The Basics: Cool- vs. Warm-Season Plants

The Basics

Traditional Planting Times

Some gardeners wait to plant everything on a certain day (e.g. Mother’s Day). It’s a strategy that has been passed down in gardening families for generations because it’s an almost guaranteed way to ensure nothing will die from frost. Our garden plants, however, greatly benefit from being planted just a little before and just a little after this typical date when the typical prevailing temperatures and soil moisture levels best meet their needs. Let’s explain why just a bit.

Genetic Heritage

Our vegetable garden plants each evolved in extremely varied environments—from hot deserts to temperate plains and even cold mountaintops. As such, each has a deep and complex genetic heritage that allowed it to evolve over tens of thousands of years to fit its climate of origin. Each plant is, therefore, hardwired to thrive in the conditions of its original climate, reaching its peak of health and productivity when it experiences its preferred range of temperature and soil moisture.

Meeting Our Plants’ Needs

For us—as gardeners wanting to create the conditions that grow the healthiest, most productive plants possible—we can’t realistically recreate the perfect climate of origin for each of our varied garden plants. They are too varied. Plus, we’re constrained by our local climates.

Luckily, however, adjusting our planting times a little before and after our last frost date—instead of on a “one-size-fits-all,” single planting date far after it—allows us to give our plants windows of near-perfect soil moisture and temperature. If we do this well, these windows are just long enough for each plant to reach its peak of growth, health, and productivity.

Gardners, therefore, gain a much better chance to grow plants at maximum health and productivity by simply planting when conditions best meet their plants’ natural preferences. But how?

This might seem complicated, but it’s really not. To make things easier, the temperature preferences of garden plants are generally lumped into two main groups: cool-season plants and warm-season plants.

Cool-Season Plants

As the name implies, cool-season plants prefer cooler temperatures and can tolerate some freezing temperatures. They include the following plants and plant families:

Allium Family: Onions, Shallots, Garlic, Leeks, Chives, etc.

Amaranth Family: Beets, Spinach, Swiss Chard, etc. 

Asparagus 

Brassica/Cabbage/Crucifer/Mustard Family: (Brassica genus:) Broccoli, Bok Choy, Brussels Sprouts, Cauliflower, Cabbages, Collards, Kale, Kohlrabi, Mustards, Napa Cabbages, Rapini, Rutabagas, Savoy Cabbages, Turnips, etc. (other genus:) Arugula, Cress, Horseradish, Radishes, etc.

Peas: Snow, Snap, Shell, etc.

Potatoes

Lettuce: Head, Leaf, etc. 

Umbellifer/Parsley Family: Carrots, Celery, Dill, Parsnips, etc. 

Warm-Season Plants

As the name also implies, warm-season plants prefer warmer temperatures and typically will be killed by frost. They include the following plants and plant families: 

Basil

Beans: Bush and Pole

Corn

Cucurbits: Cucumbers, Gourds, Melons, Pumpkins, Squash, Zucchini, etc.

Nightshade Family (except potatoes): Eggplant, Peppers, Tomatoes, Tomatillos, etc. 

Okra

Sunflowers

Sweet Potatoes

Meeting Their Needs (the easy way)

The easy way to meet these needs is to plant cool-season crops before the average last spring frost and warm-season crops only after it. 

That’s the simple story, and most gardeners do quite well following that one simple rule.

An Intro to the More Nuanced Story

However, not all cool- and warm-season crops have the same cold-hardiness. Some cool-season plants, for example, will die at sustained temperatures around 25°F (-5°C), while others will live through even the harshest winters. Similarly, some warm-season crops can be planted as soon as there is no danger of frost, while others do best if planted after the soil has warmed a little more. Also, even though they do much better in the warm season, some warm-season crops can take frosts and even freezes, and, for all of these plants, it depends on whether the plant is a mature plant or a new seedling and if it was started indoors or out.

These are part of the more complicated story….