When To Plant Flowers In Idaho

Knowing when to plant flowers in Idaho is the key to a vibrant and successful garden. Our state’s unique climate, with its short growing season and varying elevation, makes timing everything. Get it right, and you’ll be rewarded with a stunning display of color. Get it wrong, and you might face frost-damaged blooms. This guide will walk you through the best planting times for annuals, perennials, and bulbs across Idaho’s different zones.

Idaho’s planting schedule is primarily dictated by the average last frost date in spring and the first frost date in fall. These dates can shift by several weeks depending on where you live. The state is divided into USDA Hardiness Zones ranging from 4b in the colder mountain areas to 7b in the warmest river valleys. Always check your specific zone for the most accurate timing.

Understanding Idaho’s Growing Zones

Your zip code is your best starting point. Here’s a general breakdown of Idaho’s major zones and their typical last spring frost windows:

Zone 4 (Northern Panhandle, higher elevations): Last frost around June 1. A very short growing season.
Zone 5 (Idaho Falls, Pocatello, McCall): Last frost between May 15-30.
Zone 6 (Boise, Twin Falls, Lewiston): Last frost between April 30 – May 15.
Zone 7 (Warmest parts of the Boise area, low-lying western valleys): Last frost around April 15-30.

Spring Planting: After the Last Frost

This is the main event for most summer flowers. Tender annuals cannot survive a frost and must be planted only after the danger has passed.

For Zones 6 & 7: Mid-to-late May is generally safe for planting all summer annuals.
For Zones 4 & 5: Wait until late May or early June. Use cold frames or cloches to get a slight head start.
Best flowers to plant in spring (as transplants): Petunias, marigolds, zinnias, impatiens, geraniums, and begonias.

Getting a Jump Start with Seeds

Many flowers can be started from seed indoors, 6-8 weeks before your last frost date. This is especially useful for long-season plants. You can also sow hardy annual seeds directly in the garden before the last frost, as soon as the soil is workable.

Direct sow early (March-April): Sweet peas, larkspur, calendula, and poppies.
Start indoors (March): Zinnias, sunflowers, and coleus.

Fall Planting: A Secret for Success

Fall is an excellent time to plant many perennials, shrubs, and spring-blooming bulbs. The warm soil and cool air encourage strong root growth without the stress of summer heat.

Plant perennials: Early September is ideal, giving plants a month to establish before hard freezes. This includes coneflowers, daylilies, and sedum.
Plant spring bulbs: October is prime time for tulips, daffodils, and crocus. Get them in the ground about 6 weeks before the soil freezes solid.

A Seasonal Flower Planting Calendar for Idaho

Early Spring (As Soil Thaws)

Focus on hardy plants and preparation.

Divide and transplant existing perennials.
Direct sow hardy annual seeds (poppies, bachelor’s buttons).
Plant bare-root perennials and roses.
Clean up garden beds and add compost.

Late Spring (After Last Frost)

The main planting window for summer color.

Harden off your indoor-started seedlings for 7-10 days.
Plant nursery-bought annual and vegetable transplants.
Direct sow seeds for fast-growing annuals like cosmos and nasturtiums.
Plant summer-blooming bulbs like gladiolus and dahlias.

Summer

Focus is on maintenance, but you can still plant.

Keep new plantings well-watered.
Deadhead spent blooms to encourage more flowers.
In early summer, you can still plant container-grown perennials if you water consistently.
Start planning your fall bulb order!

Fall

Time to prepare for next year and plant for early spring blooms.

Plant spring-flowering bulbs throughout October.
Plant new perennials and trees in early September.
Mulch garden beds after the ground freezes to protect plant roots.
Collect seeds from your favorite annuals for next season.

Tips for Gardening in Idaho’s Climate

Our dry air and intense sun require some special considerations. Here’s a few things I’ve learned over the years.

Water Deeply and Early: Water in the morning to reduce evaporation and allow foliage to dry, preventing disease. Deep watering encourages deep roots.
Mulch is Mandatory: A 2-3 inch layer of mulch conserves precious water, suppresses weeds, and moderates soil temperature.
Watch for Wind: Stake tall plants like delphiniums early. Windbreaks can help in exposed gardens.
Soil Matters: Idaho soil often leans clay-heavy or sandy. Amending with compost every year improves drainage and fertility for all soil types.

Idaho-Proof Flower Recommendations

These varieties are known to perform well with our temperature swings and dry conditions.

Drought-Tolerant Stars: Russian sage, yarrow, blanket flower (Gaillardia), sedum, and lavender.
Reliable Perennials: Daylilies, coneflowers, hostas (for shade), bee balm, and hardy geraniums.
Easy Annuals: Marigolds, zinnias, sunflowers, cosmos, and sweet alyssum.

FAQ: Planting Flowers in Idaho

What month do you start planting flowers in Idaho?

You can start some planting as early as March or April by sowing hardy seeds or planting pansies. The main planting month for tender flowers is May, after the last frost date for most populated areas.

Is it too late to plant flowers in July?

Not necessarily! You can plant fast-growing annuals from seed, like zinnias or sunflowers, in early July. It’s also a fine time to plant container-grown perennials if you commit to consistent watering while they establish.

When should I plant tulip bulbs in Idaho?

Aim for October. Plant them when nighttime temperatures are consistently in the 40s, but the ground isn’t frozen yet. This gives them time to develop roots before winter.

Can I plant perennials in the fall in Idaho?

Yes, early fall (September) is an excellent time. The warm soil promotes root growth, and the cooler air reduces transplant shock. Just make sure to water them regularly until the ground freezes.

Gardening in Idaho is a rewarding challenge that pays off with careful timing. By aligning your planting with the states frost dates and choosing resilient plants, you can create a beautiful oasis that thrives from spring to fall. Remember to keep a garden journal each year noting what worked and when—it will be your most valuable tool for seasons to come.