Article by Steve Marple, Sammamish Resident

Starting plants from seeds is not particularly difficult if you take certain preparations. It has the advantage of making available many more varieties than are routinely available at nurseries and can save on costs too. The main considerations are: do some research to see if the seeds need a special pretreatment, use proper media / soil, have good lighting and suitable temperature. These criteria can be met at minimal cost if you just have a little space to devote to the project.

Many seeds will grow easily if planted at room temperature about 6-8 weeks before it is the appropriate season to plant outside. But some need at least a month of a cold and moist period before they will germinate (e.g., native trees and Japanese maples). Others may need different treatment like scarification (scratching the seed coat). Do a little Googling and check several sources to find the most commonly used method. I find that I will get more authoritative sources if I search by using the scientific name. The Royal Horticulture Society is a good source - RHS Germination guide. For cold moist stratification you can put seeds in some damp media in a plastic bag and store in the refrigerator for 30-90 days. My method so that I do not forget about them in the fridge is to plant them in pots or trays in late November to mid-December and keep them in a cold frame, so they are protected from critters. I scarify small to medium sized seeds by folding them in a whole sheet of medium emery cloth and rub the cloth firmly with the palm of my hand. Do it briefly, and check to see if the seeds look scuffed and some dust has formed. For large seeds you can use a file to cut a small gap through the seed coat. Other treatments may be recommended but check reliable garden resources to confirm.

A seed starting mix should be low in nutrients. This will help reduce rotting of the seedling (“damping off”); the seed provides all the nutrition needed for the initial growth. I generally use a soil-less mix like Pro-mix. Using a premium growing medium helps ensure success, and I particularly do not want failure of a carefully timed planting for the vegetable garden. Nor do I want to lose seeds that are expensive, uncommon, or received as gifts. Research will tell you if you need a more specialized medium. Some may require a sandier mix that does not stay as moist such as curry leaf tree (Murraya koenigii), as well as many alpine species and desert succulents. Some sources may recommend a layer of sand over the top of the medium to reduce drying for seeds that take a long time to germinate. Another method to keep the soil evenly moist for long periods is to use a humidity dome that fits over the nursery tray. Sometimes I cover the top of a pot with plastic wrap and secure it with a rubber band. After germination, or if any sign of mold, I will poke holes in the plastic for ventilation. Once they have germinated, I remove the plastic, because the covering blocks a lot of light and good ventilation is needed to prevent losses to fungi.

One other consideration is planting depth. Generally, it is proportional to the size of the seeds. Tomatoes I cover about 1/4”and squash about a 3/4”. Some fine seeds should not be covered at all because they require light to stimulate germination. If you are trying to reduce cost or just running out of soil, larger seeds will probably be fine planted in garden soil or used potting mix with compost. The larger size and more vigorous growth can withstand the less controlled conditions.

There are lots of different lighting options available now. Unfortunately, few windows receive enough light for ideal seedling growth. Many of us would love to have greenhouse, but that is not always possible. Small makeshift cold frames can work well for cold hardy varieties or help keep warm season plants in mid-spring when acclimating seedlings started under lights. Artificial lights work very well. Though there are many options, here I will just keep it simple. For seed starting you need 4000 – 6500K spectrum bulbs. Almost all of my seed starting has been using basic fluorescent shop lights with “Daylight” bulbs (5000K). Four 4-foot bulbs side by side are perfect for growing two standard 10x20” trays. Suspend the lights using either chains or cord with sliding knots, so the height of the lights can be easily adjusted. It is especially important to keep the lights at a distance that will provide proper intensity. For the basic T12 or T8 fluorescents I keep the bulbs within 1-2” of the top leaves. (Some high intensity lights would be positioned further from the plants.) I also use wood or foam pieces under the pots to get them closer. This helps when you have plants that are different heights. I check and adjust almost daily. It is a morning ritual that I thoroughly enjoy.

The final criterion to meet is temperature. Average house temperature will work for most species I have grown. Some germinate better at 75-80 degrees, so a heat mat will help for those. Maybe there is a warm location in your house where you could place a tray for a week to help germination but be sure to get them to good light ASAP once they germinate. Also, after germination you can turn the heat mat off. The lights will provide some warmth, but seedlings grown a little cooler will be sturdier / hardier. A basement that is about 65 degrees can be fine for starting things like tomatoes. Cooler cold frames and unheated greenhouses are great for cold hardy and half-hardy varieties. Those will germinate when the temperature is right for them. Going out most every day of late winter to early spring to look for a new variety germinating is quite exciting for me.

Pot size or planting whole trays is mostly a matter of choice partly due to space limitations. For the varieties I plant in fall that will germinate in the spring I plant mostly in gallons or shorter 8” pots, such as our native Cascade penstemon (Penstemon serrulatus) and tall Oregon grape (Mahonia aquifolium). I use that larger size for greater ability to hold a consistent moisture level. I also like a deeper pot to have more room for root growth. I rarely plant in small cells that would receive only 2-3 seeds. That is tedious, takes up more space, and I am confident that I can transplant seedlings when they become crowded in a larger pot. So, I generally plant seeds in one pot for each variety. With new varieties I guess at germination success, and sow quite a few evenly over the pot. Sometimes they end up too dense, but I can make do by thinning or transplanting the small seedlings.

New seeds of Erigeron speciosus (Fleabane)

When the seedlings grow their first true leaves, I fertilize with a half-strength mix of a balanced soluble fertilizer (20:20:20 or similar). I water as needed, and top water so that each pot is watered for its need. I will feel the soil for moisture content sometime, but I can usually tell by color if it was dried out. A high-quality soil mix has good aeration, so it does not get soggy unless extremely over watered. Especially when plants are moved outside, I make sure the trays will drain so the pots are not left in standing water. Vigorously growing seedlings like tomatoes can be transplanted to individual pots when 3-4 weeks old. Some species will need 2 months or more to be a size that can be easily handled. The stems are fragile, so always grasp them by a leaf. The new soil can contain some compost for added nutrients, but some species may benefit from a more specialized mix to meet their needs. That is particularly true for plants that need a very well-draining soil that will dry out some in between watering. Again, pot size will be a matter of choice depending on the vigor of the variety and the length of time it will stay in the pot.

One particularly important thing to remember is that seedlings started under lights or in a greenhouse will need gradual acclimation to direct sunlight. Glass and plastic block UV rays, so plants grown in greenhouse or under lights have not established their defense against the UV rays of the sun. So, to put them right out in full sun all day long will result in sunburn that will damage leaves and could be fatal. I acclimate over a period of at least a week, with just a few hours of filtered sunlight the first couple days out. One way to make this simpler is to place them where a shadow will fall on them in 2-3 hours. Cloudy days or dappled shade are good ways to get their first exposure. After 4 days they could stay in the sun for 4-5 hours (if not a shade loving species). A few days of that and they can be left out …… if the temperature is acceptable. Watch the weather and bring in tender plants if the temperature will be below 40. Cold frames can really help in this period. Do be aware that direct sun can make cold frames or greenhouse get too hot. They must be opened to let heat out. Some people position fans to blow across the seedlings to reduce humidity that could cause fungus problems, and to make the stems sturdier. I have not needed that. With good light intensity and fertilizing, the stems are short and sturdy. I start acclimation at 4-6 weeks old (when running out of space under the lights). The first rains and winds during acclimation will batter the plants a bit, but after a few hours of sun they should perk up. Keep watching the weather to decide when to plant out, or bring back in. At times I have put them under a patio table for protection from excessive rain. Most are pretty tough though, so don’t make it too difficult.

Gardening takes patience, but I find a great sense of achievement by growing plants from seed to maturity. My father and grandfather’s lessons about being thrifty are evident in my gardening practices. I do all sorts of seed saving and propagation. My yard has several nursery areas with pots of young plants including rhododendrons and daffodils I have grown from seed. So, give it a whirl and try growing those plants you have always wanted. The extras are great for trading too, which we all know is another way to get the many plants we crave.

Article by Steve Marple, with over 50 years of gardening experience in WV, KY, and now 4+ years in Sammamish.