Written By Lara Wadsworth

September gardening brings cooler air, shifting light, and new opportunities for planting and harvesting. Whether you’re in Zone 3 racing frost or Zone 10 just starting winter crops, here’s what to plant this month.

Understanding First and Last Frost Dates

By September, most zones are watching the calendar and the weather forecast carefully. For northern growers, the first frost could be just weeks or days away. For southern gardeners, the frost of winter is still a couple of months in the distance. Knowing your average first frost date (via the Farmer's Almanac or your local extension office) helps you decide what you can still plant in time to harvest.

Keep in mind that microclimates, such as shaded areas, high elevations, or urban heat islands, can shift your actual frost date earlier or later than what is generally listed for any individual zone. Adjust accordingly and stay flexible. Keep a close eye on the weather forecast for your area and take action if needed! Depending on where you’re growing, extreme heat or even an early frost can occur and should be guarded against. The best source for new gardeners to understand this information as it relates to their area is your local extension office.

What are USDA Hardiness Growing Zones?

The United States Department of Agriculture (USDA) zone hardiness map assigns a zone number to each region of the United States based on the average winter low temperatures experienced annually. Your zone number helps you determine which perennial plants will thrive in your area, taking into account the winter low temperature. However, the information you gain from identifying your growing zone should also be paired with understanding your first and last frost dates, as this will determine the length of your growing season. Remember, two regions that fall within the same zone designation could still have very different summer experiences! So, use all this data and your own experiences to choose your perfect, tailored gardening schedule.

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Zone 3 – Harvesting and Frost Protection

Your first frost is either here or right around the corner. That means that September is all about harvesting, protecting crops, and maybe squeezing in a few fast-growing, frost-tolerant leafy greens where possible. Using row covers, mulch, and cold frames can extend your growing season even after the frost has arrived, so take advantage of these options if possible.

• First Frost Date Window: Early to Mid-September
• Approximate Weeks Until First Frost: 0-2
• Seeds to Start Indoors: Microgreens and herbs like chives and basil.
• Seeds to Sow Outdoors: Fast-maturing greens like radishes, arugula, spinach, and mustard. If you haven’t already, try to get your cover crop in before the hard frosts and snows come to settle in. Winter rye and clover are great options for cooler climates.

Zone 4 – Harvesting and Quick Fall Crops

By September in zone 4, the frost can come quickly, but you still have a short window to get some cold-hardy crops. Don’t forget to protect what’s already in the ground with row covers and cold frames. Now is the time to harvest your fall crops and shift your mentality towards indoor gardening.

• First Frost Date Window: Mid-September to Early October
• Approximate Weeks Until First Frost: 1-4
• Seeds to Start Indoors: Microgreens, spinach, and lettuce for indoor harvests. There are also many herbs that can be grown and harvested indoors, like chives, basil, and cilantro.
• Seeds to Sow Outdoors: Direct sow quick, cold-hardy crops like radishes, spinach, and mache. These should be harvested by the end of the month or early October at the latest. Plant garlic this month for spring harvest.

Zone 5 – Peak Fall Gardening

September is the peak fall gardening month for zone five dwellers. Cool-weather crops will love this month of mild days and cool nights with minimal frost risk. Harvest any last summer crops you have and continue to succession plant greens and quick-growing cool crops. This is likely your last full month of gardening for the year, so make the most of it!

• First Frost Date Window: Mid to Late October
• Approximate Weeks Until First Frost: 3-7
• Seeds to Start Indoors: Only start things indoors that will stay indoors. There is not really enough time to start and transplant something. Anything like microgreens, sprouts, and indoor herbs is free game and will keep you harvesting all winter!
• Seeds to Sow Outdoors: Direct sow only crops that are cold-hardy and/or will be harvested within the month. Lettuce, spinach, radishes, beets, turnips, kale, and other greens along these lines. Plant garlic and shallots towards the end of the month for a spring or summer harvest. Early September is also a great time to plant spring-blooming bulbs!

Zone 6 – Cool-Season Planting Weather

As a resident of zone 6, September is easily one of my favorite months. The days are still warm but not too warm, and the nights are clear and crisp. Turns out there are still many plants that enjoy this weather, too! September brings near-perfect weather for cool-season planting. Your fall crops will establish quickly, thanks to the warm soil, and will grow steadily. But, there’s very little risk of bolting! Get growing!

• First Frost Date Window: Late October
• Approximate Weeks Until First Frost: 5-9
• Seeds to Start Indoors: Grow herbs, microgreens, and sprouts indoors. About 6-8 weeks before your first frost would be the last time to start fall crops indoors, such as brassicas (broccoli, cauliflower).
• Seeds to Sow Outdoors: This month is the last time to direct sow leafy greens, root vegetables like carrots, beets, turnips, and radishes, and brassicas like broccoli and cabbage. Get ready to plant alliums like garlic late in the month for harvest next year. September is also an ideal time for planting spring-flowering bulbs, such as crocuses, daffodils, and tulips.

Zone 7 – Productive Cool-Weather Crops

September in zone 7 brings a much-earned break from the intense heat of July and August. You’re still able to expect warm weather, without risk of frost, and yet you’re out of the weeds when it comes to heat stress. September in this zone can be one of the most productive months for cool-weather crops and finishing up summer crops.

• First Frost Date Window: October to November
• Approximate Weeks Until First Frost: 7-11
• Seeds to Start Indoors: Start herbs, microgreens, and sprouts for a quick, high-nutrient-density harvest. Sprouts and microgreens make a great addition to traditional summer sandwiches, BBQ, and salads. Sow brassicas like cauliflower and brussels sprouts in individual pots to prepare for fall transplanting if you haven’t already.
• Seeds to Sow or Transplant Outdoors: If daytime highs are at or below 75°F, you can start direct sowing cool-season crops such as spinach, kale, and brassicas (broccoli raab, mustard greens, etc). Direct sow root crops such as beets, carrots, and turnips. Plant quick-growing greens like lettuce.

Zone 8 – Fall Planting and Prep

Zone 8 in September is still very warm, but temperatures will begin to decline slightly throughout the month. Heat stress is probably behind you unless a heat wave comes through. September is the perfect month to start planting again and also prep for fall. There’s tons to do!

• First Fall Frost Date Window: November - December
• Approximate Weeks Until First Fall Frost: 9-13
• Seeds to Start Indoors: You can start brassicas, such as broccoli and cauliflower, indoors early in the month for transplanting in the fall. Indoors is also the perfect place to keep growing herbs, sprouts, and microgreens throughout the year. These crops can be grown year-round and add a delightful crunch and flavor to all your meals.
• Seeds to Sow or Transplant Outdoors: The mild September weather is perfect for root crops like beets, carrots, onions, radishes, and turnips. Greens and brassicas such as broccoli, kale, mustard, and collards are also well-suited to this time of year.

Zone 9 – Transition to Cool-Season Gardening

September is still quite warm for zone 9, but you are likely seeing some break from the intensity. Heat stress is a real concern, but we know that more tolerable days are around the corner!

• First Frost Date Window: November - December
• Approximate Weeks Until First Fall Frost: 10-16
• Seeds to Start Indoors: Start cool-weather crops like bok choy, kohlrabi, mustard, broccoli, and lettuce indoors to prep for fall transplanting. Don’t miss out on another great time to grow microgreens, sprouts, and herbs indoors.
• Seeds to Sow Outdoors: This is the first month since spring that you can start direct sowing and succession planting crops such as spinach, radishes, and turnips. Also, direct sow alliums (bunching and bulbing), collards, and kohlrabi (yes, you can direct sow and transplant some crops; this is a great way to keep the harvests coming).

Zone 10 – Start Planting Winter Vegetables

Finally, after the long, hot summer, September is when zone 10 gardeners can start planting things outside again! Early in the month might still be too hot, depending on where you are, so keep a keen eye on the weather and protect plants from intense sun and heat when necessary. Mulching, afternoon shade, and heat-tolerant varieties can go a long way when the temperatures are still high. The end of September is all about winter crops.

• First Frost Date Window: Does not typically experience frosts.
• Approximate Weeks Until First Frost: N/A
• Seeds to Start Indoors: Get your winter vegetables started indoors that need a bit of a head start. This includes brassicas (cabbage, kale, broccoli, cauliflower, brussels sprouts) and legumes like fava beans.
• Seeds to Sow or Transplant Outdoors: As soon as daytime temperatures stay below 80 F, you can go ahead and direct sow root vegetables such as turnips, parsnips, radishes, beets, and carrots. Be sure to keep the seeds moist! Also, direct sow legumes such as lentils and garbanzo beans.

September brings as variable weather as there are crop varieties. Whether you’re in the north and are racing the clock trying to get a few more harvests in before the frost comes, or you’re in the south and you can finally plant again after the summer’s intense heat, September is a big month for gardeners. It’s not as exciting and flashy as Spring is, but it does feature another significant opportunity to learn, grow, and harvest! Cool-weather crops love the shortening days, and harvesting in crisp air is a reward in itself. Keep planting, keep harvesting, and enjoy this sweet spot between summer’s heat and winter’s chill.

Lara Wadsworth, True Leaf Market Writer

I am a native of Southwestern Michigan, where I currently reside. I love all things plants! After completing a Bachelor's Degree in Horticulture, I found a dream work-from-home job that allows me to share my passion. Now, I spend my days writing for TLM, playing with my dog, eating delicious food with my husband, and plotting my next landscape or gardening move. I believe everyone should get down and dirty in the soil now and then. Happy Gardening!