Many folks that are interested in seed bombing are do-it yourself types and are considering making their own. I’m that way about most things. I thought I’d write a post about what to look for in a seed bomb recipe and method that will help you get the seed bombs that you need. Seed Bombs are exciting no-till tools and there are a lot of different considerations in choosing the right seed bomb recipe.
Are Seed Bombs the Solution?
I’d like to say that they always are, but that may not be the case. Think first about this. Consider the soil, the location, the effort, will the seed fare just fine on its own? In some cases, a seed bomb isn’t necessary.
Seed mixes would be so easy!
Many recipes call for a mix of seed species (wildflower mix, for example) and measure the seeds by the cup. I am skeptical. Some seeds are kind of like elephants. They are big, powerful, and rather independent, not needing a lot of nurturing. Others need to be coaxed along. If you use a handful of sunflower seeds and a hand full of columbine, guess who will ALWAYS win out? It may be convenient to use a wildflower mix, but those work best dispersed as seeds. Seed bomb dispersion requires cohabitation of seeds within a small area, so you don’t want to have competing species within one seed ball. I think it’s best to make seed balls with one species each, then throw the assortment of seed balls in your target site.
What kind of seeds?
Some of this depends on your personal preference, but a good part of it depends upon the location, climate, season, and soil. Research your site well and understand that it may take your seeds a while to germinate. Don’t plant seeds that will sprout quickly right before the frost, for example. Don’t pre-stratify the seed balls and plant before a drought. A reputable seed source will have germination instructions online or accompanying the seed. I always check a few different sources, and some non-commercial sites for information. You need to consider the natural reproductive and germination calendar of the plant, the temperature, soil, and water requirements.
If you will be using native wildflowers for your seed bombing campaign (please do!), check with a university extension service or nature conservancy in your area. They can help you select seeds that look good and really help where needed. People who work at these types of agencies LOVE inquiries from people who take native species conservation seriously. Your questions will be welcome and your interest appreciated. If run into a cranky person (these organizations are often underfunded and under staffed), just ask if there is a better time, or someone who might be able to spend more time that isn’t so busy. Plants.usda.gov is a treasure trove of information about native plants.
How many seeds go in a seed bomb?
This is an area where I am in staunch disagreement with the popularized recipes. Some recommend as much as ¼ cup seed mixed in with 4 cups soil materials! We all love a chia pet, right? Ever see a mature chia pet with flowering perennials? That’s because perennials and even annual plants don’t grow that way in nature- except maybe duckweed. The problem is that the seeds compete with each other for water, light, nutrients, and space if packed too densely and none thrive. I hate thinning baby seedlings: I feel like I’m doing something terrible, but if I don’t thin the carrots, they’ll be spindly and I won’t get much out of them. With seed bombs, we may not have the luxury of weeding. A part of the effectiveness is that they can be left. So don’t over stuff your seed bombs. I really wish folks would stop popularizing those recipes! They waste a lot of seed, time, and good intentions. I guess I am opinionated on the matter.
But really, how many seeds in a seed bomb???
I employ the rather nerdy implement of statistics to figure that out. I won’t go into that, but here are a couple of links to the type of stats I use:
If you’re as big of an egghead as me, go for it. But here’s a table of some of the numbers that I use.
Likelihood of Germination of At Least One Seed in a Seed Bomb
|Number of Seeds in Bomb||Germination Rate of Seed Ball|
|1 (The germination rate of the seed)||0.20||0.30||0.40||0.50||0.60||0.70||0.80||0.90|
You need to know the approximate germination rate. Reputable seed sellers will know this and pass it on. If you wild harvest your seeds and they look healthy and without bugs, use the rates available on line at reputable sellers, or test them yourself if you have the time.
I like my seed bombs to germinate. I add enough seeds to get the germination rate for each seed ball to 95% and that’s plenty. If I am dealing with a really low germination success seed, I accept 80% or 90%. I don’t want to end up with 10 seeds germinating in a seed ball just to get a 95% germination rate instead of 90%.
Seed Bomb Recipes and Methods
If you want to make seed bombs, you need to settle on a method. There’s the Fukuoka method of shaking the clay and compost in a pan and spritzing. This is great for bulk production. It doesn’t let you micro-manage the number of seeds in each seed bomb like I do for my commercial business, but for your purposes, that may not be necessary. Do consider the ratio of seeds to earthen material, though. I think that recipe, however classic, calls for too many seeds. It is fun to watch the seed balls form, it’s magical for kiddos to seed the seed bombs take form.
If you don’t mind the effort and aren’t making 1000 seed balls at once, mixing the matrix like dough and adding the seed to the bombs manually gives you a much finer control. You can control the number of seeds, and their location in the bomb. This is rarely discussed. Some seeds are just happy campers wherever they end up, but some like dark, some like light. What do you do if your seed requires light to germinate? What I do with my fancy commercial seed balls is use a small tool to tuck or smear the tiny seed right under the surface of the seed bomb. When using light obligate germinators, I always place at least three seeds in different locations on the seed bomb, so some will see the light of day however the seed bomb lands. That way, with only minor erosion, the seed will get the exposure that it needs. It’s a lot of work, but hey, I feel good about selling them knowing that I have optimized the germination potential.
Compost for seed bombs
Make certain the compost is sufficiently aged (should be a mix of smeary and fibric material) and has a near neutral to slightly acidic pH (pH 6-7) Don’t use straight up worm castings or bokashi. Always cut these with aged garden or leaf compost.
Clay for seed bombs
Another thing about which there has been significant discussion. Here, I have shared two articles about the oxidation state of clay as it is moistened and dries (Color of Clay, Redox Experiment). Traditionally, red clay is used, but that isn’t necessary. Make certain your clay doesn’t smell of rotten eggs. That’s an indication of hard-core reduction reactions. If it is stinky, let it air out for some time until it is dry and considerably lighter in color. Then slake it in fresh water or grind it up and follow the method that you prefer.
It is often suggested that you use local clay. This great, but make certain that you are not transporting invasive species seeds, corms, tubers, etc. Lowland areas where you might look for clay are often victims of serious invasive species problems and weedy seed banks. Here’s what I mean:
Up in Northern Michigan where I got my MS, there is a huge Purple Loosestrife invasion. It is replacing native cattail marshes. What happened in one particular locale is that the Loosestrife appeared somewhere else with its showy purple plumes, and some happy gardener in an upland landscape position transplanted some to their irrigation ditch. The seeds of that damned plant traveled via sediment transport through irrigation ditches to formerly pristine cattail marshes and took over. Just an awful mistake by a well-meaning but short-sighted plant lover.
Don’t let that be you! You really don’t want to inadvertently bomb an area with weeds. I use commercial clay (and well- digested compost) for this reason; I ship my seed balls all over the place and don’t want to take that risk at all. Can’t find clay? Try local elementary school art rooms. They often throw unfired clay scraps out. We get as much of our clay from schools as we can. Has no weeds, is quality stuff, and has the physical properties for awesome seed balls!
It’s easy to get carried away when you think you are doing good things. I know I do. Keep in mind, that with a little bit of planning, you can make your seed ball project even more effective, more green, and more memorable. I hope you found this helpful, and perhaps thought provoking. Share it with some fellow guerrilla gardeners and see what they think.