Gardening in Oklahoma: zones, frost dates and when to plant

East to west, not just north to south

The biggest divide in Oklahoma runs from east to west. The Oklahoma Climatological Survey at the University of Oklahoma puts the eastern part of the state in a humid subtropical climate and the west in a semi-arid one. Average yearly rainfall runs from about 56 inches in the far southeast down to about 17 inches in the far western Panhandle. Clouds and humidity follow the same pattern, so a garden near the Arkansas line gets far more rain and muggy weather than one in the western Panhandle, which gets more sun and less water.

The terrain matters too. The Ouachita Mountains rise in the southeast, the Ozark Plateau covers the northeastern corner, and the Black Mesa country sits at the western tip of the Panhandle. The growing season is longest in the counties along the Red River and in the Arkansas River Valley below Tulsa. It is shortest in the western Panhandle, and in the Ouachitas it is several weeks shorter than in the lowlands around them. OSU Extension’s planting dates follow the same pattern: earliest in southeast Oklahoma, latest in the northwest. Check the frost dates for your city above before you plant.

What holds Oklahoma gardens back

• Heat. Southwest Oklahoma averages about 115 days a year at 90°F or above, and western Oklahoma sees 30 to 40 days at 100°F or more. Eastern Oklahoma has fewer of those days, but its higher humidity makes summers feel harsher.
• Drought. Drought comes back again and again in Oklahoma, and the west is a bit more exposed because its rainfall varies more from year to year. Across the state, yearly evaporation from open water is greater than yearly rainfall, so plan on watering through summer dry spells.
• Spring storms. Spring brings the most violent thunderstorms, with large hail, damaging wind and tornadoes. About three-fourths of the state’s tornadoes come in April, May and June, just as the spring garden is going in.
• Soil chemistry. OSU’s summary of more than 25,000 lawn and garden samples found a median pH of about 7.1. Soils are calcareous in the Panhandle and the drier west and more acidic in the wetter east. Most samples were short on nitrogen, while most Oklahoma soils are high in potassium.

OSU Extension’s Oklahoma Garden Planning Guide

The Oklahoma Garden Planning Guide (HLA-6004) sorts crops into cool-season crops, which do best when the average daily temperature is 70°F or below, and warm-season crops, which like 70°F to 90°F. Its rule of thumb for when to sow is soil temperature at seed depth: at least 40°F for cool-season seeds and at least 50°F for warm-season ones. Its chart puts cabbage, carrots, peas, onions, spinach and Irish potatoes in from mid-February to early March. Most tomatoes, peppers, beans and squash go in during April, and cantaloupe, watermelon, southern peas and sweet potatoes follow in May.

The guide also gives practical advice for small or awkward sites. Beans, beets, broccoli, cabbage, chard, leaf lettuce, peas, potatoes, radishes, spinach and turnips will still produce in light shade. Fruiting crops may do better with some afternoon shade in midsummer. Where soil is thin, poorly drained, or too sandy or clayey, OSU points gardeners to raised beds or containers. It also says to mulch, rotate crop families each year, and choose varieties recommended for your area.

Fall is a second main season

OSU’s Fall Gardening fact sheet (HLA-6009) says some of Oklahoma’s best-quality vegetables come in autumn, when warm, sunny days give way to cool, humid nights. The catch is that planting happens in June through September. Fall tomato transplants go in during early July, and broccoli and Brussels sprouts go in from mid-July to mid-August. Spinach waits until September, and garlic is planted from September into mid-October.

Getting seeds to sprout in that heat takes some care. Bare soil can reach 140°F in summer sun. OSU suggests opening a slightly deeper furrow and watering only that trench. Cover the row with mulch, boards, screen strips or shade cloth until seedlings come up. Broccoli, cauliflower, cabbage, Chinese cabbage, Brussels sprouts and leaf lettuce are easiest started in flats and transplanted about a month later. Watch fall sweet corn for armyworms. For winter salads, cold frames and row covers let gardeners keep harvesting in January and February. OSU does not name specific fall varieties. It says many will work and advises choosing early-maturing, disease-resistant ones.

Test before you fertilize or lime

OSU’s leaflet Soil Testing, The Right First Step (L-249) explains the routine. Take slices from at least 15 spots across the garden, 6 inches deep, mix them in a clean bucket, and fill the bag with about a pint. Then take it to your county Extension office, which sends it to the OSU Soil, Water and Forage Analytical Laboratory. The report tells you which fertilizer analysis to buy, how much to apply, when to apply it, and whether you need lime. Sample before you fertilize. The planning guide suggests retesting every three to four years. On the alkaline soils of the west, a test also keeps you from adding lime the garden does not need.