I recently got an email from Sally with a familiar question. It’s the same exact question that I had when I was a beginner gardener and wondered how to start seeds:
“I’m sure this is a silly question, but I always see it recommended to plant more than one seed per hole. But why? I just got a seed starting kit with some seeds and want to make sure I’m using them efficiently. Can you help me out?”
It’s a great question, Sally! Understanding the answer to this question will improve your understanding of gardening and seed starting in general, because the answer hinges on an important concept: germination rates.
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Answer One: Seed Germination Rates
Not all seeds are created equal. Some plant species have higher germination rates than others. Even within a single plant type, some of the seeds are older than others, causing the germination rate to go down.
Imagine you’re growing arugula microgreens and the average germination rate is 90%. If you plant seeds in a 72 plant starter tray with one arugula seed per insert, you can expect only 65 of those plant inserts to actually germinate (72 x 90%).
Now imagine you plant multiple seeds at three arugula seeds per insert. Each of these seeds has a 10% chance of failing, so the probability of them all failing is 10% x 10% x 10% = 0.1%. This means that you are 99.9% likely to have the seeds in that cell germinate. So in a tray of 72 inserts, it would be extremely unlikely you would have any seeds not germinate — barring other factors that affect seed germination.
In short: Plant multiple seeds per hole to increase the chances you have perfect germination rates.
Answer Two: Seedling Selection
Just as not all seeds are created equal from a germination standpoint, not all seeds germinate equally. Sometimes you have a seed that shoots off like a rocket and becomes too leggy. If this was the only seed in your insert, you’d be forced to use it.
By planting 2-3 seeds per cell, you allow yourself to luxury of choosing the seedlings that look the strongest. All you have to do is determine which one you like the most, then snip off the other seedlings to kill them as your other plants grow.
Exceptions to The Rule
Like most things in gardening, there are always exceptions to this rule of 2-3 seeds per hole.
If you’re planting large seeds like cucumbers, melons, or pumpkins, you should only use one seed per hole. However, you can still plant seeds close together and then thin them out once they’ve established themselves. You just want to avoid crowding these large seeds together so you don’t mess up the germination process.
If you’re growing certain herbs (cilantro, dill, basil), you can get away with planting multiple seeds per hole and leaving them all there as they germinate. These plants can handle being planted right next to each other and basically become one larger, bushier plant.
Now that you know how many seeds to plant per pot, you have a deeper understanding of germination in general. For more on seed starting, please check out the simple seed starting for hydroponics guide.
So, How Many Seeds To Plant Per Hole?
There are some general rules we can glean from all of the above information. The size of the seed will help you determine how to plant. If we were to average the number of seeds based on all seed sizes, you would plant 2 to 3 overall.
Let’s break it down by seed size, to give you a good sense of the kinds of formulas you can use to start seeds. If you’re planting multiple seeds at different sizes, you can simply use this rule to get going.
- Large-sized seeds: 1 seed per hole
- Medium-sized seeds: 2 per hole
- Small-sized seeds: 3 per hole
If you want to get even more exact, you can bust out your calculator to determine exactly how many seeds to plant based on germination rate. Use a ratio to determine how many to plant.
For instance, if you have a seed packet that has an 80% germination rate, and you want to grow 20 plants, you should plant at least 24 seeds, as the remaining 4 will make up the 20% lack of germination expected.
However, since you’ll want to plant in 20 starter cells, you should probably plant two seeds per hole and thin them to the most healthy plants once they’re large enough.
Frequently Asked Questions
Q: How many seeds should I plant in each spot?
A: While it really depends on the seed type and its germination rate, on average you’re looking at 2 to 3 seeds.
Q: Can you plant too many seeds in one spot?
A: You can! However, you can always thin your multiple seeds that sprout seedlings after they sprout if needed. This will free up space for the ones you leave.
Q: Do you plant all the seeds in a packet?
A: If you’re planting over a large space, you may end up using whole seed packets. However, if you only need one or two plants, you may find you only use a few.
Q: How do I calculate how many seeds I need?
A: Multiply your germination rate to the number of plants you need, and then add a few more to up your percentage. You could also use the space you have to determine how many seeds to use.
Q: Why do you plant 3 seeds?
A: This helps you allow yourself to luxury of choosing the seedlings that look the strongest. All you have to do is determine which one you like the most, then snip off the other seedlings.
Q: What seeds can I just scatter?
A: Most wildflowers do best when broadcast. The same goes for grains. Sometimes you can broadcast beets and carrots to maximize your harvest.
Q: What seeds should not be planted next to each other?
A: Any bad companions shouldn’t be planted together. A few of the most notable are corn and tomatoes, beans and onions, cucumber and rosemary, fennel and solanaceous plants.
Q: Does seed spacing matter?
A: It does! You don’t want to overseed if you have a limited amount of seed. Also you don’t always want to have to sacrifice seedlings to make room for others.