Potatoes, roots, and alliums in alkaline soil
Potatoes, garlic, long-day onions, leeks, and storage roots for high-pH Western soils, with timing taken from extension guidance.
Roots and alliums are the crops that make a high-country garden feel like a pantry instead of a salad bar. They tolerate cool soil, they store, and most of them do not need the warm nights that tomatoes demand.
They do need a soil you have actually looked at. Alkaline, calcareous ground changes potato scab pressure, onion fertility, and how straight a carrot gets. The chemistry is explained in alkaline soil. This page is the crop plan that follows from it.
Potatoes: early, pre-sprouted, and not a long-season russet
Colorado State University’s mountain sheet is specific. Plant potatoes two to three weeks before the last frost. Pre-sprout them for two to three weeks in a warm place with indirect light. Cut seed tubers into pieces of about 2 to 4 ounces, each with an eye.
The Colorado Vegetable Guide adds the mountain judgment: potatoes are a strong choice if the cultivar is not a long-season one and the planting is early enough. A late russet that needs a long, warm finish is the wrong tuber for a valley whose median last 32°F date falls in late June.
Colorado State University’s potato program has released cultivars for this region, including Sangre and Purple Majesty. Those names mean the potato was bred with Colorado conditions in view. They are not a requirement.
Any early or midseason cultivar with a known count will beat an unnamed “baking potato” from a bin. Buy certified seed. Grocery potatoes are often treated to suppress sprouting, and they can bring disease into a bed you will use for years.
Common scab is the alkaline-soil issue. The disease is worse in dry, high-pH ground. You will not drop the pH of a calcareous bed enough to dodge it, which is the same limit CSU describes for iron chlorosis.
What you can do is keep soil moisture even while tubers are setting, avoid fresh manure in the potato bed, and resist the urge to lime a Western soil that does not need lime. Hill the stems with soil or a mulch that will not blow away, both to cover developing tubers and to blunt light frost on the new shoots.
A light frost will burn potato tops and the plant will come back if the seed piece still has energy. A hard freeze on a plant that has not emerged yet is nothing. That is why the “two to three weeks before last frost” timing works.
Water deeply and then keep the hill damp, not soggy, through flowering. Flood one week and dust the next and you get knobby tubers and hollow heart. At elevation, a crop planted in early June and dug after the vines die in September is a normal mountain schedule.
Do not wait for a bigger tuber into October if your first frost is already behind you and the vines are black.
Dig on a dry day, cure the skins, and store them cold and dark.
Garlic is a fall planting
Garlic wants to root in cool soil and then sit through winter. Plant individual cloves, point up, after the soil has cooled in autumn, around the time of the average first frost if you know that date for your station, so the clove roots without making a tall leaf that winter will tear.
On the Front Range that often falls in October. In a mountain town, plant earlier, once nights are regularly cold and before the ground locks up. Mulch after the soil begins to freeze, not before, or you insulate a warm bed and the clove keeps growing. Pull the mulch back in spring only enough to let the shoots see sun.
Hardneck cultivars are the ones northern gardens rely on. Music and German Extra Hardy are widely grown examples of that class.
They produce a scape, which you cut and eat, and they handle winters that go below zero. Softneck garlic, the braidable grocery type, is less dependable where the ground freezes hard and snow cover is unreliable.
Water through May and June. Stop once the lower leaves yellow, and harvest before the wrappers rot. Cure it out of the sun. A chinook wind will desiccate bulbs you left on a table in the open.
Elephant garlic is a leek relative. Treat it as a different crop with a milder bulb, and do not expect it to have the winter manners of a hardneck.
Onions: long-day types, and usually not from seed in the open ground
Onion bulbs form when days reach the length the cultivar expects. Colorado, Wyoming, Montana, and Idaho are long-day country. A short-day cultivar bred for southern winters will bulb while the plant is still small.
Read the packet for “long day.” Northern New Mexico sits near the boundary. Intermediate-day cultivars are the ones to try there if a long-day type is slow to bulb, and the packet is the authority, not a guess from latitude alone.
The Colorado Vegetable Guide says much of Colorado is better served by sets or transplants than by direct-sown seed, because seed needs about 110 days in the garden. Start seed indoors about ten weeks before the last spring frost if you grow your own transplants, and trim the tops so they stay sturdy.
The guide’s baseline sets plants and sets out from about mid-April to mid-May. Move that later, week for week, as your station’s median last frost moves later than a Front Range April or early May. A set planted into frozen Gunnison soil in April is wasted.
Onions are shallow-rooted and want steady water and a weeded bed. They do not compete. A drip line and an early mulch, kept off the bulbs themselves so they can dry at the shoulder, is the cultural system.
Side-dress if the leaves pale, then stop once bulbs swell. Cure them in a dry, shaded, airy place. High-pH soil is not a reason to skip onions.
It is a reason to watch fertility with a soil test rather than with a heavy manure application that grows tops and soft bulbs.
Leeks follow the onion calendar and then stay in the ground. Start them indoors in the same 8-to-10-week spirit as the mountain sheet’s note on leeks, which groups them with broccoli and Brussels sprouts at 6 to 8 weeks before the last frost for an indoor sowing, set out while the weather is still cool.
Hill them for a longer white shaft. They will stand into cold weather better than bulb onions. In a mountain town, harvest before the ground becomes a shovel problem.
Carrots, beets, parsnips
The quick roots are covered with the other hardy crops in cool-season crops. The storage versions need one more decision: soil texture. Carrots fork in clods and in fresh manure.
In Front Range clay, grow them in a deep raised bed or accept a shorter type. The vegetable guide’s advice to keep a spring seedbed damp for up to three weeks still applies, and a boards-on-the-row or a sheet of row cover helps.
Parsnips are a long count and a sweet winter root where the season from a late-April or May sowing can actually reach October. They are a Front Range and valley crop more than an alpine one. Sow them with the carrots and do not give up at week two.
Beets give greens in a month and roots in two, and they are the root to grow if your soil is rough. They are also a crop you can succession-sow on the Front Range: a spring sowing and a midsummer sowing for fall. Above 8,000 feet, one sowing once the soil is workable is enough.
A bed plan that respects pH
Do not lime this ground. Do not bury fresh manure under potatoes. Do put compost in the carrot and onion beds the season before, so the manure is not fresh at sowing.
Rotate so potatoes do not sit in the same square every year, which is ordinary disease sense and matters more where scab is already favored by pH. A simple four-year rotation is potatoes, then brassicas, then alliums, then legumes or roots. If the garden is one bed, rotate ends of the bed and accept that perfection is not available.
| Crop | Main Western limit | What to do about it |
|---|---|---|
| Potato | Scab in dry, high-pH soil; short season | Even moisture at tuber set, early cultivar, certified seed, no fresh manure |
| Garlic | Planted too late, or mulched too early | Fall planting, mulch after the soil freezes, hardneck cultivars |
| Onion | Short-day cultivar, or seed sown outdoors too late | Long-day sets or transplants; 10-week indoor start if you use seed |
| Carrot | Clay, crusted seedbed | Raised bed, steady surface moisture, thin hard |
| Leek | Not enough time from seed in open ground | Indoor start, hill, harvest before the ground locks |
Checklist
- Pre-sprout potato seed and plant two to three weeks before the local last frost.
- Keep potato moisture even. Skip lime and fresh manure in that bed.
- Plant hardneck garlic in fall. Mulch after freeze-up.
- Buy long-day onions for Colorado, Wyoming, Montana, and Idaho.
- Use sets or transplants unless you started seed about ten weeks before last frost.
- Grow storage carrots in loosened soil, and keep the seedbed wet until emergence.
- Dig potatoes once vines die. Do not wait on a larger tuber through a hard freeze.
The month-by-month placement of these jobs is on the Front Range and mountain calendars, and on the state pages if you garden outside Colorado.
Sources
- CSU Extension, Vegetable Gardening in the Mountains, potato timing and pre-sprouting.
- CSU Extension, Colorado Vegetable Guide, onion seed and transplant timing, carrot germination, mountain potato note.
- CSU Extension, Iron Chlorosis of Woody Plants, for the limit on changing pH in calcareous soil, which is the same limit that applies to chasing scab with sulfur.
- Colorado State University potato releases, including Sangre and Purple Majesty, from the university potato program.