There are several things you have to understand about seed starting.
Seeds germinate based on a change in hormones within the seed. For the most part, annual seeds germinate when the soils warm up in the spring. So home gardeners need to be able to create warm soils.
They germinate when adequate moisture is available so we have to provide appropriate moisture. Note this doesn’t mean a lot of water – it simply means what the seed requires.
I note this is generally high humidity around the seed coat, not being soaked in water.
Seed starting most annual plants is easy and this activity makes a wonderful family project that delights small children as well as their gardening parents.
I suppose non-gardening minds will ask if there is a similarity between the delights of small children and the delights of gardeners and I am pleased to be able to answer in the affirmative.
These similarities are why there are no old gardeners although you may see older people gardening
The first step in seed starting is to choose an appropriate container and while gardening magazines promote a variety of things, I use flower pots.
They work well, hold enough moisture to prevent excessive drying out and are relatively inexpensive. Most of us have a few lying around the place so they aren’t hard to come by or expensive either.
Clean them thoroughly; there is no sense starting good seed and have them die because of bacteria from old soil on the pot.
If you can’t be bothered cleaning them out – use pots that have sat in the shed for a year and the soil is thoroughly dried out. (Note there is research now suggesting cleaning isn’t necessary *if* the pot has been left empty for a year between uses.)
Bad “guys” are pretty much toast if the soil has been dry that long. What you don’t want to do is go from fresh soil to fresh soil to fresh soil – without cleaning out the pot.
Yes, you can use just about anything that holds soil but do understand that paper products such as egg cartons (beloved of back-to-earth magazines) will dry out fast and will be harder to maintain soil moisture because of their smaller sizes.
I use an soilless mix such as Promix for my seeding because it is sterile, and weed free.
Few things are as frustrating to find the plant you have been nurturing for several months on your windowsill is in reality a large weed with no redeeming floral graces. That’s experience talking.
If you really, really, want to use potting soil, (I don’t recommend it but…) put it into the pot leaving one half to one inch of space between the soil top and the rim of the pot and then slowly pour boiling water into the pot until the water runs freely out the bottom.
Let the soil cool before seeding. I don’t usually recommend potting soil because it comes in a wide variety of qualities and can compress during the growing cycle.
Do not use garden soil as it compacts terribly and seed germination rates will be much lower than in artificial soil.
Compost/Manure in the Soil
Here’s the real deal. Never use manure in a seed starting soil. So if you have one of those “wonder” soils in packages that contain it – do not use that soil for seed starting. There are simply too many problems that come along with using manure in this way.
Here’s the deal with compost. If it is fully composted, if it has been hot composted to destroy pathogens, you can actually germinate and grow your seeds in 100% compost.
If it isn’t perfect compost (and you’ll seldom buy perfect compost I note) then using it will kill off your seeds/seedlings.
If in doubt, try growing tomatoes or cress in a 100% trial pot of the stuff. If the cress grows well, it is OK to add. This is the most sensitive indicator. Tomatoes will grow in almost-OK and if they grow, then most other things will be OK to have some of this compost added to the mix. If neither of these plants does well, do not add any of that compost to the mix.
If it isn’t perfect compost (and you’ll seldom buy perfect compost I note) then using it will kill off your seeds/seedlings
There are many examples of poor compost taking out seeds, seedlings and even young plants in fields.
It is much better to avoid using compost in seed starting mixes but there are some gardeners who swear by their methods. Not me. I want consistent soil starting and growing seedlings.
Hormones and Additives
Some advanced gardeners use hormones such as Gibberellic acid-3 (GA-3) to induce germination in tough-to-start seeds.
This is pretty specialized gardening and frankly, is a waste of time for all but the very toughest of seeds. I’m not going to tell you how to do this in this introductory article because you can do more damage to regular seeds by using this stuff. I will write about it in another section.
It’s like putting jet fuel in an old volkswagen – you’ll simply kill the engine/seed rather than get extra speed.
Let me simply say that I never used GA-3 in my nursery and we grew over 1800 varieties of perennials and 6-800 varieties of annuals every year.
There are some seeds, e.g. species Rhododendrons, that can be a bit tricky to grow and the nursery solution is to scarify (mark) the seed coat so that moisture can get through the shell of the seed to induce germination.
Different kinds of acid are used in commercial seed starting of some rare plants (Liquid Plumber was a favorite) to get higher germination rates but given that you’ll be happy with a plant or two rather than 100% germination, you don’t need to use phosphoric or sulphuric acid.
One thing that has been shown to improve germination rates on home scale sowing is the use of seaweed teas. The hormones in seaweed act to stimulate seeds – natural GA-3 – and you’ll get a slightly better germination percentage if you water once or twice with this product (do it right after you sow and then forget about doing it again).
Don’t use soft water if at all possible. There’s a potential for salt buildup and seedlings don’t like salt.
Always use warm water when watering any kind of seedling flat – from annuals to perennials. The only exception to this is outside watering. While it might be a great idea, it’s a ton more compulsive than I’m ever going to be and no nursery person I know does it.
If a seed is tough enough to germinate outside, it gets what it gets.
I think you’ll find my ebook on propagation will answer a great many of your questions.