The complete seed spacing and raised bed guide

I wrote this because every "beginner gardening guide" I've ever read either hands you a one-size-fits-all spacing chart or buries the real answer inside 3,000 words of Monsanto-bashing. My goal here is simpler. Tell you how I actually plan my Indiana garden each spring (Zone 6a, last frost May 15, first frost October 1), cite the books and seed companies I trust, and give you enough specifics to not blow $60 on seed packets you won't use.

The seed spacing calculator lives on the home page. This guide is the why behind every number it produces.

Square-foot gardening, the Mel Bartholomew method

Mel Bartholomew was a civil engineer who retired, got frustrated with traditional row gardening in the late 1970s, and wrote a book called Square Foot Gardening that PBS made into a series. The third edition, All New Square Foot Gardening, is still in print and is the reference I reach for more than any other garden book on my shelf.

The method is simple. Build a raised bed. Divide it into 12-inch by 12-inch squares using string, lath, or just chalk. Each square holds 1, 4, 9, or 16 plants depending on their spacing needs. One tomato per square. Four lettuces per square. Nine spinach. Sixteen carrots or radishes. That's it.

The math works because Mel noticed that packet instructions (plant 3 inches apart, rows 18 inches apart) were field-ag advice for farmers with tractors. Home gardeners with raised beds don't need rows. A 12x12 square is exactly the right size to hold a uniform spacing grid for any common vegetable.

My first 4x8 bed in 2019 used this method. I put lettuce at 4 per square, carrots at 16, bush beans at 9. Got three times the harvest from that bed than from a 20-foot row I'd planted the same spring. Never went back.

Row garden vs intensive vs square-foot, which fits you

Three real options, three different reasons to pick them.

Square-foot gardening wins for beginners, urban gardeners, and anyone with a raised bed under 100 square feet. It cuts seed use by about 80% versus traditional rows. You never walk on the soil (so it never compacts), weeding collapses to maybe 10 minutes a week, and the grid makes succession planting dead obvious. If you've got a 4x8 box in your backyard, this is what you want.

Row gardens win when you're working a big plot (500+ square feet), you have a tiller and a wheelbarrow, or you're planting 50 tomatoes because you're canning sauce all summer. Row gardens need more seed and more weeding, but they scale. Every farm and every market garden I've ever visited runs rows, because you can't hand-tend 10,000 square feet.

Intensive gardening is Eliot Coleman and Jean-Martin Fortier territory. Same principles as SFG but squeezed 15-20% tighter, with soil amendment that would embarrass most compost operations. Works if your soil is really, genuinely biologically alive. Doesn't work on first-year beds. Think of it as SFG+ once you've gardened five seasons and actually know what you're doing.

Plant spacing: why it matters for yield

Too close and your plants compete for water, nutrients, and light. You get small fruit, leggy growth, and every fungal disease in the book. Too far and you waste bed space and give weeds a runway.

The sweet spot differs by crop. Tomatoes want one square foot per plant with trellising (without trellising, two to four). Lettuces at 4 per square foot can be harvested as cut-and-come-again. Carrots at 16 per square foot actually thrive, root vegetables don't compete for light the way leaf crops do.

What the calculator gets right that most charts don't: it accounts for planting style. A carrot in a traditional row wants 2 inches in-row, 12 inches between rows. That's 6 plants per square foot of bed. But in an SFG grid, where there are no "rows" to walk between, 16 carrots fit in the same square foot. Same seed, different geometry.

Companion planting basics

Companion planting is the practice of placing plants that help each other next to each other. Some of it is folklore. Some is well-studied. The big four I use every season:

Tomatoes and basil. Basil repels hornworm and whitefly, and a tomato-basil bed gives you caprese salad from 10 feet of dirt. Plant one basil per tomato at the base.
Carrots and onions. Carrot fly is attracted by the smell of carrot foliage. Onions mask it. And vice versa for onion maggot. This is the classic pest-confusion duo.
Three Sisters: corn, pole beans, squash. Indigenous agriculture's gift to the world. Corn gives beans a trellis, beans fix nitrogen for corn and squash, squash shades the soil and keeps weeds down.
Lettuce and radishes. Radishes germinate in 4 days, pull them at 28. Lettuce takes 50 days to head. By the time the lettuce fills out, the radishes are gone. Same square foot, two harvests.

Things that fight each other, also worth knowing: onions and beans, fennel and almost everything, tomatoes and brassicas, walnut trees and nearly every vegetable (juglone toxicity, look it up).

Seed packet math, why 2 to 3 seeds per hole

Germination rates for fresh seed are usually 85-95%. For older seed (more than 2 years in a warm garage), that drops to 60-70%. If you plant one seed per hole and it fails, you've lost a spot for the season.

Two to three seeds per hole gives you a statistical guarantee of at least one germinating. Then you thin, cut the weaker seedlings with scissors at soil level once they have true leaves. Don't pull. Pulling disturbs the roots of the one you're keeping.

This is why the calculator's "seeds needed" is 2-3x the plant count for most species. Corn is 2. Carrots are 3. Celery and parsnips and anything slow to germinate, also 3, because germination is iffy even on fresh seed.

A $3 packet of 800 carrot seeds covers four full 4x8 beds at SFG density. You're buying more seeds than you'll ever use. That's fine. Store the rest in a labeled mason jar in the fridge.

When to thin, and how to do it without crying

Thin when the seedlings have their first set of true leaves. The first leaves that come up are cotyledons (seed leaves); true leaves appear second and look like miniature versions of the adult plant.

Pick the strongest-looking seedling in each cluster. Cut every other seedling at soil level with scissors or a thin blade. I use my Felco pruners for anything thicker than a toothpick. Don't pull, pulling pulls the root mass of the keeper too.

Eat the thinnings. Baby lettuce, radish greens, beet tops, they're all edible and delicious. My 2022 salads were almost entirely thinnings until May.

Soil prep and bed depth

Mel's Mix is the SFG orthodoxy. One-third compost (ideally five different sources blended), one-third peat moss or coco coir, one-third coarse vermiculite. It's expensive the first year, basically free from year two on because you top-dress with your own compost.

Bed depth matters. Six inches works for radishes and lettuce. Twelve inches is the sweet spot for most vegetables. Eighteen inches gives you room for carrots, parsnips, and long-rooted tomatoes. If your bed is sitting directly on grass or concrete, don't skimp, the plants don't know there's a lawn three inches beneath them, but they do know when they hit the bottom of the box.

I use Bluelab's pH meter every spring before planting. Most vegetables want 6.0-7.0. If you're below 6, add lime. Above 7, add elemental sulfur. One of the easiest ways to turn a mediocre bed into a productive one.

The formula this calculator uses

You paid nothing for the calculator so you get the math for free:

SFG mode: plants = floor(bed_sqft) × density_sqft[species]. Densities from Mel's All New Square Foot Gardening, 3rd edition.
Row mode: rows = floor(bed_width / row_spacing), per_row = floor(bed_length / in_row_spacing), plants = rows × per_row. Row and in-row numbers from Johnny's Selected Seeds technical data.
Intensive mode: same as SFG but multiplied by 1.15. Reflects the ~15% density increase Eliot Coleman uses in his greenhouse beds.
Seeds: plants × seeds_per_hole. Most vegetables get 2 or 3; corn gets 2; chives get 5.
Packets: ceil(seeds / packet_avg). Packet averages from Burpee, Baker Creek, and High Mowing retail listings.

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Regional timing, last frost and first frost

The most important number in gardening, more important than spacing, is your last-frost date. It's the day after which you won't get a killing frost. Zone 6a in Indiana (where I garden): May 15. Zone 5b in Wisconsin: May 25. Zone 8 in Atlanta: late March.

The Old Farmer's Almanac lets you type a ZIP code and get a personalized planting calendar. I use it every year for the specific, hyperlocal last-frost date. Direct-sow timing is almost always expressed as "X weeks before last frost" or "after all danger of frost passes."

Cold-hardy crops (peas, spinach, lettuce, radishes, arugula, kale) go in 4-6 weeks before last frost. Warm-season crops (tomatoes, peppers, eggplant, cucumbers, squash, beans, basil) go in 1-2 weeks after last frost. Corn, after last frost. That's the whole calendar.

Common mistakes

Planting too early. I lost two full flats of tomato transplants in May 2023 because a late freeze I didn't see coming hit a week after my "safe" last-frost date. Now I wait until Memorial Day weekend every year. Zone 6a safety margin.

Not thinning. If you leave five seedlings in one hole, you get five spindly plants that produce nothing. Thin.

Planting corn in rows instead of blocks. Corn is wind-pollinated. A single row will produce half-empty ears. A 4x4 block (16 plants minimum) is the standard.

Over-seeding carrots. Carrot seeds are tiny. People always dump half the packet in one square foot. Buy a pelleted seed line from Johnny's and use a seeder; saves you an hour of thinning per season.

Skipping the soil test. Most garden "problems" are soil pH or nutrient problems in disguise. Test once a year.

Go plant something

That's the whole method. Pick a style. Size the bed. Pick your plants. Run the numbers. Sow 2-3 per hole. Thin ruthlessly. Water consistently. Harvest until the first frost takes it all.

Ready to plan a specific bed? Head to the calculator and plug in your numbers. Got a question this guide didn't answer? Check the FAQ or reach out, I read every note.