For many home gardeners, it's fun to get a head start on the upcoming garden season by starting seedlings indoors. Growing quality seedlings indoors requires high-quality seeds, a germination medium/potting mix, containers, proper temperature and moisture conditions, and adequate light.
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A seed is a miracle waiting to happen. The embryo comes pre-packaged with a food supply and the vital genetic information needed to become a plant, much like its parents.
Seeds exist in a state of dormancy, absorbing oxygen, giving off carbon dioxide, and slowly using up their stored food reserves. During this process, the seed continually monitors the external environment, waiting for ideal conditions specific to the particular seed. Once the ideal conditions occur, the seed breaks dormancy and germinates. The seedling gathers energy through its leaves through photosynthesis and absorbs nutrients and water from the soil through the roots.
As gardeners, the goal is to provide the optimal environment for germination and seedling growth.
Resist the urge to start seedlings too early in the season. It is easy to get excited for spring, but starting seed too early leads to challenges later. Seedlings planted too early get large and lanky by the time they can be planted outside, and lanky seedlings do not transplant well. Additionally, the plants that grow from spindly or lanky seedlings will not be as high-quality as those started from smaller, stockier transplants.
The crop time (number of weeks from sowing to planting outdoors) is printed on the seed packet. You can also get the crop time for several popular flowers and vegetables in this article: Germination Requirements for Annuals and Vegetables.
Use this number to determine when to sow seeds indoors. If you intend to plant outdoors on May 15th, the sowing date indoors would be the number of weeks listed in this column before May 15th. Most annual and vegetable transplants are planted in the garden after the threat of frost has passed for the summer. For much of Iowa, this is the last week in April or the first week in May. Find your frost-free date in this article: Frost Dates in Iowa.
Some cool-season vegetables and annuals like broccoli, cabbage, or pansy, tolerate light freezing temperatures and can be planted 2 to 4 weeks before the average last frost date.
The germination medium should be lightweight, porous, and free of disease pathogens. Excellent germination media are commercially prepared soilless products, such as Jiffy Mix or Redi-Earth. When transplanting seedlings into individual pots or cell packs, use a high-quality, well-drained potting mix.
Various containers can be used to germinate and grow transplants. Gardeners can purchase flats, trays, cell packs, pots, compressed peat pellets, and other commercial products. Cut-off milk cartons or plastic jugs, paper cups, and egg cartons can also be used to start seeds.
Previously used flats, trays, and pots should be cleaned and disinfected before use. Wash the containers in soapy water, then disinfect them in a solution of one part chlorine bleach and nine parts water. Holes should be punched in the bottom of milk cartons, jugs, paper cups, and similar containers to allow for drainage.
The size of the seeds and their germination requirements largely determine the type of container and sowing method. Fine seeds, such as begonias and petunias, are typically sown in flats or trays. After germination, the seedlings are transplanted into individual containers. Some seed companies offer coated or pelleted seeds for ease of handling and planting, especially for species with fine or tiny seeds.
Large seeds, such as marigolds and tomatoes, are commonly germinated in flats. However, they can also be sown directly into individual containers, eliminating the need to transplant the seedlings.
When sowing seeds in flats or trays, fill the container with the germination medium within 1 inch of the top. Firm the medium and water thoroughly, then allow it to drain. Fine seeds and those that require light for germination are sown on the surface of the medium and then lightly pressed into the germination medium. Cover all other seeds with additional media to a thickness of one to two times the seed's diameter.
After sowing the seeds, water the medium by partially submerging the container in water. When the surface becomes wet, remove the container from the water and allow it to drain.
Watering from below prevents the washing of seeds on the surface of the medium. The medium can also be moistened with a rubber bulb sprinkler. The fine mist from the rubber bulb sprinkler will not disturb the seeds or the medium.
When sowing seeds into individual containers, plant two or three seeds per container (peat pots, pellets, soil blocks, etc.). Place the containers in a flat and water.
Seeds require a certain temperature to germinate. Each plant has a specific optimum and a range within which germination will occur. The closer the temperature is to the optimum, the quicker germination will occur.
Most seeds germinate when the soil temperature is between 68° and 86°F. Once germination occurs, the optimum growing temperature for the seedling is about 10°F cooler than the optimum germination temperature.
Moisture is critical for germinating seeds. They like a moist but not soggy environment. Seeds require oxygen and, if kept waterlogged, may rot. On the other hand, if the soil dries out, the seed will lose whatever water it has absorbed and die.
Finding the middle ground can be difficult and comes easier with practice. After sowing the seeds, mist the tray with water and cover it with plastic wrap, a plastic bag, glass, or plexiglass to seal in moisture.
As soon as the seed germinates, remove the covering. Ventilation and air circulation are also important to discourage damping off diseases.
Check the seedlings frequently (as much as daily) for moisture. Thoroughly water the seedlings when the soil surface becomes dry to the touch.
If using a commercial potting mix containing a slow-release fertilizer, fertilization should not be necessary. Applying a dilute fertilizer solution once every two weeks should be sufficient for those potting mixes that don't contain a slow-release fertilizer.
Some seeds need light to germinate, but many do not. Seed packages will usually indicate what your particular selection requires.
It is important to follow the directions given on the package for planting depth. Seeds planted too deep may not receive the light they need to germinate (if applicable), will not have enough stored energy to reach the soil surface, and may die in the process.
After germination occurs, seedlings require about 12 to 16 hours of light a day. Intense light is necessary to prevent spindly or leggy seedlings. If you are growing under lights, ensure the light source is 4 to 6 inches above the plants. In a sunny window, turn the seedlings regularly to avoid leaning.
Learn more about providing supplemental light for seedlings indoors in this article: Growing Indoor Plants Under Supplemental Lights.
If you are sowing seeds in furrows or flats, transplant individual seedlings into cell packs when the first true leaves appear or when they are large enough to handle.
Seedlings started indoors should be fertilized regularly with a dilute (1/4 strength) water-soluble fertilizer. This will help to produce stockier transplants, provided enough light is available.
Before planting in the garden, harden seedlings by gradually acclimating the transplants to the outdoors. Start by putting them outside on cloudy days or in a shaded location, then after a few days, work them into more light and exposure.
The best time to plant in the garden is overcast skies or late afternoon. Water immediately after transplanting. If plants wilt, provide protection with an open milk carton or row cover for a few days.
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When do you start your seeds indoors? Sow too early, and the plants may be ready to leave the pot before it&#;s warm enough. Sow seeds too late, and they won&#;t have enough time to reach maturity. We&#;ll provide some tips as well as a chart on how many weeks to sow indoors before last spring frost.
The Hindi word for seed is bija, which translates literally to &#;containment of life.&#; This is an apt description of these tiny miracles that contain everything needed to make a new plant. This time of year, we are up to our elbows in dirt, starting more seeds indoors each week!
There are many benefits to sowing seeds indoors:
As a general rule, seeds are started indoors about six weeks prior to your last frost date.
During a cold spring, it&#;s better to delay sowing a little to ensure the soil temperature is warm enough than to jump the gun and get disappointing results.
Here are three different tools to find out the best date to start your seeds where you live.
Not ALL seeds should be started indoors. In fact, most vegetables grow perfectly well when started outdoors and even prefer not to be transplanted. Ultimately, it&#;s important to consider how each type of vegetable grows in addition to where you&#;re growing it.
Consult the table below to see which crops are typically started indoors and which are typically started outdoors. Remember that there isn&#;t a hard-and-fast rule about what you can start indoors and outdoors; it varies by your experience, personal preference, location, and the plant itself. In general, we find that:
While it&#;s easiest for most people to count back the number of weeks for seed-starting, it&#;s all based on average or typical frost dates, and frost dates do shift from year to year.
So, if you want to get down to the nitty-gritty and improve seed germination, it&#;s actually soil temperature (not air temperature) that really controls seed germination. Pick up a soil thermometer (available at garden centers) to get a reading.
CROP
MIN. TEMP.
OPTIMUM TEMPS.
MAX. TEMP.
°F
°C
°F
°C
°F
°C
Beans
60
16
60-85
16-29
95
35
Beet/Beetroot
40
4
50-85
10-29
85
29
Cabbage
40
4
45-95
7-35
85
29
Carrot
40
4
45-85
7-29
95
35
Celery
40
4
60-70
16-21
85
29
Chard
40
4
50-85
10-29
95
35
Sweet Corn
50
10
60-95
16-35
105
41
Cucumber
60
16
60-95
For more information, please visit Leak-Proof Seedling Trays.
16-35
105
41
Eggplant/Aubergine
60
16
75-90
24-32
95
35
Lettuce
35
2
40-80
4-27
85
29
Melon
60
16
70-95
21-35
100
38
Onion
35
2
50-95
10-35
95
35
Parsnip
35
2
50-70
10-21
85
29
Pea
40
4
40-75
4-24
85
29
Peppers
60
16
65-95
18-35
95
35
Spinach
35
2
45-75
7-24
85
29
Squash & Pumpkin
60
16
65-95
18-35
100
38
Tomato
50
10
70-95
21-35
95
35
Turnip
40
4
60-100
16-38
105
41
Most vegetable crops have a minimum germination temperature between 36°F and 60°F (2°C and 16°C), but there is also an optimal range. This is where the difference between cool-season crops (spinach, lettuce, cabbage, etc.) and warm-season crops (eggplant, cucumbers, tomatoes, peppers) comes into play. For example, parsnips will germinate best between 50°F and 70°F (10°C and 21°C), but eggplant will germinate best between 75°F and 90°F (24°C and 32°C), tomatoes between 61°F and 86°F (16°C and 30°C), and peppers between 64°F and 95°F (18°C and 35°C).
For starting seeds, you only need a seed-starting mix, containers, and a strong light source (but more on that later).
Let&#;s start with the potting mix. Generally, you will have no trouble using an all-purpose potting mix. Drainage is good, but if your potting mix seems to have a lot of big chunks of wood or rocks, we recommend sifting it through a screen before using it for seed starting. Seedlings&#; roots will struggle in soil that&#;s not fine enough.
For very small or delicate seeds, we recommend using an actual seed-starting mix, which is a potting mix designed especially for starting seeds. We also like seed-starting mixes because they&#;re low in nutrients; the seed itself is already full of nutrients. Avoid peat-based mixes, if possible. We use more sustainable alternatives, such as coconut coir.
Do NOT use soil from outdoors. It&#;s often too heavy or compacted for seedlings&#; roots and may contain pests or diseases!
You can sow into pots, plug trays, or recycled containers. Each has its advantages:
Well-made containers and trays of rigid plastic can potentially last for many years, but if you want to avoid plastic, look for alternatives made of biodegradable fiber.
Sowing in a pot or a plug tray is really easy to do!
Watch this video to see the seed-starting advice in this article come to life. Ben will show you how it&#;s done so that you can sow like a pro!
We&#;re all impatient and want to see those seedlings push through quickly! The best way to achieve that is to give your seeds as close to ideal conditions as possible, which usually means a little warmth.
See our article on &#;How to Improve Germination.&#;
Poor light levels are often the killer in wintertime, rather than cold, at least for cool-season crops. So, if it&#;s early in the season and you don&#;t have a suitable outdoor protected structure such as a greenhouse or cold frame, it might be worth investing in some full-spectrum grow lights.
Placing seedlings on a windowsill rarely gives the same light as outdoors. You can try turning seedlings daily to help them grow more upright, but more often than not, the result is leggy seedlings that are bent in all directions and that will struggle to recover.
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or fluorescent light fixture that&#;s &#;full-spectrum&#; (i.e., produces light in the full range of the visible spectrum&#;like the Sun) can usually be found for under $40 at a local hardware or department store.See our articles on using grow lights and the best grow lights for growing vegetables indoors.
After you start your seedlings and they have two pairs of leaves, you&#;ll often need to transplant them into their own pots. Don&#;t delay transplanting your seedlings, as you don&#;t want them to become overcrowded, which can cause all sorts of issues, such as leggy seedlings or disease.
Seedlings of tender crops must be gradually introduced to outside conditions before they are planted in the garden, a process known as &#;hardening off.&#; Suddenly moving plants from a stable indoor environment to one with wide variations in temperature, light, and wind can seriously weaken&#;or kill&#;plants!
For most plants, start hardening off about 7 to 10 days before the final frost date for your area. Check our Planting Calendar for safe dates to plant outside and work back from there. Withhold fertilizer and water them a little less often during this period.
Here&#;s how to harden off your seedlings:
See our video on How to Harden Off Plants.
If you&#;re not able to be around to bring your seedlings back and forth from the outdoors, another option is to place your seedlings into a cold frame and gradually increase the amount of ventilation by opening vents progressively wider each day. Make sure to shut them down completely before dark. (See how to make a cold frame for cold-weather protection.)
Consult the Almanac&#;s library of Vegetable Growing Guides, which provides planting, care, and harvesting information for each of the common vegetables, fruits, and herbs.
What seeds are you starting this season?
If you want to learn more, please visit our website cheap seed starting trays.