold hardening you can put the seedlings in an individual container. If you are going to wait until well into the growing season to put your plants outdoors, you do not need to harden off with cold. The second part of this involves exposing the seedlings to direct sunlight. If you don’t want to deal with bringing a lot of your larger plants in and out of the house before putting them outside, you can expose the seedlings to sunlight when all plants are small and can be dealt with more easily. As long as you keep the seedlings moist, expose them to some direct sunlight when they are small. If all your seedlings fit in a single tray you could put a seedling tray on an open window for increasing amounts of sunlight exposure each day. Any time the plants get direct sunlight the better, even if you just put them on the floor for an hour of sunlight through a window. Make sure the window is open, plants need unfiltered sunlight. As with cold, early sunlight exposure will enable plants to tolerate sun later in life compared to plants that were never exposed to any sunlight (Like those under artificial lights). The minimalist hardening but is not the most efficient but it does not require much work and it increases plants hardiness and growth. Think of this method as inoculating your plants like giving them a vaccine. Plants may still be stressed by the move outdoors, but it will be less severe.
Weather permitting, when your plants start to get bigger, it is time to put them outside. Keep an eye on the weather and listen to the low temperature prediction. Don’t make the rookie mistake of putting plants outside the first warm day in April. If you have the luxury to decide the day you transplant your plants outside, transplant the seedlings to the garden on an overcast day to ease the shock of transition from pot to ground. If a light rain is falling, so much the better. Once you have hardened off your plants they are ready for the outside. Some people suggest limiting nutrients to seedlings while hardening off. I am not sure I agree, if you add fertilizer to you plants when you transplant them to the garden. I suggest you try using a ½ strength nutrient solution when transplanting. Moreover, you should add compost and nutrients to your plants throughout the summer as they grow.
When you transplant the plants into the ground prepare a hole twice the size of the pot. If the plants are root bound (roots are a white mass in the shape of the pot) you should gently break up the roots so they will grow out and down and not keep twisting around each other. When you put the plant in the ground, you should try to keep the base of the plant (where the stem meets the soil) at the same level. With normal soil conditions, don’t pile up a mound of dirt to cover the stem or leave expose roots, plant the plants at the soil level. If you are planting in a wet area, you may have to plant your plant in a mound so it drains better and doesn’t become soggy and get “root rot”. On the other hand, if you think water may be limited, you may plant the plant in a slight depression so that water collects with the rain. This is called creating a microclimate, and can allow you to help each individual plant in your garden have fewer limiting factors in the growing season. Fill in the hole with soil around the transplanted plant. Lightly pack the soil down, because you don’t want any air pockets around your plant. It is a good idea to water you plants thoroughly when transplanting, this will also help pack in the soil and get rid of air pockets.
When you transplant you plant you should add a diluted (½ strength) high phosphorus fertilizer to encourage root growth and make sure the plant has lots of water. When the plants start to put out new leaves add a high nitrogen fertilizer to encourage vegetative growth. Add a high phosphorus fertilizer with minimal nitrogen when flowering/fruiting begins to encourage strong fruiting/flowering. I like to add organic compost. Adding compost to the hole of annual plants when you transplant them will slowly give them nutrients over the growing season, and the ability to hold onto water throughout the growing season. Adding compost into the transplant hole will help almost every plant grow better and yield more (look for future articles on composting). By starting your plants indoors and with proper hardening off your plants should continue to grow vigorously after you transplant them outdoors, adding weeks or months to the growing season and increasing your overall yields.