Seed Starting Guide: Grow Plants from Seed

Starting plants from seed is one of the most rewarding gardening skills. Growing from seed offers hundreds more variety choices compared to buying transplants at the garden center, is significantly more economical than purchasing started plants, and gives you complete control over growing conditions from the very beginning. This guide covers everything from seed selection through transplanting.

Choosing Seeds

Select varieties that match your climate and growing conditions. Check the days to maturity listed on seed packets and make sure your growing season is long enough in your climate for the variety to produce before frost. Look for disease-resistant varieties labeled with codes like V for verticillium wilt resistance and F for fusarium wilt resistance in tomatoes.

Choose open-pollinated varieties if you plan to save seeds from year to year. Heirloom varieties are open-pollinated varieties that have been grown and passed down for generations, often with superior flavor and unique characteristics. Hybrid F1 varieties offer hybrid vigor with higher yields and better disease resistance but will not grow true from saved seed.

Seed Starting Supplies

Use clean containers with drainage holes at the bottom. Cell packs, six-packs, and propagation trays with clear humidity domes work well for most seeds. Recycled yogurt cups and egg cartons are usable if you poke drainage holes in the bottom. Thoroughly clean and sanitize any reused containers in a solution of one part bleach to nine parts water.

Use a sterile seed starting mix rather than garden soil or potting soil. Seed starting mix is a fine-textured, soilless blend of peat moss or coconut coir, perlite or vermiculite, and sometimes a small amount of fertilizer. The fine texture provides good seed-to-soil contact, while the sterile components prevent damping off and other soil-borne diseases that kill seedlings.

Consider using bottom heat with a seedling heat mat placed under propagation trays. Most seeds germinate best when soil temperature is seventy to eighty degrees Fahrenheit, which is warmer than typical indoor air temperatures. Heat mats raise soil temperature twenty degrees above ambient and significantly improve germination speed and percentage for warm-season crops.

Planting Seeds

Fill containers with moistened seed starting mix and firm gently. Plant seeds at the depth recommended on the packet, generally two to three times the seed diameter. Tiny seeds like lettuce and petunias are surface sown and need light to germinate, so press them into the surface without covering. Large seeds like beans, squash, and sunflowers are planted one inch deep.

Label every container with the variety name and planting date. Use waterproof markers or plastic plant tags inserted into each container. Unlabeled seedlings look identical, and once they start growing you will not be able to identify which variety is which until they fruit or flower.

Water from the bottom by setting containers in a tray of water and allowing capillary action to pull moisture up through the drainage holes. Bottom watering prevents the soil surface disturbance and splashing that dislodges seeds and promotes disease. Remove containers from the water tray once the surface glistens with moisture.

Light and Temperature

Provide bright light immediately after seeds germinate. Seedlings need fourteen to sixteen hours of strong light per day to grow compact and stocky rather than tall and leggy. A sunny south-facing window provides enough light only in late spring. For earlier starts, use grow lights positioned two to four inches above the seedlings and adjust upward as they grow.

Maintain proper temperatures for healthy seedlings. Most seedlings grow best with daytime temperatures of sixty-five to seventy-five degrees Fahrenheit and nighttime temperatures ten degrees cooler. Cooler nighttime temperatures produce stronger, more compact seedlings. Too much heat causes soft, leggy growth that does not transplant well.

Transplanting

Harden off seedlings before moving them to the garden permanently. Hardening off gradually exposes seedlings to outdoor conditions — wind, direct sun, and temperature fluctuations — over seven to ten days to prevent transplant shock. Start by placing seedlings outside in a sheltered, shady location for one to two hours and gradually increase exposure each day.

Transplant hardened seedlings to the garden on a cloudy day or in late afternoon to reduce water stress. Water transplants immediately after planting with a diluted liquid fertilizer solution to provide a gentle nutrient boost. Protect new transplants from intense sun, wind, and pests with row covers or shade cloth for the first few days after planting.

Hardening Off and Transplanting

Hardening off gradually acclimates indoor-grown seedlings to outdoor conditions. Start the hardening off process seven to ten days before the planned transplant date. Place seedlings outdoors in a sheltered, shady location for one to two hours on the first day. Choose a calm day with mild temperatures and no harsh wind or direct sun exposure.

Gradually increase outdoor exposure each day, adding one to two hours and moving seedlings into gradually more sun and wind exposure over the week. Bring seedlings indoors at night or when temperatures drop below 50 degrees Fahrenheit. Protect seedlings from heavy rain, strong winds, and temperature extremes during the hardening off period. After seven to ten days of gradual exposure, seedlings should be ready for full-time outdoor conditions and transplanting into the garden.

Transplant hardened seedlings on a cloudy day or in the late afternoon to reduce transplant shock from sun exposure. Water the seedlings thoroughly a few hours before transplanting so the root ball holds together during handling. Dig holes slightly larger than each root ball at the appropriate spacing for the mature plant size. Gently remove each seedling from its container, supporting the stem rather than pulling on the top growth.

Place each seedling in its hole at the same depth it was growing in the container, except for tomatoes which can be planted deeper to the first set of leaves to develop additional roots along the buried stem. Firm soil gently around each seedling and water immediately with a diluted liquid fertilizer solution, using half the recommended strength to provide a gentle nutrient boost without burning tender new roots.

Common Seed Starting Problems

Poor germination is most often caused by soil temperature that is too cold, seeds planted too deep or too shallow, soil that dried out before germination, or old seeds with low viability. Check seed packet dates and replace seeds older than two to three years for most vegetables and flowers. Use a soil thermometer to verify proper germination temperature and adjust with heat mats for warm-season crops.

Damping off disease kills seedlings by rotting stems at the soil line. Prevent damping off by using sterile seed starting mix, clean containers with good drainage, avoiding overwatering, providing air circulation with a small fan set on low, and thinning seedlings promptly to prevent overcrowding. Water from the bottom rather than overhead to keep the soil surface drier.

Leggy seedlings with tall, thin, weak stems indicate insufficient light or excessive heat. Move grow lights to within two inches of seedlings or increase daily light exposure to fourteen to sixteen hours. Reduce room temperature if running above 75 degrees Fahrenheit during the day. A light breeze from a small fan strengthens seedling stems by gently swaying them, which triggers hormone responses that produce thicker, stronger stems.

Yellow or purple leaves on seedlings usually indicate nutrient deficiency. Once seedlings have their first set of true leaves, begin feeding with a diluted liquid fertilizer at half strength every one to two weeks. Purple coloring on leaves and stems indicates phosphorus deficiency, often caused by cold soil temperatures that prevent roots from accessing available phosphorus in the seed starting mix.

Frequently Asked Questions

When should I start seeds indoors?

Count back from your average last frost date. Warm-season crops like tomatoes, peppers, and eggplants need six to eight weeks indoors. Cool-season crops like broccoli, cabbage, and lettuce need four to six weeks. Melons and cucumbers need only three to four weeks because they grow quickly and do not transplant well if root-bound.

Why are my seedlings leggy?

Leggy seedlings that are tall, thin, and flopping over indicate insufficient light, light that is too far away, or temperatures that are too warm. Move grow lights to within two inches of the seedlings or increase the duration of light exposure. Reduce temperatures to the preferred range if they are running warm.

What is damping off and how do I prevent it?

Damping off is a fungal disease that causes seedlings to rot at the soil line and fall over. Prevent it by using sterile seed starting mix, clean containers, avoiding overwatering, providing good air circulation with a small fan, and keeping soil surface dry between waterings.

Do I need to use a heat mat for seed starting?

Heat mats are not required but significantly improve germination for warm-season crops like tomatoes, peppers, eggplants, and melons. Cool-season crops like lettuce, broccoli, and kale germinate fine at typical indoor temperatures and do not need heat mats.

How do I know when seedlings are ready to transplant?

Seedlings are ready to transplant when they have two to three sets of true leaves (not the first seed leaves / cotyledons), have a well-developed root system visible at the bottom of the container, are about four to six weeks old, and have been hardened off for outdoor conditions.

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