Seed (Sexual) Propagation

Development of Seeds

Seeds develop from the sexual reproductive parts found in the flower. The female part is called the pistil and the male part is called the stamen. (You can find a full description of the creation of seeds, including details about pollination and fertilization, in “Seed to Seed 101” article on page 7 of our pdf. newsletter Seeds: The Promise of Life.

Seed Germination

Inside a seed lies a new plant, also known as the embryo, which lies in a dormant state protected by a seed coat until conditions arise that stimulate it to sprout, or germinate. Most seeds just need moisture and warm temperatures for germination.

When you purchase seeds, the planting instructions on the back of seed packets should tell you if they require special treatment such as those described below. If you have collected seeds from other sources, conduct an Internet search to discover if they have special requirements for germination. You can find germination requirements of many common garden seeds here: Starting Plants from Seeds

Special Treatments for Breaking Dormancy

Some seeds need special treatment for germination to begin. Two common treatments needed include exposure to cold temperatures, called stratification, and exposure to conditions that cause chemical or mechanical damage to the seed coat, known as scarification.

Seed dormancy is a survival mechanism. For example, many seeds of northern plants need a certain amount of exposure to cold temperatures, or chilling, before they germinate. That way, even though their seeds drop to the ground in the fall, they won’t begin growing and produce tender young seedlings right before winter arrives. One common method gardeners use to mimic this process is storing seeds in the refrigerator in a container of moist peat moss for a period of time.

Some seeds have very tough seed coats that prevent moisture and air from entering and stimulating germination. This ensures that not all seed will germinate in a given year so that if the seedlings from one year don't survive, there's still a "bank" of seeds to try again in future years. In nature, acids in the soil and microbial activity soften seed coats over time, and for some species, such as those native to prairies, fire can do the trick. But because we want seeds to germinate on our schedule, not necessarily on nature's timetable (nor do we want to set fire to our gardens!), so we use use tools such as sandpaper or a nail file to nick and scratch seed coats, such as those of moonvine and lupine, so that moisture and air can penetrate and start the germination process. Another option is to soak seeds overnight in a solution of 1 tablespoon of vinegar per cup of water.

Other examples of special conditions seeds require for germination include dark conditions (pansy) or exposure to light (lettuce).

Planting Seeds

The next question is where to start your seeds. You can plant seeds directly outside, or you can start them inside and transplant them outdoors. There are a couple of advantages to starting plants indoors: students get to see the process up close and day by day, and it extends your growing season by allowing you to grow plants in a hospitable place until growing conditions outside are favorable.

In addition to the special treatments we discuss above, there are two other variables to consider when planning your seed starting:

Seedlings’ temperature tolerance. Some seedlings, including salad greens and peas, are hardy and can tolerate cold temperatures, so you can plant them early in the spring, or in warmer climates, right through the winter! Others, such as zinnias and beans, aren’t cold hardy so wait to plant them until the chance of frost has passed. On the flip side, when you plant fall and winter gardens, wait until temperatures cool before sowing seeds of heat-intolerant seedlings.

Transplant Tolerance. Seeds that germinate and grow better when sown directly into the garden include beans, sunflowers, lettuce, radishes, cucumbers, squash, and peas. Others are more successful if you start them inside for transplant into the garden, including tomatoes, impatiens, and mint. There are still others that aren’t fussy either way, such as marigolds and zinnias. This information should be spelled out on seed packets.

To start seeds outside:

Have your garden beds prepared ahead of time. For guidance on preparing soil and laying out garden beds, read Planning for Sustainability, Part 3.
Read the instructions on the back of seed packets to determine when to plant them.
Work with students to mark rows and beds, then sow your seeds, following the instructions on the seed packet for planting depth and spacing between seeds.
After planting, water the bed using a gentle soaking spray. A strong stream of water may cause seeds to float to the lowest part of the garden. Check to make sure moisture penetrates a few inches into the soil.
Monitor your plants and keep the soil moist but soaking, since excess moisture can cause seeds to rot. If too many seedlings emerge in the same spot, consult the seed packet for information on proper spacing, and removed enough extra plants that the seedlings are spaced appropriately. This is called thinning.

To start seeds inside:

Use shallow containers (2 to 3 inches deep) with drainage holes. You can purchase seed starting supplies or use something as simple as an egg carton or yogurt containers with holes punched in the bottoms for drainage. It is easiest to monitor moisture in containers that are no more than a few inches deep...and they require less potting medium!

Fill the containers with a lightweight soilless potting mix. These mixes are made primarily of ground peat moss and have been sterilized, so they are less apt to contain weed seeds, fungi, and bacteria that may hamper growth than garden soil. These mixes also provide good drainage, providing both the aeration and moisture seedlings need. As an experiment, try different types of growing media and have the students observe the success rates.

Moisten the soil before placing it in containers. If you can squeeze a handful of the mix and water comes out, it is too wet and you’ll need to add more of the dry mix. The ideal moisture level feels like a well-wrung sponge.
Plant seeds according to the instructions the packets. If you do not see instructions about how deep to plant your seeds, a simple rule is to plant them 1 1⁄2 to 2 times deeper than the width of the seed.
Water after planting using the mist setting on a spray bottle.
Place trays in a location that receives 6 to 8 hours of direct sunlight or under fluorescent lights for 12 to 14 hours per day (unless seeds need dark conditions to germinate). Under lights, keep seedlings within 2 or 3 inches of the bulbs.
Most seeds germinate best in warm and humid conditions. Comfortable room temperatures (65 to 72F) are adequate for most seeds, but if your area is cooler, you can increase the soil temperature with a heat mat (see Resources for product information).

If the air in your classroom is very dry, you can maintain adequate moisture for germination by creating a tent over the pots or with clear plastic wrap. Prop the plastic wrap off the surface of the planting mix using popscicle sticks or other “posts” so seedlings don’t stick to it. As soon as your plants have their first set of true leaves (the very first you see are called seed leaves, and all that grow after that are true leaves) you can remove heat mats and moisture tents.

Check daily to make sure the mix is moist. With the right conditions, most garden seeds should germinate in 7 to 14 days (unless otherwise noted on the seed packet).

Germination Rates

Most seed packets will list another piece of information: germination rate. This indicates the percentage of seeds in the packet that are likely to germinate when planted according to directions. Even a packet of the freshest seeds, germination is unlikely to be 100%. Seeds may experience damage from the environment (too dry, too wet); some may not be mature; others may possess genetic defects that hamper growth. Measuring the germination rates and then translating that into charts or graphs can make an excellent math lesson. It can also spark inquiry: ...

Care for Seedlings

Seedlings need to be monitored frequently. Instead of setting up a specific watering schedule, check the soil and water only when it becomes dry to the touch, but don’t wait so long that seedlings wilt or they may not bounce back – they aren’t as resilient as full-grown plants.

During this time, the new plants need adequate light. If you notice the seedlings are looking leggy or growing towards windows, then they are not getting enough light. If this occurs in your classroom, ask students to come up with ideas for providing more light to plants (e.g., using reflective surfaces).

Providing Nutrients to Your New Plants

Seeds contain both the embryo (the baby plant) and stored energy that “feeds” the seedling as it germinates and starts to grow. You won’t need to provide any nutrients in the form of fertilizer until the plant has 3 or 4 true leaves. Apply fertilizer only as directed on the product label – more is not better. Too much fertilizer can cause plants to grow new leaves faster than they grow new roots and you will end up with unhealthy and stressed out little plants!

Transplanting Seedlings

When seedlings have 3 to 4 true leaves, you can transplant them into larger pots or into your garden. If you plan to put them right in the garden, you first need to provide them with a transitional time called hardening off. Gradually expose your plants to the elements day by day. First, place them in a shaded area sheltered from direct wind, and bring them indoors at night. Increase their sun and wind exposure a little at a time, and eventually leave them out overnight.

Outdoor conditions also increase evaporation and transpiration (the process of plants giving off moisture), so make sure the potting mix doesn’t dry out! After a week or so, go ahead and plant them in their final home.

You students might also enjoy experimenting with propagating ferns or mosses from spores – read Fond of Fronds for inspiration and instructions.