Tips For Planting of Warm-Season Crops
By now, for most parts of Oklahoma, the cool-season crops should be in the ground (you still could be succession sowing radishes weekly) or about to go into the ground soon. (Broccoli is an exception. Some of us have found that planting broccoli in latest March or earliest April reduces the likeliness of recurring cold weather causing bolting or buttonheads.) Now it is time to turn our focus to planting the warm season crops.
Warm-season crops fall into two general categories: tender and very tender. In general, tender crops can be planted at or near the last frost date and very tender crops need to go in a bit later when the soil is a bit warmer.
By date, here's the OSU-recommended planting dates for tender and very tender vegetable garden crops:
MARCH 25-APRIL 30:
Sweet Corn, planted from seed or plants, tender
Snap beans (bush or pole), from seed, tender
Eggplants, from plants, very tender (planting towards the end of the recommended period lessens flea beetle damage most years)
Tomatoes, from plants, tender
APRIL 10-30 OR LATER:
Cucumbers, from seed or plants, very tender
Okra, from seed, tender
Peppers, from plants, tender (planting later in recommended period gives better results most years)
Pumpkins, from seed, tender
Summer squash, from seed, very tender
Lima Beans, from seed, tender
Cantaloupes, seed or plants, tender
Watermelons, seed, very tender
MAY 1-JUNE 10:
Southern Peas, seed, tender
Sweet Potatoes, plants (slips), very tender
MAY 15-JUNE 15
Winter Squash, Seed or Plants, very tender
BEANS: Most beans germinate best if planted after soil temperature at planting depth is staying consistently at or above 60 degrees. Beans planted into colder soil tend to germinate poorly or rot before germination and if they germinate in the colder soil they still are very vulnerable to diseases in cooler soil temps.
For Lima Beans, wait until soil temps are staying at 65 degrees.
Beans produce poorly when air temps are high, so planting on-time or slightly early can pay off in better yields and earlier yields as long as the soil temps are in the right range for seed germination.
CANTALOUPE/MUSKMELON/OTHER MISCELLANEOUS MELONS BUT NOT WATERMELONS: Sow seed or transplant young plants raised in plantable pots only after soil temps are staying consistently above 60 degrees. If your soil is still cold and you are eager to plant, lay down black plastic on a prepared seedbed for about 7-10 days to preheat the soil and raise its temperature a little. Planting into cold soils can give poor germination rates and weak seedlings that remain stunted and slow to grow and produce.
To start your own seedlings indoors, sow seed indoors into plantable pots to reduce transplant shock about 2-4 weeks before your anticipated transplant date. Harden off properly before transplanting into garden.
CUCUMBERS: Sow seeds or transplant your indoor-raised seedlings in plantable pots only after soil at planting depth is remaining at or above 60 degrees. Ideal transplants would be about 3 weeks old and planted in plantable pots or peat pellets to minimize transplant shock. As with beans above, you can lay down black plastic in advance of planting to warm up the soil. If you diret-seed into cold soils, germination rates may be poor and the young seedlings very vulnerable to disease.
EGGPLANTS: Very sensitive to cold weather and frost. While tomato plants can tolerate air temperatures right down to just above freezing, eggplant can be damaged by those cold temps. It generally is recommended that eggplants can go into the ground about 2 weeks after tomato plants. Plant after air and soil temps are stable and soil temps are staying consistently above 65 degrees.
OKRA: Plant seed when soil temperature at planting depth is consistently staying at or above 68 degrees, and only after you've had at least 5 consecutive days with nighttime low temps staying above 50 degrees and unlikely to drop below that level very much again if at all. Okra does best when direct-seeded. If using transplants, use plantable pots. To increase germination rates, you can pre-soak your okra seed in room-temperature water for 24 hours before direct-sowing into the ground, or if desired, heat up water to 110 degrees but no hotter and pre-soak the okra seed for 90 minutes before direct-sowing.
PEPPERS: For best and earliest yields, always use transplants instead of direct seeding. Plants that are direct-seeded tend to not produce well until fall's cooler temps arrive because by the time they're large enough to produce, they're having to fight really high air and soil temperatures although the heat is more of a problem for sweet peppers than hot peppers. (High humidity plays a role in this too.)
Like tomatoes, peppers are very heat-sensitive, and have very specific temperatures at which the best fruitset occurs. In general, and once again this applies more to sweets than hots, peppers set fruit best when nighttime air temps remain above 60 degrees and daytime highs remain below 80 degrees. Luckily for us here in Oklahoma, most peppers set fruit somewhat better at higher temps since we often are exceeding 80 degrees pretty early in the year.
Transplants that are about 8 to 10 weeks old and are 6-8" tall are optimal, but taller and older transplants will work as long as they aren't rootbound.
Peppers are set back by cold temperatures so plant them a couple of weeks after tomatoes.
Pepper plants that are exposed to temperatures in the 40s for only a brief time can remain stunted and produce poorly for their entire life. Some will outgrow the cold-related stunting but still not produce well, and some will outgrow the stunting but not produce well until the fall. Thus, it is advisable to not let your plants be exposed to temps in the 40s if you want an early crop and a big crop.
You can transplant pepper plants into the ground once soil temps have remained above 55 degrees for 3 or more consecutive days. I usually wait until soil temps are 65 degrees or higher and I get better and EARLIER yields from those late plantings that from earlier plantings.
PUMPKINS: Technically these can be planted any time after the last killing frost. However, they'll germinate fastest and produce best if you wait until soil temperatures are staying consistently above 70 degrees.
SOUTHERN PEAS: These are the warm-season peas like blackeyed peas, purple hull pink eye peas, lady, crowder, cream or zipper peas. (The green English peas, shelling peas, or sugar snap peas are cool-season crops that should have been planted in February through mid-March.) Southern peas should be planted only after soil temperature at planting depth is staying consistently above 65 degrees.
SQUASH, SUMMER: These are very cold-sensitive. Plant only after all danger of frost is past and soil temperatures are staying at or above 60 degrees at planting depth and daytime high temps are staying above 65 degrees consistently.
SWEET CORN: Because there are several different types of sweet corn, the planting of sweet corn has become more complicated than it used to be and you need to know what type of corn you're planting in order to know what soil temperatures it needs for best germination and growth.
In each instance, plant only after soil temperatures at planting depth are staying consistently at or above the temperature recommended for that specific type of corn. Corn seed planted into soil temps below 50 degrees tends to rot and germinate poorly if at all.
TRADITIONAL SWEET CORN (SU): This is the traditional sweet corn grown for hundreds of years and there are both open-pollinated and hybrid types. This has traditional corn flavor and is not terribly sweet. The amount of sweetness varies from one variety to another. This is the most cold-tolerant corn. SU corns have sugars that convert rapidly to starch after harvest. Seeds can germinate at soil temperatures as low as 50 degrees but it is better to wait until soil temperatures at planting depth are staying consistently above 55 degrees.
SUGARY ENHANCED (SE, SE+, EH or Enhanced Heritage) corn is bred to have increased tenderness and more sweetness than SU corn types. Its sugars convert to starches more slowly than SU's. No isolation from SU corn is needed. SE corn germinates at or above 60 degrees, and 65-70 is even better.
SHRUNKEN GENE (SH): Commonly referred to as supersweet corn. These are easily recognized because the dry kernels (seed corn, for example) have a shriveled or shrunken appearance. The presence of the SH gene gives the corn much more sweetness and a much slower conversion of starches to sugars after harvest. These SH corn varieties must be isoalted from SU, SE and Synergistic corn varieties to prevent cross-pollination which will give you tough, starchy kernels. In a home garden planting, isolation of 25' is usually recommended or time isolation of 2 weeks between pollination times. Supersweet corns should be planted only after soil temperature is consistently staying at or above 70 degrees.
SYNERGISTIC (AKA TRIPLESWEETS): These ears are 75% SE kernels and 25% SH kernels so they have the tenderness of the SE's and the extreme sweetness of the SH's. These can be grown with other Synergistic varieties, SE's and SU's but cannot be grown with SH's or they will cross-pollinate and all your corn will be starchy and not sweet. Triplesweets can germinate at 65 degrees but 70 degrees is even better.
SWEET POTATOES: Plant from slips after soil temperatures at planting depth are staying consistently at or above 60 degrees.
TOMATOES: Transplant into the ground as soon as possible after the last frost date but only once temperatures have stabilized enough that a return to frost and freezing temperatures is unlikely. Be prepared to cover up the plants to protect them from any possible late frosts or freezes. Planting only after soil temps are at a stable 50-55 degrees at planting depth is recommended.
For the best yields, you want your plants in the ground early enough that they can flower, pollinate and set fruit earlier in the season while temperatures are in the right range. Once it is excessively hot, especially in combination with excessive humidity, pollination and fertilization can be impeded.
You'll get the best bloom and fertilization resulting in good fruit set when the nighttime lows are staying above 55 degrees but below 72-75 degrees. Plants that produce bite-sized tomatoes (grape, cherry, currant, plum or pear-shaped) are not impacted by high temperatures and high humidity to the extent that plants producing larger tomatoes are.
While tomato plants can endure cold soil temps and even cold but above-freezing air temps, they can be damaged or killed by freezing temperatures or frost, so always cover them up if those conditions threaten after plants are in the ground.
Once daytime temperatures are exceeding about 92-95 degrees and night-time temperatures are exceeding 72-75 degrees, fruit-set can be affected as those higher temps can cause blossom drop. That is why we risk planting earlier and covering up plants if late frost or freeze threatens.
WATERMELONS: These need warm weather to germinate and grow. Plant only after soil temperatures have stabilized at or above 70 degrees and all danger of frost has ended.
I hope the above info helps. Although we humans like to use a planting calender, plants don't grow because the calendar says they should. They grow when planted at the right soil and air temperatures, so if your soil/air are at the right level and are staying there consistently, that's the right time to plant no matter what the calendar says. In our state, though, you always have to be prepared for an occasional cold spell even after the last frost date and may need to cover up plants to protect them during an occasional late cold spell.
You may wonder how much later you can plant than the recommended dates or soil temperatures and that is a very complicated topic to discuss because every vegetable is affected negatively by hot temperatures in some way. For example, bean blossoms can drop off the plants at high temperatures, thereby reducing yields. Very hot air temperaturs can impede corn from pollinating/fertilizing properly so that you may get cobs but few corn kernels on sweet corn that pollinates/fertilizes at high temps.
On the crops that have a planting date with the words "or later" added, you can plant later than the recommended dates but your success with later plantings can vary depending on how hot the weather is and how early in the plants' growth it occurs.
Finally, if you are in an area plagued by drought, later plantings may not produce well in extreme heat and extreme drought, so the earlier you can plant the better. In a drought year, I push myself to plant as early as is reasonably possible and I water well and fertilize well (without overfertilizing) to try to push the plants to produce as early as possible before the heat shuts them down.
Also, you should know that in a drought year like much of the state is currently facing, pests tend to arrive early and often, so be prepared to go after them vigorously and to protect your crops from them so that your veggie crops can produce a good yield despite the drought and pests.
You can check the OK Mesonet for your county's soil temps, or use a kitchen thermometer or soil thermometer with a metal probe to check your soil's temperature at planting dept. With beds raised above grade level, you usually will have warmer temps earlier than with grade-level beds.
I've linked the OSU Garden Planning Guide for you as it contains not only planting dates, but also contains in-row spacing, spacing between rows and other helpful info.
Here is a link that might be useful: OSU Garden Planning Guide