What Do My Seed Need for Good Germination?
In a word, seed need the correct soil temperature, moisture, humidity and light to germinate. Supplying these requirements correctly is not too difficult, but it is necessary. Let us look at supplying these needs in a little more detail.
1. Correct Temperature – When you think about it, it makes perfectly good sense that different seed would require different temperatures; after all, you would not expect a tropical seed adapted to a uniformly warm-to-hot environment to thrive in cool temperatures! In general, few plants germinate at sustained temperatures below 40° F. or above 104° F., although short exposures to these extremes will not harm them.
When horticulturists refer to temperature, they are talking about the temperature of the soil, not of the air. The soil temperature is usually 5-10° F. cooler than the air temperature because the soil is cooled by evaporation from its surface. This means that although the room temperature may be 70° F., the temperature of the growing medium you are expecting your seed to germinate in is several degrees lower.
Gentle bottom heat is usually needed to give your seed the optimal temperature for best germination; your seed will start quicker and be more uniform and the seedlings bigger if you give them bottom heat. In the past, the top of the water heater or refrigerator was often used as a sowing table for the flats of seed, and this is certainly better than nothing, though it is difficult to control the temperature. However, modern refrigerators are often built into cabinetwork, and water heaters tend to be in places that are not conveniently accessible, so they are becoming much less practical to use in today’s homes.
I recommend using a waterproof seedling heat mat with a thermostat. This has proven to be the most reliable and easily controlled way to give your little seedlings exactly what they need. Be sure to use a heating mat with a thermostat. You can set it for the temperature your seed require and it will monitor the temperature for you, making sure you don’t accidentally bake your precious seedlings or slow them down by being too cool or alternating between warm and cool. Some seed germinate best with a soil temperature alternating between 68 and 86° F. They actually need to mimic conditions outside, when the temperature is around 68° F. for 16 to 18 hours a day, and 86° F. for 6 to 8 hours a day. You can give these seed what they need by setting your heat mat for 86° F., leaving it on during the day, and turning it off at night. You can also grow these seed under lights, which give off warmth and create the higher temperature during the day that your seedlings need. Simply turn the lights off at night to allow the temperature to drop. If you have a suitable sunny window, seed flats may also be placed there to provide this daily difference in temperature.
If the recommended temperature for your selected seed is 55° F., germinate your seed indoors in an unheated garage, attic, basement or porch, which must, of course, have a source of natural or artificial light. Outdoors, this temperature is achieved in early spring. Sow directly into seed beds, or set flats outside in a spot protected from sun and wind. You can also use a cold frame, if your area is subject to unexpected cold spells.
While we are talking about temperature, let me mention that some species need a different temperature for germinating than they do for seedling growth.
2. Proper Moisture and Humidity – Proper moisture and humidity are also vital elements in seed starting success. The planting medium must be kept evenly moist, but never water logged. If there is too little moisture, then germination will not occur; too much and you’ll watch your precious seedlings rot.
Before planting your seed, your growing medium should already be moist. If you are using a soilless seedling mix, you can add water to the amount of mix you need to fill your containers and gently mix it before placing in the container. If you are using a seed starting system, the planting plugs might already be moist. If they are beginning to dry out, place them in their tray and pour water into the tray. Let them soak until the tops of the planting plugs are moist but not water logged. Now you’re ready to plant!
Though many gardeners have success with gentle top watering for their germinating seed and seedlings, we recommend that you ALWAYS water your seedlings from the bottom. You should top water your planting medium to moisten it before you plant your seed. This but after that, place your seed-growing container in a tray and put water in the tray, so the growing medium can soak up what water it needs from the bottom. Bottom watering will minimize disease, keep the soil evenly moist without over watering, and prevent accidentally dislodging or washing away small seed that are just getting started.
Germinating seed need to be kept uniformly moist, so they need protection from drying air. If you are growing in a Bio Dome, simply place the dome over your newly planted seed to keep the growing sponges from drying on the top. If you are using a tray filled with growing medium, you can use something like labels or drinking straws in the corners of the tray to hold a sheet of clear plastic over the top to keep moisture in without touching the emerging seedlings.
I must warn you, however, that while seed need this humid atmosphere to germinate, seedlings need good air circulation to thrive, so as soon as you see your seedlings emerge, take the cover off and rely on the growing medium to supply the moisture needed, not the air.
3. Proper Light – Providing your seed the correct lighting conditions is as important as giving them the temperature and moisture they need. Some seed require light to germinate, some require darkness, and some don’t care. If light is needed by your seed, simply plant them on top of the medium and do not cover. If darkness is required, completely cover the seed with planting medium, unless they are too fine to be covered. If this is the case, place your seed flat or Bio Dome in total darkness or cover them with something that will block the light, such as black plastic or several layers of newspaper, until emergence has started.
Once your seed have germinated, the seedlings must have light. Light is necessary for plants to convert water and carbon dioxide into sugar (which plants use as food), in a process known as photosynthesis. If light intensity is too low, which often happens during the short days of winter or during prolonged cloudy periods, the plants will be unhealthy, tall and spindly. Plants have the greatest need of intense light when they are seedlings than at any other stage of life. When starting your seed indoors, using fluorescent lights or growing in a greenhouse is best, but if you do not have these available, an unshaded south window will suffice.
4. Good Records – Labeling your seed when you sow them and then when you plant them in the garden is really a must; I can hardly stress the importance of labeling too much, and I must admit I speak from personal experience! No matter how good your memory, or how careful you are to keep your seedlings segregated, something will inevitably mess up your system – your spouse will help water, and inadvertently move one, or company is coming and your seed starting area gets ‘cleaned up’ with dire results for your organization. So please use labels and record not only the name of the seed you sow, but also the sowing date, which will help to keep track of when they are ready to be transplanted to a lot or to your garden. If you use a plastic or wooden label, you can use the same labels when you plant your little plants out into the garden, and you have an easy record of what you did, for ease in planning next year.
If you are really into growing from seed, you can keep a record book of when you planted, how long it took to germinate, whether you started your seed too early or too late, or whether you grew too few or too many.
Now you have assembled everything you need for starting seed – the next step is actually sowing your seed!