13 March 2009
Starting seeds: About the seeds this time
I've been thinking a lot about seeds lately -- it is that time of year, so I thought I'd write a post about getting the things to germinate.
Vegetables and your standard annual flowers are simple to germinate -- get them wet and warm, and hay presto, they sprout, all at once, often within a week. That is because these plants have been bred to do that by us humans -- we like seeds that sprout as soon as we plant them, so we selected for varieties to germinate that way.
But in the wild, it doesn't work that way. Many seeds ripen and fall to the ground in the fall -- but it is better to wait until spring to sprout. Small seeds need to be close to the soil surface to survive -- if they try to sprout when buried too deep, they won't have the strength to reach the surface. Many plants spread their germination out over time -- if all the seeds sprout at once, they can all be killed off at once by a drought, hungry rabbit, or human with a hoe. So they use various method to keep their seeds dormant until just the right time. Here's how they do it, and how you convince them to germinate when you want them to.
Light is an important factor in the germination of a lot of seeds, most common being seeds that require light to germinate. These are usually very small seeds that can't push up from deep underground, so light signals that they are at the surface of the soil, where they can germinate and grow successfully. Incidentally, this is a way weeds persist in the ground for a long time. When you plow or till, weed seeds get buried, and then each time you turn the soil over, some are brought to the surface, are exposed to light and sprout.
For seed that aren't weeds, they are easiest to sprout by sprinkling them directly on the surface of your soil, and then lightly covering them with a sheet of plastic wrap until they get growing so they don't dry out.
Some seeds, on the other hand, require dark to sprout. These are usually larger seeds, and want to be deeper in the soil so they don't dry out while small. For most seeds, these need for darkness can be met simply by planting them deeper in the soil and not placing them under your lights until they've come above ground.
A hard seed coat:
Many seeds have very hard seed coats which prevent them from absorbing any water at all -- so while they may be planted in moist soil, on the inside they are as dry as they were in their packet. Over time, this hard coat is slowly broken down by fungi and microbes in the soil, or weakened by passing through the digestive tract of a bird, eventually the water seeps in, and the seed sprouts. This trait seems to be common with large seeded tropical plants like cannas and bananas, and almost universal in plants in the bean family. I used to spend HOURS making little holes in the hard coats of these seeds with a nail file or a sharp knife or nail clippers -- which is horribly tedious -- and often I'd accidentally cut too deep and damage the inside of the seed. Then I learned the easy way: Put the seeds in a bowl. Pour boiling water over them. Wait a few hours, leaving the seeds in the water as it cools. The seeds will swell up much larger than before (like dry beans when you soak them before cooking). If any of the seeds didn't swell up the first time, fish them out, and douse them with boiling water again. It seems extreme, but works like a charm! The packet of canna seeds I ordered this year said they germinate irregularly over a period of weeks or more. I doused them with boiling water, planted them, and 90% were sprouted within a week.
A period of cold temperature (technically, this is called "cold stratification." you may be more familiar with it as: "winter"):
Most seeds ripen in the later summer and fall, but in cold climates, the best time to sprout is in the spring. So many winter hardy perennials, and almost all winter hardy trees and shrubs produce seeds that won't sprout until after they've spent time in cold temperatures. One way to achieve this is to plant the seeds outside in the fall, and let nature take its course -- but in my experience, hungry mice and birds end up taking their course as well -- so I prefer to use my refrigerator. Now, my partner is not too fond of pots of soil and seeds in the fridge (he prefers to keep food in there for some strange reason) so I put my seeds on a couple layers of coffee filter (I used to use paper towels -- but they fall apart too easily. Coffee filters hold up much better. I don't actually drink coffee, so I buy filters just for seed starting) moisten them with water, pop them in a zip-lock bag, write their name and date on the outside, and throw them in the fridge (not the freezer! That's too cold). 3 months in the fridge is enough for most plants, but I check on them every few weeks, when I think about it. If I seed little white roots starting to pop out of the seeds, I pull them out and pot them up.
Those three techniques will allow you to successfully germinate most seeds -- and usually a quick google search with the name of the plant and the word "germinate" will turn up the exact requirements for that specific seed.
But seeds like to keep you guessing... fully three years ago I first placed some Japanese maple seeds in the refrigerator. After three months, I took them out and planted them. One seed sprouted. I thought, okay, bad seeds, and kinda forgot about them. They sat outside that next winter, and the next spring, a dozen more sprouted. There are still a bunch of seeds which look good, so I'm fully expecting another batch to sprout this spring -- who knows how long they will keep it up. Once again, my garden is teaching my patience.