Sowing Seeds

Spring seems to arrive a little sooner when gardeners start their own plants indoors. Starting plants from seed is a very rewarding activity and there are many locations throughout the house suitable for starting seeds. The two critical factors are the amount of light and the soil temperature. Generally, a consistent temperature near 75 degrees F. (21-24 degrees Celsius) is ideal. This can be achieved on top of the refrigerator or on a heating pad. For adequate light consider the area under cupboards with fluorescent lights, a window sill, or on the floor of a solarium.

The first leaves of seedlings emerging

The first leaves of seedlings emerging

When should annual flower seeds be sown? Sowing dates are a little flexible because many factors have an influence on a seedlings rate of growth. These include light levels, day and night temperatures, fertilizer application, soil type, timing of transplanting, moisture levels, and seed age. The best advice is to use the dates below only as a guideline and make a chart that is specific to your conditions. The following sowing dates are determined by counting backward from the average frost date – in this case I have picked the first week in June. If you feel that it will be safe to plant annuals outside during the last week of May, then subtract one week. One of the fastest growing annual vines are morning glories which, if sown on May 1st, will probably be ready for outdoor planting within 4 weeks. Cosmos should be sown on April 18th, 6 weeks before planting outside. There are several annuals, such as amaranthus, sunflower, sanvitalia, and zinnia, that should be sown on April 12th. Annuals that require 8 weeks of indoor growing time should be started on April 5. These include melampodium, mimulus, nicotiana, marigold, and nasturtium. Brachycome, gomphrena, portulaca, and thunbergia are annuals that should be sown near the end of March. Annuals that require 10 weeks to reach a size where they can be planted safely outside are ageratum, celosia, cleome, annual phlox, and salvia. Sow annual dianthus, coleus, impatiens, and nierembergia indoors on March 15. Gazania, lobelia, petunia, and dusty miller require 12 weeks before they can be moved outdoors and should be sown in early March. Snapdragons, ivy geraniums, seed geraniums, and pansies are some of the slowest growing annuals and require 14 or more weeks before they can be planted outdoors. Start them around the middle of February. The slowest growing group of annuals includes begonia, browallia, and eustoma. These plants need to be sown in early February so that they can have 16 or more weeks to grow before being planted outside.

Starting seeds is addictive and can quickly fill up your light garden or windowsill. Finding space and containers is sometimes a challenge. The most important requirement of a seeding container is to hold soil. Commercial seeding trays can be purchased as a complete kit (full-sized or window sill version) which contains a water-holding tray, cell pack inserts, and a clear plastic lid. Gardeners don’t necessarily have to purchase new equipment for this project if they have some shallow plastic or clay pots handy. These can be used with a plastic bag to simulate a miniature greenhouse. Some gardeners who find they have more seeds or seedlings than pots or flats become very resourceful. They use margarine or yogurt containers, egg shells halved or styrofoam cups. All of these can make great seeding containers as long as they have drainage holes.

Tomato seedling

Tomato seedling

Soil is critical to the success of starting seeds. It must be sterilized, light-weight, and have a fine texture. It also must have a small nutrient supply, hold moisture, but also provide good drainage. The simplest way to meet these requirements is to purchase soil that is manufactured specifically for starting seeds.

Should seeds be covered? Most seeds will germinate well if they are covered with a thin layer of soil. This soil provides moisture and darkness which is required for good seed germination. Seeds that need absolute darkness should also be covered with a dark garbage bag to block all light. Upon germination, all seedlings will stretch looking for light. Check your pots daily for emerging green stems or roots. This is the beginning stages of germination. Change the dark cover to a clear plastic one if more than 40% of the seeds have germinated.

A small group of plant seeds must remain uncovered in order to germinate well. They require light to trigger germination. Some of the annuals that are in this group are: ageratum, begonia, brachycome, browallia, coleus, sweet alyssum, mimulus, nicotiana, petunia, portulaca, salvia, dusty miller, and torenia. When sowing these seeds, scatter them on the soil surface. Gently press them down into the soil so that they have good contact but are still exposed to the light. A clear plastic cover will keep the humidity high near the seeds and increase germination time.

A few annuals need to be lightly covered with soil to have the best germination results. They have to be as close to the surface as possible for exposure to light and they have to have a thin covering of soil to maintain humidity and moisture. Keep the soil surface moist. Some of the plants that should be lightly covered are: amaranthus, Madagascar periwinkle, cleome, dianthus, heliotrope, impatiens, melampodium, nierembergia, marigolds, verbena, and pansy.

For other annuals not listed above, check their seed packet or a reference book for instructions. Most likely they will be in the group that needs to be lightly covered.

Germination and growing temperature requirements are often different from one annual to another. In a lot of cases the minimum night temperatures are the critical ones. Refer to seed packets, seed catalogues or a reference book for information on what temperatures are ideal for optimum plant growth.

Some annuals don’t like to be disturbed – they resent being transplanted and will sulk. These plants should be sown into a bigger pot to reduce the amount of shock during transplanting. Some seeds can be sown into fiber pots to avoid disturbing them later since they can be planted outside while still in the pot. Eschscholzia, morning glory, nasturtiums, phlox, sunflowers, sweet peas, thunbergia, and zinnia should be sown directly into their final indoor growing container. Place 2-3 seeds in each cell or container and later thin the seedlings to a single strong plant.

A few other annuals produce plants that are too small to handle, such as portulaca and sweet alyssum. Several seeds of these can be sown into each final growing container. No thinning is needed in this case.

Proper misting and watering are important for good germination and plant health. Check the soil moisture level at least once a day. If a seed is allowed to dry out after it starts to soak up moisture, it will die. Use tepid (room temperature) water that has been allowed to sit for 12 hours. For delicate seedlings, a mister will gently apply water to the soil without causing injury from wash-outs or fallen plants. If pots or trays of seeds are very dry they can be placed into a laundry sink with a shallow amount of water. The pots will gradually soak up water from the bottom and the seeds or seedlings will not be disturbed. Once the top of the soil is barely moist, remove the pots from the sink and drain them.

Novice gardeners and children can have great success with starting seeds indoors by selecting plants that are easy to grow. Marigolds, geraniums (takes a long time), nasturtiums, sunflowers, morning glory (soak the seeds overnight), and sweet peas (rub the seed with sandpaper then soak the seeds overnight before sowing) are excellent suggestions.

Sow more seeds than you need for the garden. The average germination rate is between 60 and 70 percent, so plan to sow 50% more seed than you require as plants. If you end up with more plants than you can use, donate them to a local horticulture or gardening society or give them to your neighbours.

Extra seeds can be stored for next year in a sealed glass jar in the refrigerator. This helps keep the moisture levels high in the seeds. Remember seeds are living things. Viability will decrease at different speeds for each seed type. Some seeds can be stored for up to 5 years while others should be purchased fresh every year. Begonia, impatiens, onion, peppers, and viola have shorter storage viability than tomato, sweet pea and zinnia.

The transplanting process is very simple with two main rules. Never hold your seedlings by the stem. Use your fingers to grasp only the leaves in order to avoid causing serious damage to the fragile stem. Use your transplanting tool or your finger to create a hole in the centre of new container to the depth of your seedlings roots. Carefully separate one seedling from the rest and plant it into the hole. Plant it at the same depth as it was growing. Gently firm the soil around the roots to remove any air pockets. Once again, be careful of the fragile stem. The second rule is to water the seedling after transplanting. Then place it under a light garden or cover it for a few days if it is on a sunny windowsill.

First two images copyright, last two images courtesy of and used under the creative commons license.

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