When To Plant Watermelon In Iowa

If you’re planning to grow watermelon in Iowa, timing is everything. Knowing exactly when to plant watermelon in Iowa is the first step to a succesful harvest.

Our state’s unique climate, with its relatively short growing season, means you can’t just put seeds in the ground whenever you feel like it. Planting too early risks a late frost killing your seedlings, while planting too late means your fruit won’t mature before the first fall frost. This guide will walk you through the optimal timing and methods to get those sweet, juicy melons from your garden to your table.

The golden rule for planting watermelon in Iowa is to wait until all danger of frost has passed and the soil has warmed up significantly. Watermelons are tropical plants that despise cold, wet conditions.

For most of Iowa, the safe planting window for watermelon seeds or transplants outdoors is from late May to mid-June. A more specific target is after your area’s average last spring frost date. Here’s a quick regional breakdown:

Southern Iowa: Can often begin planting around May 15-20.
Central Iowa: Aim for May 20 to June 1.
Northern Iowa: Safest to wait until June 1-10.

Always check your local 10-day forecast before planting. A single cold night can set plants back for weeks.

Understanding Soil Temperature is Key

Air temperature is important, but soil temperature is critical. Watermelon seeds will rot in cool soil, and transplants will just sit there, stunted.

Your goal is to plant when the soil temperature at a 4-inch depth is consistently at least 70°F (21°C). You can use a simple soil thermometer to check this. To help warm the soil faster in spring, you can:

Cover the planting area with black plastic for 1-2 weeks before planting.
Use floating row covers to trap heat on the soil surface.
Plant in raised beds, which naturally drain and warm up quicker than ground soil.

Starting Seeds Indoors vs. Direct Sowing

You have two main options for getting your watermelon plants started: planting seeds directly in the garden or starting them indoors first.

Starting Watermelon Seeds Indoors

This method gives you a head start, which is valuable in Iowa. To do it right:

Start seeds indoors about 2 to 4 weeks before your planned outdoor transplant date. Don’t start them earlier, as watermelon seedlings grow quickly and become root-bound.
Use large, individual pots (3-4 inches) to minimize root disturbance.
Plant seeds ½ inch deep. Keep the soil warm (80-85°F) for best germination.
Provide plenty of light from a south window or grow lights.
Harden off seedlings for a week before transplanting by gradually exposing them to outdoor conditions.

Direct Sowing Seeds in the Garden

This is the simpler method and avoids transplant shock. Follow these steps:

Wait until the soil is reliably warm (70°F+).
Create small hills or mounds of soil about 3 feet apart. This improves drainage and warms the roots.
Plant 4-6 seeds per hill, about 1 inch deep.
Once seedlings have two true leaves, thin them to the 2-3 strongest plants per hill.

Choosing the Right Watermelon Variety for Iowa

Selecting a variety with a shorter “days to maturity” is crucial for Iowa’s climate. Look for varieties that mature in 80-90 days. Some excellent, reliable choices include:

‘Sugar Baby’: A classic icebox type (75-80 days). Small, round, sweet fruit perfect for smaller gardens.
‘Blacktail Mountain’: Known for its cold tolerance and quick maturity (70-75 days).
‘Sweet Dakota Rose’: A larger, oblong melon bred for northern climates (80-85 days).
‘Yellow Doll’: A yellow-fleshed, early-maturing variety (70-75 days).

Avoid varieties that need 100+ days to mature, as they will likely not ripen fully before our first fall frost.

Step-by-Step Planting and Care Guide

Once your timing is right, here’s how to plant and care for your watermelons.

1. Site Selection and Soil Prep

Watermelons need full sun—at least 8 hours of direct sunlight daily. They prefer loose, well-draining soil rich in organic matter. Work in several inches of compost or aged manure a few weeks before planting. The soil pH should be slightly acidic to neutral (6.0 to 6.8).

2. Planting Layout

Give plants plenty of space! Vines can sprawl 10-20 feet. Plant in hills or rows with the following spacing:

Hills: Space hills 4-6 feet apart in all directions.
Rows: Space plants 2-3 feet apart in rows that are 6-8 feet apart.

3. Watering and Fertilizing

Water deeply and consistently, providing 1-2 inches of water per week. Use drip irrigation or soaker hoses to keep foliage dry and prevent disease. Avoid overhead watering. Feed plants with a balanced fertilizer when vines start to run, and then again when fruits set. Too much nitrogen late in the season will produce lots of leaves but few fruits.

4. Pest and Disease Management

Common issues in Iowa include cucumber beetles (which spread bacterial wilt), squash vine borers, and powdery mildew. Use row covers early in the season to exclude beetles, but remove them when flowers appear for pollination. Rotate your crops each year to prevent soil-borne diseases.

Knowing When to Harvest

Your watermelons are ready about 80-90 days after planting, depending on the variety. Here’s how to tell if they’re ripe:

The curly tendril closest to the fruit turns brown and dry.
The spot where the melon rests on the ground changes from white to a creamy yellow.
The rind loses its glossy shine and becomes dull.
It makes a deep, hollow sound when you thump it (this takes practice).

Use a sharp knife to cut the stem, leaving about an inch attached to the fruit.

FAQs: Planting Watermelons in Iowa

What is the latest I can plant watermelon in Iowa?
Aim to have seeds in the ground by June 20 at the absolute latest for central Iowa. Later than that, and the risk of an early fall frost ruining the crop becomes to high.

Can I use mulch for watermelon plants?
Yes! Mulching with black plastic or straw is highly recommended. Black plastic warms the soil, suppresses weeds, and keeps fruit clean. Straw mulch also helps with weeds and moisture retention.

How do I protect young plants from a late cold snap?
Have cloches, frost blankets, or even old milk jugs (with the bottoms cut out) on hand. Cover the plants in the evening if a frost is forcasted, and remove the covers in the morning.

Why are my watermelon vines flowering but not producing fruit?
The first flowers are usually male and don’t produce fruit. Female flowers come later. If you see female flowers (they have a tiny melon at the base) but they wither, poor pollination is likely. Encourage bees by planting flowers nearby, or hand-pollinate by transferring pollen from male to female flowers with a small brush.

Growing watermelon in Iowa is a rewarding challenge that hinges on perfect timing. By waiting for warm soil, choosing fast-maturing varieties, and providing consistent care, you can enjoy the taste of homegrown watermelon right from your own backyard. Remember, patience in May leads to sweet rewards in August and September.