Seed-starting season has arrived! I love this time of the year when gardening begins again with the seeding of new transplants indoors. Seed starting gives gardeners full control over their transplant production and a vastly wider range of flower and vegetable species and varieties from which to choose.
While getting set up to grow your own transplants can have some initial setup costs, it is also possible to cut corners in many areas and still end up with healthy, attractive new plants that are ready for the garden. If gardening is something you plan on doing for years to come (I sure hope it is!), a setup to raise transplants (and to root cuttings) can save money in the long run, while increasing your access to more types of plants.
Let’s take a look at a few tips and techniques for starting seeds indoors. My goal is to help you achieve success and the personal enjoyment that come with growing your own new plants, while avoiding some of the common pitfalls that I, and others, have made in the learning process.
The Best Time to Begin
Seed catalogues have been arriving in the mail since last fall, and if you are fortunate, your local garden center may offer a great selection as well. If you can’t find what you’re looking for locally, seed shopping online should be able to fill that gap. Order soon since supplies of popular varieties will often sell out.
Determine how long it takes to grow a transplant of a particular species and count back from the date you plan on planting it outdoors to determine the time to start seeds indoors. See the chart for time to grow transplants of some common vegetables and flowers. Keep in mind that indoor temperature, lighting and consistent moisture content in the growing mix will have a major affect on the time it takes to reach a good size for transplanting.
I sometimes allow an extra week in case there are setbacks or if indoor conditions are not ideal. You can always move transplants into a slightly larger pot if they outgrow their seeding container before they can go out into the garden. Tomato enthusiasts often begin very early, with plans to repot the transplants once or twice so they already have blooms or even small fruit at planting.
Start with Viable Seeds
Seeds vary significantly in how long they remain viable, and storage conditions dramatically affect viability as well. Heat and moisture are two big no-no’s that can shorten seed life. Store seeds in the refrigerator or freezer in an airtight container with a desiccant packet inside to ensure maximum viability.
I always have packets of leftover seeds around and sometimes they are several years old. Nothing is more disappointing than planting a flat of seed and getting little to no germination. There is a simple germination test that will cue you in on the percentage you can expect to germinate.
To do this test, fold a paper towel once or twice and wet it. Squeeze out some excess moisture and then put 10 seeds in the middle of the moist towel, fold it over and place it in a zip-closure bag.
Place the bag in a location with the temperature suitable for that species (about 75 degrees is a good guess for most species) and check it periodically to determine the percent germination. You can use this percentage to sow extra seeds accordingly.
Containers for Starting Seeds
There are a multitude of container options for starting seeds from store-bought 6-packs, peat pots and seeding flats to an endless array of homemade and recycled items like egg trays, eggshells, paper pots (my favorite), soil blocks (small cubes of soil pressed together with a soil-block tool), food containers, Styrofoam cups with holes for drainage, and on and on.
The bottom line is that the seeds don’t care what they are in as long as there is a good growing medium, excellent drainage and adequate size to form a decent root system. For my initial germination, I mostly use a combination of (1) planting trays that are sturdy enough to last several seasons, (2) paper pots that I make at home and (3) very small food containers.
The reason I like small food containers is that I can start a lot of seeds in a very small container. Once the seedlings are up, they can be easily separated and reset into their larger growing containers. This hedges your bet against erratic germination, and you end up with exactly the number of plants that you want.
If you reuse plastic trays or containers, first wash off any remaining soil. Then dip them in a 10% bleach solution for a minute or two. After soaking the plastic, you can simply tap off the excess bleach water or rinse them if you wish. I should note that if you are soaking terracotta or other porous materials, you may want to leave them soaking for 30 minutes to allow extra time for the bleach solution to reach well into the materials and kill pathogens.
There are a wide variety of soilless growing media on the market for starting seeds and potting plants. A good mix holds moisture but drains well. This is accomplished by including a blend of ingredients that holds moisture (peat, coir or finely screened compost) with something to keep it loose and add air space (perlite or vermiculite).
Seed-starting soil is ground or screened to a very fine texture, making it easy to plant even small seeds at the ideal depth and to ensure good seed-to-soil contact. It tends to be more expensive per cubic foot but is worth it if you are planting very small seeds. To cut costs when planting a larger container with seeds, I use potting soil to fill most of the container and top that with seed-starting mix for the last inch or less.
Store-bought soilless growing media are generally free of harmful fungal and bacterial pathogens. If you make your own seed-starting media, or reuse media, you’ll want to pasteurize it to avoid the disappointment of losing a crop of tender seedlings to diseases. See the sidebar “Oven-Pasteurizing Growing Media or Garden Soil” for more how-to info.
Dry-growing media tend to repel water, making it very difficult to moisten after you plant the seeds. If your growing medium has dried in its container, spray some water on it as you mix it to get it moist before adding it to the planting containers.
Best Planting Depth
Seeds should be planted at the proper depth for best results. While most seeds germinate best under dark conditions (some even being inhibited by light), some seeds need the red wavelength in light to germinate. See accompanying chart, “Optimum Light, Temperature and Time for Selected Garden Plants,” for more details.
The seed packet should specify planting depth. In the absence of that information, as a general guide, plant seeds so they are covered with media about 3–4 times their width deep. Very tiny seeds and seeds that need light to germinate should be scattered on the surface of pre-moistened media and lightly pressed in or can be covered with a light sprinkling of finely ground vermiculite.
Watering Newly Planted Seeds and Growing Transplants
After planting, water the mix well so the seeds can absorb water. This begins the germination process. Remoisten as needed to prevent the surface from drying out until the seeds are up and growing.
Note that seeds of some ornamental plants, such as Texas mountain laurel, pride of Barbados and our beloved bluebonnets have hard seed coats that tend to resist infiltration of water. Nicking the outer seed coat will allow water in much faster, which will greatly speed up germination. Others, such as okra or spinach, can be hurried along by presoaking in warm water overnight.
During the initial germination phase, take care to not dislodge seeds with a strong stream when you water. I have found that a misting nozzle or plastic bottlecap sprinkler (fits on standard drinking-water or soft-drink bottles) that gently applies water works very well, especially for small seeds and those seeds planted at or near the surface.
Once the seeds and soil are moist, cover them to help prevent the surface from drying out. You can purchase seeding trays with clear hard plastic covers or place the tray in a clear bag (such as a drycleaner bag) and use plastic labels, coffee stirrers, popsicle sticks or plastic food utensils to hold the bag up off of the soil surface. When the seedlings have germinated, remove the bag. Covering with a clear plastic material is especially helpful when the trays are on a heating mat, which causes the media to dry out faster.
After germination I generally switch to bottom watering. Simply place the containers in a tray that holds water and add a little water to the tray to wick up into the media. Bottom watering can help avoid keeping the media too soggy as long as you don’t leave standing water in the bottom of the tray.
Lighting Is Key to Success
Good quality light may be the most important of all the requirements for growing a great transplant. Sunlight is best, but often very limited in most home settings, even by a fairly bright window. Without adequate light, your seedlings will be spindly and weak. Artificial lighting is a decent replacement for the few weeks needed to grow a transplant.
Light needs to be sufficiently bright and include the proper wavelengths (mostly red and blue) for best results. You can spend a lot on lighting and many new options are available in energy-efficient LED lighting. However, I’ve grown a lot of seedlings under two 4’ shop lights with one cool white and one warm white T-12 florescent tube each, suspended just 2” above the seedlings. To take it up a notch, high output T-5 fluorescent fixtures in the proper wavelengths are more efficient in light output for the energy used than T-8 or T-12 tubes.
Use a timer set to run the lights 14–16 hours a day during the time needed to grow transplants. For more details on quality lighting for growing transplants, check out my article on lighting for seed starting (TG January/February 2020).
Best Temperature for Seed Germination
Seeds have their preferred temperature ranges for germination. As conditions move colder or warmer, germination slows or comes to a halt. The accompanying chart gives the range for some common vegetable and flower species.
When starting seeds in a cooler than ideal location, it helps to use a seed-starting mat to regulate the temperature of the growing media. You can purchase a thermostat with a probe to place in the growing container to turn the mat on and off as needed to avoid over- or underheating.
Transplants also have their ideal growing temperature following germination. This is often 5–10 degrees cooler than the germinating temperature. I often start seeds in a garage, where air temps can be rather cool. I find that they do quite well with their roots in adequately warmed soil and their tops in the cool garage air. This method also tends to create a slightly stockier transplant.
Adequate Nutrition for Seedlings
Once seedlings germinate and the first true leaf appears, begin to provide a little extra nutrition to boost them along. You can begin a feeding regimen. A very dilute solution of a product made for dissolving in water, or a mix of fish emulsion and seaweed, works fine.
Use the lowest label rate, or even less, once a week. It is easy to burn tender seedlings, especially with a salt-based synthetic fertilizer solution. If there is compost in the growing media, it will also be releasing some nutrients slowly over time.
Building Stocky Transplants
Remember that pushing the seedlings with extra nitrogen can be detrimental, especially when growing in warm temperatures and lower than ideal light levels, which all contribute to lanky, weak transplants.
Plants growing in a still indoor environment develop stems that are very weak. As with an outdoor tree, wind movement causes the stems to develop tissues that are much stronger and makes for a stockier transplant. Use your hand or a soft brush or pencil to brush over the seedlings once or twice a day to move them around and stimulate this strengthening response. If family or friends see you “petting your plants,” tell them that you are performing thigmomorphogenesis and they’ll be very impressed and think you really know what you’re doing (or they may still think that you’re just going crazy).
Preparing Transplants for the Outdoors
Your plants, living the easy life indoors, need some help transitioning to the outdoor environment, where chilly night temperatures, wind or hot daytime temps with blazing sun await them.
The process of hardening off involves gradually introducing them to outdoor conditions for a few hours a day, leaving them progressively longer into the cool evenings in the early-spring garden, or gradually transitioning them from bright shade to more direct sun in summer. Given a week or two of this process, they will be much better able to make the transition.
Protective structures such as a hoop tunnel or tomato cages covered with row-cover fabric block the wind and lessen the cold in spring or can provide temporary shade to block the hot mid- to late-day sun in summer, which can also make the transition smoother.
It is time to get out those seed-starting supplies and get a head start on the spring garden. Growing your own transplants is one of the many fun and rewarding parts of home gardening. Here’s to a bountiful 2021! tg
Oven-Pasteurizing Growing Media or garden Soil
If you wish to reuse growing media or to blend your own by using finely screened compost or garden soil, it is best to kill any harmful fungi, bacteria or weed seeds that might be present. While a microwave can be used for very small amounts of media, uneven heating and overheating can be a problem. An oven is the best technique. However, overheating media or soil in an oven can leave some “earthy” odors. A temperature of 140 degrees for a half hour is enough to kill most fungi, while most harmful bacteria and insects may require 160 degrees and weed seeds require 180 degrees.
- Preheat oven to the target temperature.
- Place premoistened (not wet) media or garden soil up to 4” deep in a large pan.
- Cover pan and soil with aluminum foil.
- Place pan in oven away from heating element and check the temperature of the media or soil periodically with a meat or candy thermometer placed through a hole in the foil.
- When the temperature of the media or soil reaches the target temperature, leave it in the oven for 30 minutes longer before removing it to cool.
An alternative option is to place the media in a large plastic bag made for oven baking, such as a turkey bag. Make a hole in the bag for air to escape and for inserting the thermometer.
Optimum Light, Temperature and Time for Selected Garden Plants
Light media Seed to
Vegetables Preference* temp (°F) Transplants
Broccoli D 75 5 to 7
Cabbage D 75 5 to 7
Cauliflower D 75 5 to 7
Collards D 75 5 to 7
Cucumber D 85 2 to 3
Eggplant D 85 7 to 8
Lettuce L 55 4 to 5
Muskmelon D 85 2 to 3
Onions DL 75 8 to 10
Peppers D 85 7 to 8
Squash D 85 2 to 3
Tomatoes D 85 5 to 6
Watermelon D 85 2 to 3
Dill L 65 5–6
Basil D 70 4–6
Parsley D 75 6–8
Alyssum DL 70 3–5
Calendula D 70 7–8
Celosia DL 70 8–9
Cosmos DL 70 6–8
Dianthus (annual pinks) DL 70 6–7
Gaillardia (annual) DL 70 7–9
Impatiens (I. sultani) L 70 4–6
Marigold DL 70 6–7
Moss rose (portulaca) D 70 4–6
Petunia L 70 5–7
Phlox (Phlox drummondii) D 65 5–6
Rudbeckia DL 70 6–7
Scarlet sage (Salvia splendens) L 70 5–6
Snapdragon L 65 5–7
Vinca (rosea periwinkle) D 70 7–8
Zinnia DL 70 3–5
*Light Preference: D—Seeds germinate best in darkness; DL—No light requirements; L—Seeds germinate best in light.
By Robert ”Skip” Richter
Brazos County Horticulturist
Texas A&M AgriLife Extension Service