Potatoes in grow bags

What experts say

Every extension source in this research advises certified seed potatoes, not grocery potatoes.

Maryland Extension: store-bought potatoes "may be symptomless disease carriers". Illinois: "results occasionally may be acceptable, [but] the risk of introducing a nematode, disease, or other pest is much higher".

The three real risks

1. Disease you can't see Store potatoes aren't inspected for planting. Diseases that ride inside tubers include:

• Viruses (mosaic, leafroll). They spread to later crops by aphids and can't be cured.

• Bacterial ring rot. One infected potato can contaminate up to the next 20 cut with the same knife.

• Blackleg and soft rot, black scurf, seed-borne scab, and late blight.

• Some soil diseases, once introduced, persist for years (Tier 2 source).

Certified seed isn't perfectly clean either. But it comes from a state program with tissue-culture starts, field inspections, virus tests and zero tolerance for ring rot.

2. Sprout-inhibitor residue

• How common: chlorpropham (CIPC) is the most common US storage sprout inhibitor. University of Idaho says nearly all market potatoes are treated after mid-winter. An industry estimate cited by EPA puts it on 55–65% of stored US potatoes.

• What it does to seed: in Idaho field trials, small amounts (1.3–2.5 ppm) delayed sprouting. Larger amounts stopped some plants from coming up, and yields fell 26–94%. Even tiny residues slowed emergence and cut yield in a UK study.

• Washing: removes only 33–47%. Peeling removes most, but you can't plant a peeled potato.

• Organic potatoes: chlorpropham isn't allowed on certified organic potatoes. Organic stores may use clove or mint oils, which only burn back sprouts and leave little residue.

• Food safety: EPA found no human health concerns from chlorpropham on stored potatoes. The issue here is planting, not eating.

3. Old, unknown potatoes

• Age: store potatoes are often long-stored. Old seed sprouts heavily and gives many small potatoes.

• Variety: the bag rarely names the variety.

• Southern growers: the russets common in stores grow poorly in the South.

If you plant store potatoes anyway

These lower the risk but don't remove it:

• Choose organic potatoes to avoid chlorpropham.

• Choose firm, unblemished potatoes, ideally ones already sprouting on their own. Reject any with soft spots, rot, sunken blemishes with rusty flesh, or black crusty specks (black scurf).

• Clean the knife between potatoes if you cut them (10% household bleach).

• Plant small whole potatoes when you can, into warm soil.

• Grow them in bags of fresh potting mix rather than your garden. This is our practical suggestion, not something the sources tested: it keeps any problem out of your garden soil, but used mix should be thrown out, not spread on beds.

Chitting (pre-sprouting)

What it is: sprouting seed potatoes in light before planting, so the plants come up faster and are ready sooner.

How (2–6 weeks before planting):

1. 1

   Spread whole potatoes in one layer, eyes up, in a tray or egg carton.

2. 2

   Keep them at room temperature in bright, indirect light, not direct sun.

3. 3

   Plant when sprouts are short, sturdy and green, up to about 1 in long.

4. 4

   Don't cut potatoes before chitting, because the cut surfaces dry out.

What to expect:

• Faster start: in a Maine study, 87–96% of chitted plants were up by day 18, against 21–37% of unchitted ones.

• Earlier harvest: Utah State says 7–14 days earlier.

• Not more potatoes: chitting doesn't reliably raise the total crop. Use it for earliness, especially in short-season areas.

Cut or keep whole?

• Ideal piece: 1.5–2 oz, about the size of an egg or a 1.5-in cube, with at least one eye (two or more is better).

• Small potatoes: under about 3 oz, plant whole. Small whole seed is often the best choice for home growers.

• Too small: throw out pieces under 1.5 oz.

• Fewer cuts: keep the number of cuts to a minimum. More cut surface means more rot risk.

• Heal the cuts: let pieces sit 1–3 days in a warm (about 60–70°F), humid, airy spot so the cuts cork over. Healed pieces are less likely to rot in cool, wet soil. Sources disagree on how much this matters.

• Don't wash seed potatoes.

• Fall plantings in the South: use small whole potatoes only. Cut pieces rot in hot soil.

Planting (Fedco's no-layering method)

1. 1

   Put about 3 in of moist compost or mix in the bottom.

2. 2

   Set seed potatoes on it, eyes up, about 6 in from the sides.

3. 3

   Cover with 12–16 in of mix, leaving 3–4 in of space at the top.

4. 4

   When sprouts come up, add straw or hay mulch to cover any potatoes that form near the surface.

Layering method (UNH): start with 6–8 in of mix, plant, cover with 6 in more, then keep adding mix as the plant grows, leaving about 6 in of leaves showing. This works too, but adding many extra layers doesn't add potatoes (below).

Do tall bags and "potato towers" make more potatoes?

No good evidence says so.

• Where tubers form: at the tips of underground side stems (stolons), mostly near the seed piece.

• UMD Extension: potato "towers" "don't usually work".

• Hilling trial: in a University of Alaska field trial, hilled potatoes beat unhilled ones and had fewer green tubers. But higher hills didn't give significantly more good potatoes than low hills.

• What limits yield: a potato breeder explains that yield is limited by leaf area, not buried stem. In tall towers you mostly find a long bare stem with potatoes clustered near the bottom.

• The famous claim: "100 pounds in 4 square feet" hasn't held up in reported tries.

• Who disagrees: some extension pages still say burying more stem makes more potatoes, without data.

Bottom line: about a foot of mix over the seed, plus mulch, is enough. Extra height mostly adds watering trouble.

Water

• When: water when the top 1–2 in of mix is dry, until some drains out the bottom.

• How often in summer: bags may need water daily in hot, dry weather.

• Keep it even. Dry-then-soaked swings cause knobby, cracked, hollow or chained potatoes.

• Most critical time: from flowering on, when tubers are growing.

• Late season: ease off as vines yellow.

Keep roots cool

• Tuber formation: best at soil temperatures of about 60–70°F; tubers fail to form near 80°F soil.

• Second growth: soil as warm as 75°F can trigger it.

• Straw mulch keeps soil about 10°F cooler.

• In hot climates: use fabric or light-colored bags, mulch, and afternoon shade, and time planting so tubers form before summer heat.

Feed

• Container feeding (UNH): once shoots appear, use a balanced soluble fertilizer every couple of weeks, with phosphorus at least equal to nitrogen (e.g., 5-10-10).

• Don't overdo nitrogen. Too much "encourages excessive foliage growth and delays tuber growth".

• Compost alone runs out. Compost nutrients usually don't last the whole season.

Scab and pH

Common scab (rough, corky patches) is only skin-deep; the potatoes are fine to eat.

• What makes it worse: dry mix during tuber set, soil pH of 6.0–7.5, and fresh manure, lime or wood ash.

• What helps: even moisture, slightly acidic mix, and clean seed.

Pests and diseases

Colorado potato beetle:

• Cover early: put row cover on before beetles appear.

• Hand-pick: drop adults and larvae into soapy water, and crush the yellow-orange egg clusters on leaf undersides.

• Mulch: thick straw helps.

• How much damage is too much: plants handle about 30% leaf loss before flowering, but only about 10% once potatoes are sizing.

• Sprays: the beetle resists many insecticides. Bt and neem work only on small larvae.

Late blight (cool, wet weather; dark water-soaked spots with white fuzz underneath):

• Pull infected plants into a sealed bag. Don't compost them.

• Potatoes from the plant are usable if they aren't infected themselves. Check them often in storage.

Aphids spread viruses. Hose them off or use insecticidal soap.

Refrigerator? FDA advises not refrigerating potatoes, because cold storage can raise acrylamide when they're fried, roasted or baked. A 2021 UK study found 15 days at 41°F didn't raise the sugars involved. We follow FDA: use a cool closet, basement or root cellar. To cut acrylamide, soak slices 15–30 min before frying or roasting, and cook to golden, not brown.

Apples and pears: Utah State says don't store potatoes near them, because their ethylene gas can make potatoes sprout. USDA notes that ethylene can either speed or slow sprouting depending on the amount.

Saving your own seed: Texas suggests holding small spring potatoes for a fall crop in a cool, humid spot, and not saving seed more than 1 year. Illinois advises against saving seed unless it's an heirloom you can't buy.

This is an example built from the steps above. Adjust it to your planting window (see By Zone).

Costs not yet researched.

1. 1

   Planting grocery potatoes into your main garden. It risks introducing diseases that stay in the soil.

2. 2

   Scrubbing store potatoes and assuming the inhibitor is gone. Washing removes less than half.

3. 3

   Planting into cold, wet soil, below about 45–50°F. Seed pieces rot.

4. 4

   Cutting potatoes into tiny pieces under 1.5 oz, or with no eye.

5. 5

   Using garden soil in a bag. It compacts and drains poorly.

6. 6

   Building a 3–4 ft tower expecting layers of potatoes.

7. 7

   Black plastic in full sun in a hot climate. It cooks the roots.

8. 8

   Letting the bag dry out, then flooding it. You get knobby, cracked, hollow potatoes.

9. 9

   Heavy nitrogen. You get big leaves and few potatoes.

10. 10

    Leaving potatoes exposed to light, in the bag or in storage. They turn green and toxic.

11. 11

    Digging storage potatoes right away. Skins scuff and rot in storage; wait 2 weeks.

12. 12

    Composting blighted plants.