How to Take Care of Mums Outside: Planting and Winterizing

The Golden Rule: Hardy Garden Mums vs. Florist Mums

Before you dig a single hole, you must identify which type of chrysanthemum you are holding. The most common mistake beginners make when learning how to take care of mums outside is planting greenhouse-forced florist mums in the garden, only to watch them die at the first frost. According to the University of Minnesota Extension, understanding the genetic difference between these two categories dictates your entire care strategy.

If you bought your mum from a supermarket or floral shop in a lightweight plastic pot with no drainage, it is likely a florist mum. Treat it as an annual. If you bought it from a nursery or garden center labeled as a "garden mum" or "hardy mum," follow the perennial care guide below.

Site Selection and Soil Preparation

Mums are shallow-rooted and highly susceptible to crown rot if left in standing water. Choose a location that receives a minimum of six hours of direct, unfiltered sunlight daily. Morning sun is particularly critical, as it dries dew from the leaves, reducing the risk of fungal diseases.

Testing and Amending Soil pH

Chrysanthemums thrive in slightly acidic to neutral soil, specifically between pH 6.5 and 7.0. If your soil pH drops below 6.0, the plants will struggle to absorb phosphorus, leading to stunted growth and poor bud set. Test your soil using a digital meter or a mail-in kit from your local cooperative extension. To raise the pH of acidic soil, apply pelletized garden lime at a rate of 5 pounds per 100 square feet, working it into the top 6 inches of soil two weeks before planting.

For heavy clay soils, do not just add sand; this creates a concrete-like texture. Instead, amend the planting bed with a 50/50 mix of native soil and organic compost to improve drainage and aeration.

Planting Timeline and Spacing

While garden centers sell mums in the fall, spring planting is the secret to perennial success. Planting in April or May gives the root system 5 to 6 months to establish before winter dormancy. Fall-planted mums expend all their energy blooming rather than rooting, resulting in a 90% winter mortality rate in zones 6 and below.

• Spacing: Plant hardy mums 18 to 24 inches apart. They will fill in this space by their second year.
• Depth: Plant them at the exact same depth they were in their nursery pot. Burying the crown even half an inch too deep invites rot.
• Watering in: Immediately after planting, apply a root stimulator containing indole-3-butyric acid (IBA) and water deeply to collapse air pockets.

Watering and Fertilizing Regimen

Mums require consistent moisture but despise wet foliage. Aim for 1 inch of water per week, including rainfall. Utilize drip irrigation or a soaker hose to deliver water directly to the soil line. Overhead watering with a sprinkler promotes Septoria leaf spot and powdery mildew.

Fertilizer Schedule

Feed your mums with a balanced, water-soluble fertilizer (like a 10-10-10 or 20-20-20) every two weeks from spring until early July. In late July, switch to a high-phosphorus bloom booster (such as a 5-10-5 or 10-30-20 formula) to encourage dense flower bud formation. Stop fertilizing entirely by late August. Late-season feeding promotes tender new growth that will be instantly killed by the first autumn frost.

The Pinching Schedule for Maximum Blooms

If you want dense, dome-shaped mums covered in flowers rather than tall, leggy stalks with three blooms on top, you must pinch them. Pinching removes the apical meristem (the dominant growing tip), forcing the plant to branch out laterally.

1. Initial Pinch: When the plant reaches 6 inches tall in late spring, use clean bypass pruners to snip off the top 1 inch of every stem.
2. Secondary Pinches: Every time the new lateral branches grow to 5 or 6 inches, pinch off the top half-inch. Repeat this every 2 to 3 weeks.
3. The Hard Stop: You must stop pinching by July 4th (or exactly 100 days before your region's first expected fall frost). Pinching after this date will remove the microscopic flower buds that are already forming, resulting in a mum that produces only green foliage in the fall.

Winterizing: How to Guarantee Spring Return

Many gardeners believe mums are not hardy because they fail to return in the spring. In reality, they are killed by the freeze-thaw cycle, not the cold itself. When the ground repeatedly freezes and thaws, it pushes the shallow roots out of the soil—a phenomenon known as frost heaving. Once the roots are exposed to freezing air, the plant dies.

"The goal of winter mulch is not to keep the soil warm, but to keep it consistently frozen and prevent the heaving action that tears mum roots from the earth." - Iowa State University Extension

Step-by-Step Winterization

• Do Not Cut Back: Leave the dead, brown foliage on the plant throughout the winter. The brittle stems act as a natural scaffold that catches snow, providing excellent insulation for the crown.
• Wait for the Freeze: Do not apply mulch in October. Wait until the ground has frozen solid, usually after the first hard freeze in late November or December.
• Apply Loose Mulch: Pile 4 to 6 inches of loose, airy mulch over the crown. Straw, pine needles, or evergreen boughs are ideal. Avoid heavy shredded hardwood bark or wood chips, which mat down, trap excess moisture, and cause the crown to rot.
• Spring Uncovering: Gradually pull the mulch away in early April as the soil begins to thaw and new green shoots emerge from the base.

Common Outdoor Mum Pests and Diseases

Even with perfect care, outdoor mums can attract specific pests. Monitor your plants weekly during the growing season.

• Aphids: These tiny green or black insects cluster on new growth, sucking sap and transmitting viruses. Spray them off with a strong jet of water, or apply insecticidal soap (mix 2 teaspoons of pure castile soap per quart of water) directly to the undersides of the leaves.
• Powdery Mildew: Appears as a white, dusty coating on leaves, usually in late summer when humidity is high and air circulation is poor. Treat early with an organic copper-based fungicide or a neem oil solution applied in the evening to prevent leaf burn.
• Chrysanthemum Nematodes: Microscopic worms that cause leaves to turn brown and crinkle between the veins. There is no chemical cure for home gardeners; infected plants must be dug up and destroyed in the trash, not the compost bin.

Frequently Asked Questions

Can I leave my potted mums outside in the winter?

No. Potted mums are highly vulnerable to freezing because the roots are exposed to ambient air temperatures on all sides of the container. A mum that survives Zone 5 in the ground will easily die in a pot in Zone 7. If you must overwinter a potted mum, move it to an unheated garage or shed where temperatures stay between 32°F and 40°F, and water it lightly once a month.

Why are my outdoor mums turning brown in the center?

A brown, dead center with green growth only on the outer edges is a sign that the mum needs to be divided. Mums should be dug up and divided every two to three years in early spring. Discard the dead center woody portion and replant the vigorous outer shoots.

Do deer eat garden mums?

According to The Old Farmer's Almanac, mums are generally considered deer-resistant due to their strong scent and slightly toxic, bitter-tasting leaves. However, in areas with high deer pressure and scarce winter food, deer may still browse the tender spring shoots.