Top 5+ tips to know when propagating trees and shrubs from seeds

Gardeners love plants. We love to try new plants. We delight in trading plants.

What we generally hate to do is spend money on plants. We’ll just say that as a group, gardeners tend to be ... shall we say ... thrifty?!

With that in mind, it has always perplexed me that so few gardeners take advantage of the free system Mother Nature has provided all of us who so uniformly tend to have squeaky wallets.

Sure, many of us are completely happy to pick up a few packets of basil or zinnia seed and start a few trays on a sunny windowsill or in a hastily thrown together plant rack in the basement. But when it comes to growing the woodier members of the garden from seed, we seize up as if someone just handed us a laser and suggested we self-perform Lasik surgery.

Like just about all garden tasks, growing woody plants from seed isn’t all that complicated. With few exceptions, it takes just a little basic knowledge, and of course, a constant dose of humility.

Seeds of some temperate woody plants contain germination inhibitors to help ensure seeds germinate at the most opportune time of year. Removal of the red/orange fleshy pulp of this big leaf magnolia (Magnolia macrophylla) can help speed up germination.

Here is what to know about how to propagate trees and shrubs from seed:

What is seed maturation?

Most woody plants native to temperate regions of the globe (those with distinct summers and winters) tend to mature their seeds late summer or fall. But when fully mature, many seeds still won’t germinate because Mother Nature has trained many plant species to use one of several mechanisms to make sure the seeds don’t germinate in September/October only to get nuked by the onset of winter weather.

Most of those seeds require some bit of cold weather exposure or other mitigating factor to overcome the inhibition and set the stage for a more sensible spring germination.

A small handful of common woody garden plants (hydrangeas and clethra) produce seeds that are ready to germinate as soon as they are mature. These tend to be species that don’t put a lot of effort into disseminating their seed in fall the way maples (whirling “helicopters”) and hollies (bird distribution) do.

When should seed be collected?

This can be a little tricky. You want to collect seeds when they are fully mature but before they dry too much. Seed of some species, maples for example, can enter a very frustrating state of double dormancy if you let them get too dry.

If you’re a detailed, process person, you can buy an accurate (read: expensive) balance and track the weight of a sample of seeds each week through the fruiting season. When they stop gaining weight, the seeds are generally ready. But it’s much easier (and cheaper) to watch the seed color. When seeds/fruit start to yellow or show a touch of brown, they are generally ready. In fleshy seeds, like magnolias or crabapples, peak fruit color (red/orange) generally marks seed maturation.

How to tell if a seed is dormant

Dormant seeds are mature. It’s just that there is something keeping them from germinating. No matter how nice and cozy a setting you give them, they’ll just sit there and smugly refuse to do anything. You have to know what type of dormancy you’re dealing with to overcome it and get the seeds to germinate.

Most temperate region perennial plants produce seeds with an internal chemical dormancy (generally a hormone balance) that must be overcome before germination can occur. And in most cases, it is the winter temperatures (or the fluctuation of cool/warm temperatures through the cooler seasons) that eventually causes those internal hormone levels to change. Once the hormone levels hit their target levels, the seed is ready to germinate.

But in order for the cold to do its thing, the seeds must first absorb some water to get the hormone level machinery in the mood to do its seasonal dance.

If you’re trying to germinate an acorn or an apple seed, it’s pretty straightforward. You harvest the mature fruit, clean the seed, and plant it out in the soil. In that cool and damp soil, the seeds will absorb plenty of water, which, in turn, will start the cold weather clock. Generally, 60 to 90 days of cool after water absorption puts the seed hormone levels in the right ballpark for germination.

But it isn’t always that straightforward. There are two additional wrenches that some plants throw in the germination gears.

What is an impermeable seed coat?

Some seeds complicate matters by producing a hard seed coat that does not allow water to get into the seed. And if the water doesn’t get through that coat, the hormone level clock never starts. The impermeable seed coat strategy generally helps survival of the species by making sure all of a season’s crop don’t germinate in the same year. The process of weathering and/or microbial action gradually breaks down that seed coat to let the water in, but that process may take several years.

To overcome the impermeable seed coat strategy you have to break the seed coat. This can be done commercially by soaking seeds in concentrated sulfuric acid — not a process I would recommend at home. You can rub seeds between a few blocks of wood with coarse sandpaper glued to the planks. With some large seeds like Kentucky coffee tree (Gymnocladus dioius), you can gently crack the expoxy-hard beans in a vice or file a shallow notch in the coat —anything that breaks the seal and doesn’t damage the delicate embryo beneath.

What are seed germination inhibitors?

A third type of dormancy involves the plant producing seeds with a germination inhibiting compound in the outer, usually fleshy layer of the seed. In magnolias, the bright red/orange fleshy coating can keep those seeds from germinating for several years, until weathering breaks down and/or weathers away those compounds. Gardeners can circumvent that process by manually removing that pulp before providing the cold, damp treatment.

And if you’re paying attention at all, you can probably guess that some plants employ one strategy. Some plants employ two strategies. And still other plants stack all three, just to keep us humble.

Paul Cappiello is the executive director at Yew Dell Botanical Gardens, 6220 Old Lagrange Road, yewdellgardens.org.