Seed Starting 101

Here are highlights from a seed starting presentation I gave to our local Master Gardeners a few years back.

seed starting supplies

seed starting supplies

Starting plants from seeds is an easy, economical way to get new plants. It can often be the only way to get the specific varieties you want to grow. You can start seeds for many flowers, herbs and vegetables – even trees and shrubs. A seed is a plant just waiting to happen, and all we need to do is create the proper environment for it to grow.

broccoli seedling, with seed shell still attached

I’m often asked questions about starting plants from seed. There’s certainly a lot of different ways to go about it, and every gardener has slightly different methods and materials they like to use.  But most of the basics are very similar. A little bit of planning can ensure success with your seed starting, and perhaps head off some of the common problems before they happen. And if you’re new to gardening, there are a few things you want to address even before you open that first seed packet.

  • Light – young seedlings need 12-16 hours of light per day. For that you need a bright sunny windowsill (south is best) or fluorescent lights. Seeds and seedlings should be kept 1-2 inches from the lights. Use plant grow bulbs or a combination of 1 cool white and 1 warm white bulb. I use fluorescent lights with an automatic timer set to be on for 14-16 hours per day.
  • Containers -need to be clean and sturdy. Plastic is easy to clean and sterilize, plus it is lightweight and reusable. You can use pots, cell paks, or flats. Other choices include wood flats, peat pots and pellets, and homemade soil blocks. Plastic cups and the bottoms of milk cartons can also be used.
  • Growing medium -needs to be well-drained, light and fine-textured. You can use a commercial mix or make your own. Avoid using soil  straight from the garden, because it’s heavy and can harbor diseases and insects. A commercial mix containing peat moss, vermiculite and perlite works well (I use Pro-mix). Coco coir, which is made from coconut husks, is gaining popularity as an economical and earth-friendly alternative to peat moss.

There are also a few other things to research before you start sowing those seeds. Much of the information can be gleaned from the seed packet itself, or from a good gardening book. Johnny’s Selected Seeds catalog has a lot of data on individual seeds and their needs, plus their seed packets are some of the most informative I have ever seen. And “The Vegetable Gardener’s Bible” by Edward C. Smith has good information about each vegetable’s requirements from sowing to growing. Some of the things you need to know include:

  • When to plant – use the seed pack, catalogue or book to help determine the timing.
  • How deep to plant seed/how much to cover. Some seeds need light to germinate, while others need darkness.
  • Temperature needs (hot, warm or cool). Use a heating mat or cable to warm the soil for heat lovers such as tomatoes, peppers and eggplant.
  • Germination time – how long do the seeds take to germinate
  • Seed viability. Seeds typically last from 1 to 5 years depending on species. Store seeds in a cool dry place. You can test viability using a damp paper towel in a ziploc bag.
  • Special considerations – some seeds need to be nicked or soaked, or need a cold treatment before they will germinate.

handful of lettuce seeds

Once you’ve done your research, it’s time for the fun part – actually sowing some seeds. Here are a few tips and considerations about the process itself:

  • If using pots or containers, fill with mix to within an inch of the top. This leaves some room for watering later on. Either pre-moisten the mix or water after filling the container but definitely moisten it before seeding. If using small cell packs, fill almost completely with soil.
  • Sow seeds thinly and uniformly (try not to put too many in one spot). The number of seeds depends on the size of the container. Sow several seeds in pots and larger containers (you might sow a dozen or more small seeds in a 4 inch pot), and sow one or two per cell pack, peat pot or soil block. You can thin or cull seedlings later as needed.
  • Cover seeds with seed starting mix, vermiculite or sphagnum moss. For seeds that need light to germinate, don’t cover seed at all. For soil blocks and peat pellets, sow the seed to the proper depth.
  • Water gently or mist. Bottom watering is also good – place container in pan of water. You want to make sure the seed has good contact with the planting medium.
  • Containers or flat may be covered with clear material like plastic wrap or cover to retain moisture and heat. Remove covering as soon as germination is underway. Don’t put covered containers in direct sunlight, or the seedlings will get cooked.
  • Label what you plant. Remember, all tomato seedlings look about the same when they’re coming up!
  • Don’t allow seeds to dry out before germination. Check daily and water as needed.

lettuce seedlings in cell pack

Once you’ve got the seedlings up and growing, here are a few considerations to keep them healthy and happy:

  • Keep seedlings in good strong light to keep them short and stocky. Tall, spindly seedlings are a sign they’re not getting enough light. If using fluorescent lights, adjust the distance to keep the growing plants within 2″ of the lights.
  • Maintain a good growing temperature , typically around 70F during the day and 60-65F at night. Check specifics for each plant.
  • Do not over water, water only when needed. Soil should begin to dry out between watering, but never allow seedlings to wilt.
  • Fertilizer – plants will need fertilizing. Begin 2-3 weeks after germination, or when true leaves start to appear. Use half strength or weaker liquid fertilizer every two weeks (I use a fish emulsion and seaweed combination like Neptune’s Harvest)

Problems you may encounter, and how to deal with them:

  • Damping off – seedlings fall over at the soil line. Caused by fungus disease, may be worse with high temps, poor light, excess moisture, lack of air circulation. Avoid by using sterile mix, clean containers. Can be treated with fungicide if caught early.
  • Poor germination – old seed, soil too wet or dry, temps not right, seed covered too deep
  • Tall, leggy plants – not enough light, too hot

Special considerations and terms:

  • Stratification – cold treatment necessary for some perennials, trees or shrubs
  • Scarification – tough seeds need to be nicked or scratched
  • Soaking – some seeds need to be soaked in water before sowing

Transplanting and Thinning:

  • Should be done after seedlings have one set of true leaves, and are large enough to handle
  • Transplant to individual pots or cells, or thin to one per pot or cell
  • Use soilless growing mix or good quality potting soil (mix Pro-mix with compost or potting soil)
  • Dig up or separate plants, disturb roots as little as possible
  • Handle plants by root ball, or leaves, never by stem
  • Make hole in center of soil mix, plant seedling at same depth (or slightly deeper for some species)
  • Gently firm soil and water
  • Label plants

Hardening Off & Growing On:

  • Plants grown indoor are tender and need adjustment before moving or planting outdoors
  • Allow at least 2 weeks to harden off
  • Gradually acclimate plants – move to shady spot, then gradually expose to sunlight
  • Protect from wind, and extreme heat or cold
  • Cold frame can be used for growing on and hardening off
  • Repot in larger container if planting is delayed to avoid rootbound plants



“Park’s Success With Seeds”

“Seed Starter’s Handbook” by Nancy Bubel