You’re at the store this spring, and colorful flowers catch your eye. It’s natural to want to drive straight home and plant them. Bring on the curb appeal!
But hold up…
Before you plant your flowers, it’s a good idea to help them adjust to outdoor conditions first.
In garden lingo bingo, this is known as “hardening off” or “acclimating” your plants. Many flower plants have been growing inside in greenhouses. They’ve had spa-like conditions indoors. And they’re all soft and tender from these comfy conditions.
Gradually exposing them to outdoor conditions helps toughen them up to wind, sunlight and outdoor temps. For example, their cell walls grow thicker. And they can better resist the stresses of being outside.
Think of this like preparing for the start of ski & snowboard season.
Sure, you could hit the slopes without getting your legs ready. But if you haven’t been working out, your legs are going to be ON FIRE after your first day out. You may be too sore to ski or ride for the rest of the week.
If you cross-train first, your muscles will be in much better shape. You’ll have more endurance.
Cross-training your flowers gives them more toughness too.
What happens if you don’t harden off your plants?
It can be a shock for young plants.
(And shock is not what we’re going for!)
Plants that haven’t been hardened off can struggle in multiple ways, says the University of Nebraska Extension. For example, these plants:
- Are vulnerable to sunburn, windburn (moisture loss and tissue death), and dehydration from freeze damage.
- Can break or shred easily.
- Can become stunted or die.
I see sunburn on a lot of newly-planted flowers around the Denver area in the late spring. It looks like their leaves have been bleached.
You’ve paid good money for your flowers. You want healthy plants—not flowers that look like they’re struggling, right?
Here’s how to gradually harden flowers to the outdoors:
Put your flowers outside in a protected spot for a few hours. What’s a protected spot? Look for someplace that doesn’t get direct sunlight and is sheltered from the wind, like a covered porch right up next to your home.
Then, bring them back inside or into an attached garage.
Give them a little more exposure to wind and sunlight each day. Add a few hours each day. You’ll build up their endurance, so they’re ready for planting. If your flowers like sunlight, start with shade… then morning sunlight… then work your way up to afternoon sunlight. Also work them up to staying outside overnight by the end of your hardening off period.
Try to keep your flowers from getting dried out and wilting. It’s easy for flowers to get dried out in their small nursery pots. If you pick them up and they feel light in your hand, they may need water. But if you see a few leaves turning yellow and they feel heavy when you pick them up, they may be getting too much water. Slowly reduce the amount of water you give them—but not so much that they look droopy.
Keep your eye on temperatures. If temperatures are dipping down near freezing, keep your plants inside.
Don’t fertilize them. Hold off on encouraging new growth.
Worth noting: Some flowers and veggies like warmer air and ground temperatures. They won’t do well if temps drop below 50-55 degrees. If you aren’t sure what your plants prefer, just ask at the garden center.
How many days should you harden off plants?
Some agriculture extension offices say 7-10 days. Others say 5-7 days. And some say 10-14 days.
I usually go about a week for the plants I’m buying from the store.
Ways to make it easy, peasy to harden flowers:
Make your plants easy to carry. Put your plants in a laundry basket, an open Amazon box or another container, so you can quickly move them inside and outside.
Roll, baby, roll. I have a neighbor who puts her plants on a small wagon that she wheels in and out of her garage each day.
Schedule reminders or set an alarm on your phone. That way, you’ll remember to take your flowers out. More importantly, you’ll remember to bring them back inside. (An alarm can be so helpful!)
Get creative if you head to a job every day. Maybe you want to start cross-training your flowers on the weekend. Then, if you’re going to leave them out all day during the week, start with protected spots.
What kinds of flower plants should you harden off?
- Flower plants that have been growing inside. This includes annuals (the one-season-only flowers) and perennials (the flowers that come back each year). Even some of our toughest plants in the intermountain west need to adjust to outdoor conditions. If you’re buying plants at an indoor nursery, it helps to harden them off. If you see plants outside at a store and you aren’t sure when they arrived from the greenhouse, I’d err on the side of hardening them off.
- Flowers you’re planting in the mountains. According to the Colorado State University (CSU) Extension, many mountain plants are grown in nurseries below 6000 feet. These plants may not be ready for mountain conditions yet, so it helps to harden off these plants. If your flower plants have been locally grown and they’re growing outside, they’re likely hardened off already.
- New seedlings you started inside your home and are transitioning to the outdoors. These plant babies are more likely to need more time to harden up. I’d give them 10-14 days.
If your flower plants have been growing outside already, you should be good.
Related topics that may interest you:
- 3 misconceptions about when to plant flowers that can lead to dead plants (ack!)
- When should I plant perennials in the spring in Colorado?
- Proven ways to protect your flowers from frosts and freezes
