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Common Mistakes Made While Growing Seeds Indoors

Guidance on Watering, Lighting, and Other Growing Factors

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The Spruce / K. Dave

It is quite economical to start seeds indoors, especially when the seedlings grow into robust plants. However, growing seeds indoors can be challenging. To significantly increase your chances of success, avoid these common seed-starting mistakes.

Watch Now: Mistakes to Avoid When Growing Seeds Indoors

Not Supplying Enough Light

Seedlings need a lot of light to grow into sturdy, healthy plants. No matter what anyone tells you, chances are that you do not have enough natural light in your home to grow robust seedlings. Even a south-facing window usually will not do. You can, however, use artificial light to achieve the right amount of light required by seedlings. To do so, obtain grow lights explicitly designed for plants. Or, for a more economical solution, purchase large fluorescent shop lights outfitted with one warm bulb and one cool bulb.

Suspend the lights from chains so that you can raise the lights higher as the seedlings grow. Keep the lights as close to the seedlings as possible without touching them (2 to 3 inches). When seedlings first appear, keep the lights turned on for 12 to 16 hours per day. To reduce your hands-on time, use a timer to turn the lights on and off automatically.

Applying Too Much or Too Little Water

The amount of water you supply can make or break seedling growth. Watering is one of the most challenging aspects of seed starting. Because seedlings are so delicate, there is very little room for error when it comes to watering. You must keep the sterile seed-starting medium damp but not wet.

To increase your chances of getting it right, here are a few things you can do:

  • Create a mini-greenhouse to keep soil moist: cover the container with plastic until the seeds germinate.
  • Water from the bottom to enable the seedlings to soak up water through the container drainage holes. There is less chance of over-watering when you use this approach. Add water slowly for 10 to 30 minutes, and use your finger to touch the top of the soil to ensure that moisture has reached the top of the container.
  • Check soil moisture at least once a day.
  • Buy a self-watering, seed-starting system.

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The Spruce / K. Dave

Starting Seeds Too Soon

Many plants do not tolerate cold temperatures, and exposing them to chilly air or cold soil will stress them out. Chas Gill, who runs the Kennebec Flower Farm, agrees that one of the biggest mistakes people make when starting seeds is starting the seeds too early. Stressed-out plants are more susceptible to pests and disease. Most plants are ready to go outside four to six weeks after you start the seeds.

Planting Seeds Too Deeply

Seeds are finicky when it comes to how deep they are planted. Some seeds need complete darkness to germinate and others require light to germinate.   Proper planting depth is usually provided on the seed packet. If there is no information on the packet, the rule of thumb is to plant seeds two to three times as deep as they are wide. Determining depth can be a challenge, but if you are not sure, err on the shallow side.

For seeds that need light to germinate, make sure the seeds are in contact with the seed starting medium but are not covered. To do this, gently press the soil medium to create a firm surface. Then, place the seed on top of the medium and gently press down, making sure the seed is still exposed.

Moving Seedlings Outdoors Too Soon

There is no benefit to a tough-love approach with seedlings when they are young. They will either instantly die or become weak and then fail to thrive. Even the most stalwart plants, when young, need a considerable amount of coddling and attention.

When your seedlings are large enough to plant outdoors, you need to prepare them for the transition by hardening off.   Hardening off gradually prepares them for outdoor conditions like wind, rain, and sun. The hardening-off process is simple, though it can be time-consuming; it involves exposing your plants to the elements gradually. The first day of hardening off, place your seedlings outdoors for one hour, and then bring them back indoors. Gradually increase the amount of outdoor time every day for 6 to 10 days. You will need to make some judgment calls based on the outdoor temperature and the fragility of your seedlings. If it is a particularly cool day or very rainy, you will want to decrease the time of that hardening-off session.

Sowing Too Many Seeds

When sowing seeds, begin modestly if you are a beginner. If you sow more seeds than you can reasonably maintain, it will become challenging to nurture the seedlings into adulthood. Depending on the type of plant you want to grow, you might be able to direct-sow seeds in outdoor containers or in the ground when outdoor temperatures warm up.

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The Spruce / K. Dave

Keeping Seeds Too Cool

For seeds to germinate, most must be kept warm: about 65 to 75 degrees Fahrenheit. A favorite place to keep seeds warm in order to germinate is on top of the refrigerator. Or, you can purchase seed-warming mats to place under the seed trays. Once a seedling emerges, they can tolerate fluctuating temperatures (within reason). Whatever type of light you use, natural or artificial, make sure it produces enough heat to keep the plants in the 65- to 75-degree range.

Failing to Label Seeds

To be able to identify seedlings as they grow and to know when they will be ready for transplanting, you should label the seed containers as you are sowing. For every type of seed sown, use popsicle sticks or plastic plant markers and permanent ink pens to record the plant name and date sown. Insert the plant labels into the soil near the edge of the container or tray.

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The Spruce / K. Dave

Giving Up Too Soon

Starting seeds can be a difficult process. However, one of the most satisfying benefits of this labor of love is eating a tomato or marveling at the flowers that you nurtured from day one. Growing plants from seed takes dedication, attention, and time. Recognize that you might make mistakes along the way, but you should not give up. The results outweigh the challenges along the way.

Growing seeds indoors isn't hard, yet keeping them alive can be challenging. You can save a lot of money by starting plants from seeds.

Growing Seedlings Under Indoor Grow Lights

Anyone who’s tried growing plants from seed knows that proper lighting is critical to producing an abundance of stocky, green seedlings. For those of us lucky enough to have a south-facing window with 12+ hours of full sun, lighting isn’t an issue. But for the rest of us, an indoor lighting system of some kind is a necessity.

This primer on indoor lighting for seed starting will help you choose the options that work best for you.

Light Color

Light color is also referred to as color temperature, with cool light describing the blue end of the spectrum and warm light being the red end. Sunlight contains the complete spectrum of light, including all colors of the rainbow.

Although plants use the full spectrum for photosynthesis, red and blue light seem to be most critical. Red light stimulates vegetative growth and flowering (but if a plant gets too much, it will become tall and spindly). Blue light regulates plant growth, which makes it ideal for growing foliage plants and short, stocky seedlings (but too much will result in stunted plants).

You can tell which color a grow light produces by looking at its Kelvin rating. Lamps with a rating of 5000 Kelvins will appear bluish, while those with a 2500 Kelvin rating will be reddish.

Light Intensity

The intensity of light that a plant receives is determined by the wattage of the bulb and the distance between the plant and the light source. So, for example, a brighter bulb that’s farther away from the plant could provide the same light intensity as a dimmer bulb that’s closer to the plant.

Different plants have different light intensity needs, but most seedlings grown for the garden will need higher intensity light to flourish. In general, the leaves should be about 2 – 4 inches away from the light source (assuming use of a fluorescent bulb – see below).

Duration of Light Exposure

There’s still debate about how many hours of supplemental light is ideal when starting seeds and growing plants indoors.

Most vegetables and garden plants require at least 16 to 18 hours of light each day; without enough light, they get pale and leggy. The conventional advice was to turn lights on for 16 hours each day. However, some growers maintain that 24 hours of consistent light every day provides a better outcome when growing seedling (i.e., there’s no need to give seedlings a nightly rest but this advice doesn’t necessarily apply to full-grown plants).

It’s certainly easier to leave your grow lights on all the time and that’s what I do. If you choose to go with 16 hours on, 8 hours off, put the lamp(s) on a timer so you won’t forget to turn the lights on or off.

Types of Bulbs

You can choose between incandescent, fluorescent, LED, and high-intensity discharge (HID) bulbs, each of which has its own pros and cons. Choose the grow light that works best for the type of plants you want to grow and where you plan to grow your seeds.

Incandescent Bulbs

These include halogen bulbs and are the type of light bulbs still used in most homes (although they’re getting harder to find now that stores are carrying only more efficient bulbs, such as CFLs and LEDs).

Incandescent bulbs are a good source of red light, but a poor source of blue, meaning that plants will likely become spindly when grown under incandescent light.

Incandescent bulbs, and especially halogen bulbs, also produce a lot of heat in relation to the amount of light they give off; plants growing too close to the bulb can be easily burned.

Generally speaking, these are not the best type of lamps for growing seedlings.

Fluorescent Bulbs

These types of bulbs produce two to three times more light than incandescent bulbs for the same amount of energy and are the most inexpensive lights for indoor gardening. However, they usually require bulky external ballasts (like, for example, overhead shop lights) so aren’t as easy to work with as incandescent and LED bulbs.

Cool white bulbs are a good source of blue and yellow-green light, but are a poor source of red light. Plants grown under cool white bulbs will be stocky or even slightly stunted. Warm white bulbs emit plenty of orange and red light, but less light in the blue and green spectrum. These bulbs, when used alone, result in tall, spindly plants. If you are growing seedlings under two-bulb fluorescent fixtures, you can usually achieve a good color balance by combining one cool white and one warm white bulb.

A set of stacked shelves with fluorescent T5 lamps (or LED lamps) makes it easier to grow a large number of seedlings.

Full-spectrum fluorescent bulbs produce a balance of cool and warm light that replicates the natural solar spectrum, although these are less energy efficient than other fluorescent bulbs and tend to produce more heat. But, given the wider range of light frequencies emitted by these bulbs, they are a good choice for growing seedlings.

T5 lamps are fluorescent lamps that are 5/8″ of an inch in diameter, making them much less bulky than typical fluorescent bulbs. These are the lamps you’re most likely to find in grow light kits.

When using fluorescent lamps, be sure that all plants get ample light. For a typical seedling tray, that means using 2 bulbs, ideally with a reflector hood over them to focus all the light on the seedlings below.

LED Grow Lamps

Unlike other bulbs which produce light across a broad spectrum, light-emitting diodes (LEDs) give off light within a narrow band. LEDs designed for growing plants emit light in the two bands that plants need – red and blue. The result is a purple glow that not everyone likes.

LEDs are mercury-free and won’t shatter like glass. These bulbs are long-lived (up to 5x longer than fluorescent lamps) and very energy efficient, but they cost considerably more than fluorescent bulbs.

Research is still ongoing to determine which combination of light frequencies are best for plant growth and how LED grow lights compare to fluorescent bulbs in producing healthy seedlings. I haven’t seen a definitive answer on this one yet but more and more companies are producing LED grow lights and seed-starting kits, and even commercial growers are slowly moving in that direction.

High-Intensity Discharge Lamps

These lamps are used by commercial growers and serious horticulturists. These energy-efficient bulbs generally emit twice the amount of light (lumens) as a fluorescent bulb. However, the bulbs and special fixtures are considerably more expensive than those needed for incandescent or fluorescent lights. They also tend to be high-wattage bulbs, so you need to be sure your electrical system can handle the load. Some of these lights burn so brightly that they must be located in a special room and you’ll need to wear eye protection when working around them.

Metal halide lights emit an intense, bluish-white light that is excellent for growing plants. The foliage stays green and vigorous, and plants are usually stocky and strong. Metal-halide lights are currently the number one choice for serious indoor gardeners. Mercury vapor lamps emit a bluish, relatively well-balanced, high-intensity light. High-pressure sodium bulbs are usually used to promote flowering and fruiting but, when used exclusively, they produce leggy, weak-stemmed plants.

What’s the best option?

For the average home gardener starting seedlings indoors, a fluorescent or LED lamp will usually be the best choice to ensure that your plants get the quality, intensity, and duration of light they need to stay in peak condition.

A description and comparison of different grow lights for starting seeds and growing plants indoors. Covers incandescent, fluorescent, LED and high intensity discharge lamps, as well as issues around light color, intensity and duration.