Guest Contributor: Cassey Anderson, Horticulture Agent, Colorado State University Extension
Ask 10 different gardeners about their approach to gardening and you’ll get 10 (maybe even 15!) different responses on methods and applications or special tips and tricks. That said, there are others that most if not every gardener can agree on. Let’s get into some of the big ways that incorporating science into your garden can help you have a better yield, more success, and understand more about the plants you grow!
USDA Hardiness Zones
Do you know how to use the USDA Hardiness zone to help plan your garden space? Hint: it’s a guideline, not a guarantee! The USDA hardiness zone is a resource that is incredibly helpful as a starting point when considering more permanent plants in your garden. While the zone can influence what you can plant (I’m looking at you German Butterball potatoes), many more annuals (most vegetables are annual plants) can be started early and transplanted in cooler zones.
But first off, what IS a USDA hardiness zone? In the broadest sense, it is an average of temperatures over several decades of data. The USDA provides an incredible interactive map that lets you “zoom” into your local area to determine your local hardiness zone. Your zone is determined by the annual extreme minimum temperature in increments of 10-degree F. The zones are then divided into a and b sections of 5-degree increments. The main thing to note is that they are based on the lowest average temperature, not the most extreme possible low temperatures. The map was updated dramatically in 2023, and most areas have moved up 1-2 zones.
So, if you are thinking about planting a perennial shrub (or tree) that is marginally hardy in your area (say you are a zone 5b and it is rated for zone 6) you may run the risk of losing your plant if you get an intensely cold spell in the winter that goes below the general average. When looking at annual plants for your vegetable garden you may notice that something like a tomato is rated at a zone 8 or 9, but we can grow down to zone 3 or 4 by starting early inside and transplanting them. This works well for annual plants but isn’t such a great option for woody plants as they must stay out in weather extremes beyond their evolutionary ability to manage. So, use the USDA map to figure out your zone, but realize that it may be more of a start line than a final say.
Soil Temperature
Let’s keep going on the discussion about temperature. Soil temperature is one of the best ways to assess whether your garden is ready for planting. Most crops can germinate warmer than they like to grow, so for seeds you may want to pre-germinate in the home by soaking your seeds in a wet paper towel in a sealed container or bag. This can give you a bit of a head-start when direct sowing seeds.
To know where your soil temperature is, you can take a measurement in the morning (around 7 or 8 a.m.) and use an easy-to-read thermometer, it can be digital or standard. Typically, you want to take the temperature 4-6” deep to ensure that you’re getting the most accurate reading for the rooting area of the seed or plant. If you purloin a thermometer from the kitchen, it is better to replace the kitchen thermometer and keep the old one as your dedicated soil testing thermometer as soil can harbor pathogens you don’t want to be sticking into your Sunday night roast.
When planning to transplant, soil temperatures are equally important. Let’s consider everyone’s favorite garden vegetable, the tomato. Granted, the tomato is a bit of a garden princess, but if you plant a tomato out when soil temperatures are below 50F you’re likely to get a sulky tomato that may not ever fully recover from its sulk. Check out the Colorado Vegetable Guide from Grow & Give to find recommended planting temperature for your crop of choice, or check out your local Extension resources.
Soil Nutrient Testing
Let’s consider the soil as well. Soil is the, pun intended, ground upon which all good vegetables grow in most gardens. If you don’t know what your soil is like, then you may add unnecessary nutrients or amendments. So, for every gardener the first recommendation for a new space, or a recently amended garden, is to perform a soil test.
Soil testing is best done in a professional laboratory and there are many to choose from. You can select a private lab, or one associated with your local state university. Soil testing labs perform soil tests all the time and provide reliable, accurate information about what your soil profile looked like at the time of sampling. Tests are relatively reasonably priced as well, usually ranging from $25-45. It’s good practice to test your soil regularly, at least every 5-7 years or each time you make a significant change such as amending the soil.
To take a soil test we recommend that you sample 5-8 areas in the space you’re managing the same (so vegetable garden space would be distinct from ornamental beds or lawns). Sample about 6” deep, collect your samples and mix them together. Sift out any larger particle sizes, rocks or organic material, and let the soil dry before sending them to the lab. Most labs need about two cups of material to get accurate readings. You can send your samples in a plastic container or sealed plastic bag.
Once you know the results of your soil test, you can set forth by adding (or not) any additional nutrients your soil is lacking. One of the most common amendments most gardeners use is compost. (Learn about marking your own compost here.)
There are two general types of compost: manure-based and plant-based. Both can be incredibly useful for increasing the organic material in your soil and feeding the plants you want to grow. However, it can be easy to add too much. If you are working with a manure-based compost you likely do not want to add more than 1-2” of compost in a year and may wait a year or two before amending again as they are higher in salts. A plant-based compost will be lower in salts so you can potentially add a little more in a season, but only if necessary.
Temperature and Crop Selection
Finally let’s discuss different crop types for setting up your vegetable garden. Vegetables can be divided into two broad categories: cool season and warm season. The name gives you a good idea of when each crop grows best. This is why once the heat of summer hits your peas shrivel and your lettuce bolts (sends up a flower stalk).
Cool season crops
Cool season crops are typically tolerant of light frosts, or temperatures down to about 28F. They can germinate and grow in cool soils, but often begin to complete their life cycle when temperatures warm up. They are the first crops you are likely to plant in the springtime, sometimes as soon as you can get in the garden to work the soil. In many parts of the country, you can get two harvests of cool season crops by doing a second planting in mid to late summer.
Warm season crops
Warm season crops need higher soil and air temperatures to grow and thrive. These are what we call “tender” plants, they must be planted after the risk of frost has passed. In many zones you may need to start your plants inside, or purchase seedlings and then transplant them outside for a long enough time to harvest.
Planting a combination of warm and cool season crops can guarantee a longer season of harvest and let you sample a true diversity of crops in your garden for your own purposes and for donation!
As always, reach out to your local Extension office for further information if you have any questions.
Gardening in Colorado? Check out Grow & Give www.growandgivecolorado.org and in particular our Colorado Vegetable Guide https://growgive.extension.colostate.edu/colorado-vegetable-guide/ for more crop information on all of the above plants.
What if every gardener planted just one extra plant to share?
One small donation can have a tremendous impact. Just imagine, if every gardener planted one extra plant to share, collectively, we would have an abundant source of fresh, healthy produce available to be distributed to families experiencing food insecurity in our own communities! The free Fresh Food Connect mobile app connects you to a local hunger relief program, then manages and tracks your donations of homegrown produce throughout the season. Download the app today!