This article details how to plant potatoes to ensure that they will produce a ton of tasty tubers. Potato plants require fertile, evenly-moist, loose soil, so it's very important to provide the appropriate fertilizer in the root zone, and to deeply loosen and amend the soil with compost so that the developing tubers have a soft place to expand into.
The goal is to provide growing conditions that optimize development of both top growth (not too crowded, excellent soil fertility) and root and tuber growth (deep, loose, friable soil with even moisture). The following method balances both.
Start with seed potatoes, not potato seed. (See How to Grow Potatoes for an explanation.) Let them green up and start to sprout.
Fork up the whole bed (or row) you're going to plant, mixing in appropriate amounts of fertilizer and compost, then open a trench down the middle with a hoe. Put your prepped seed potatoes in, cover them with 3" of soil, and then over the course of the next few weeks, slowly backfill an inch of soil at a time up over the growing stems.
This lets the potato shoots quickly reach the light and start developing sugars, while also slowly burying the lower stems, which in the absence of light will convert to growing tubers rather than leaves. Slowly backfilling also protects the developing potatoes from the light so they won't become green and toxic. Since the tubers form mostly above the level of the cut seed piece and the feeder roots, it also means that the developing potatoes will have a nice loose bed of soil in which to develop.
How to Plant Potatoes Step One:
About 6 weeks before your last frost date, wake up your seed potatoes.
If you live where it freezes in the winter, the earliest you should plant out potatoes is about 4 weeks before your last frost date, provided your soil temperature is above 50° F. You will be planting them about 3" deep initially, so they won't be damaged by frost unless it freezes after the shoots emerge above ground. (If they do happen to get a frost-nip after that, they will recover, but yield will be reduced.)
So about 6 weeks before your average last frost date, take your seed potatoes out of their cool, dark storage location, spread them on trays (or box lids, cookie sheets, etc.) and put them in a bright, room-temperature location, though not in direct sunlight. Within a couple of weeks they will have turned green and started growing shoots (actually "stolons") out of the eyes, at which point they are ready to plant. While you're waiting for the spuds to wake up, you can think about or start your soil preparation. (If your soil is too wet or cold to work and your seed potatoes have already sprouted, keep them cool and dark until your soil is ready to receive them)
How to Plant Potatoes Step Two:
About 4 weeks before last frost (a day or two before plant-out), prep the soil in the potato bed (or row).
Never dig wet soil because you will destroy the soil structure, which is important for healthy soil ecology. Ideally the soil should be about as moist as a wrung-out sponge when being worked. For maximum yield and nutrition, the entire bed or row intended for potatoes should be loosened as deeply as possible with a spading fork, while incorporating both a ½" layer of good-quality compost and an appropriate dose of complete organic fertilizer. If you are mixing your own fertilizer blend, you may want to leave out the lime component, because lime may enhance susceptibility to potato scab disease.
You need to get the fertilizer down where the plant's feeder roots will grow, which is mostly at or below the level where the tubers will form. This may be daunting if the soil has never been gardened before, or if it's been rototilled, which actually damages soil ecology and creates a hardpan beneath the level of the tines. You can grow potatoes without proper soil preparation, but your results will likely be disappointing.
This is a case where double-digging is ideal. In double-digging you remove the first spade-depth's of soil, then open up the next level down with a spading fork, mixing in compost and fertilizer. Then the top spade-depth's worth of soil that you removed is put back in, also with compost and fertilizer added. (If you have not worked the soil at all before, soil preparation can take considerably more time than "a day or two before".) I have a YouTube showing how to double-dig here.
Good soil preparation with compost and fertilizer assures good nutrition for the plant, and the loosening will make it easier for the plant to fill out large tubers. It will also make it easier for you to harvest when the time comes.
How to Plant Potatoes Step Three:
The day before planting out, cut your seed potatoes into chunks.
Using a clean sharp knife, cut your seed potatoes into chunks that are somewhere between golf-ball or egg-sized, and that each have 1-3 eyes. (Very small potatoes that only have 1-3 eyes don't need to be cut up.) If the pieces have more than 3 eyes, too many shoots will come up, crowding the plants and lowering overall yield. Be very careful not to break off any of the sprouting shoots.
How to Plant Potatoes Step Four:
On planting day (about 4 weeks before your average last frost date), dig a trench down the center of the fertilized and loosened bed.
Using a standard hoe, open up a deep trench down the entire length of the bed (or row), pulling soil up and making a ridge of soil at the top edge of the trench. Do this from both sides, so a ridge of soil borders the trench on each side. (An alternate method is described at the end.) We'll use these ridges of soil later in the process.
Plant prepared seed potato chunks in the bottom of the trench about 8-12" apart, with the emerging shoots pointing up. If you live where it's hot and dry, the 12" spacing will give the plants less competition for water and will make them more drought resistant.
Cover them back up with some of the removed soil till they are covered about 3" deep with soil.
How to Plant Potatoes Step Five:
Hilling Up the Growing Potatoes As the Season Progresses
Those ridges of soil on each side of the trench will be slowly be pulled back into the trench and "hilled up" against the growing stems, only an inch at a time over the next several weeks. This strategy allows plenty of light to reach the stems (which are really stolons) now, when they are just little and are working on developing their first leaves. But later, after they have a good set of leaves up top, hilling soil up over the lower stem will make it switch over from growing leaves to growing roots and tubers, maximizing yield.
After the shoots have emerged out of the ground and grown up about 4", reach over the plant with your hoe and pull about an inch more soil up over the stem. Do this from both sides of the bed.
As the plants continue to grow, every 5-7 days hill them up with another inch or so of soil. Smaller, more frequent hilling is much better than covering a lot at once. Never cover more than about a quarter of the new growth at one time, or you'll set production way back. By about the time the plants start to flower, you'll have a beautiful row of potato plants whose bases are growing in a deep, soft mound of well-fertilized earth where the new potatoes will be forming.
Alternate to the trench method:
You can forego the trench and instead dig 3-4" deep holes every 8-12" down the center of the prepared bed or row, planting the seed potatoes in the holes and completely covering them. Then instead of pulling the soil from the ridge back down into the trench, you will hoe up soil from the edges of the surrounding bed to hill up the plants. Either way, you'll be using your hoe to move soil.
Now that you know how to plant potatoes the most optimal way, I wish you a bountiful and delicious harvest!
And be sure to check out my upcoming articles (arriving before 2-1-21) on How to Harvest Potatoes, How to Store Potatoes, and Are Green Potatoes Safe to Eat?