Seeds to Start Indoors 4–6 Weeks Before Your Last Frost

Learn which seeds to start indoors 4–6 weeks before your last frost date, including brassicas and greens, plus tips for healthy transplants.

Pak choi and Chinese cabbage seedlings growing in seedling trays under grow lights.

As spring draws closer, the pace of seed starting begins to shift.

The 4–6 weeks before your average last frost date is an important window for starting fast-growing crops indoors, especially cool-season vegetables that grow quickly and transplant well. These seeds don’t need a long time under lights, but starting them indoors helps ensure strong, healthy seedlings ready for the garden.

In this guide, you’ll learn which seeds to start indoors during this window and how to time them for successful transplanting.

Start With Your Average Last Frost Date

As with all seed-starting schedules, timing begins with your average last frost date.

This date helps you determine when it’s typically safe to plant outdoors in spring and serves as the foundation for your seed-starting schedule. To find yours, enter your ZIP code at PlantMaps.com or Garden.org, then count backward to identify when to start seeds indoors.

If it’s still a little early for your area, save this post for reference. Once the timing is right, you’ll be ready to plant with confidence.

For a deeper look at frost dates and how to build a complete planting plan, see: How to Create a Seed Starting Schedule Based on Your Last Frost Date.

Seed starting schedule and seed packets arranged on a kitchen table for spring garden planning. — Creating a seed starting schedule helps you time indoor sowing based on your last frost date.

Why These Seeds Are Started 4–6 Weeks Before Frost

Seeds started during this window grow quickly and are often ready for transplant soon after sprouting.

Crops started 4–6 weeks before frost:

Germinate rapidly.
Grow well under lights without becoming overgrown.
Can be transplanted outdoors earlier than warm-season crops.

These crops are ideal for sowing indoors if you want strong seedlings without a long indoor growing period.

Seeds to Start Indoors 4–6 Weeks Before Your Last Frost Date

The following crops are well suited to starting indoors during this time. They grow quickly and benefit from transplanting as young, vigorous seedlings.

Brassicas to Start Indoors

Many members of the cabbage family grow best when started indoors and transplanted into the garden while temperatures are still cool. This approach helps prevent pest damage and encourages steady growth.

Broccoli

Broccoli grows quickly and transplants well. Starting it indoors helps ensure sturdy seedlings that can be planted out early for a productive harvest.

Brussels Sprouts

Brussels sprouts need a longer growing season, but they don’t require a long indoor start. Starting them during this window helps them establish without becoming root-bound.

Cabbage

Cabbage is a classic indoor-start crop. Young seedlings transplant easily and thrive in cool spring weather.

Chinese Cabbage (Napa Types)

Chinese cabbage grows quickly and prefers cool conditions. Starting seeds indoors helps ensure even growth and reduces stress that can lead to bolting.

Cauliflower

Cauliflower benefits from steady growth early on. Starting seeds indoors helps produce uniform plants that handle transplanting better.

Kohlrabi

Kohlrabi grows quickly and does best when transplanted while young. An indoor start helps avoid stress from cold or uneven spring weather.

Pak Choi / Bok Choy

Pak choi grows quickly and prefers cool weather. Starting it indoors allows you to transplant young seedlings early for tender spring harvests.

Tatsoi

Tatsoi is a cold-hardy Asian green that grows quickly in cool conditions. An indoor start helps ensure uniform growth and allows for early transplanting before plants bolt in warming spring weather.

Chinese cabbage and bok choy seedlings growing in seedling trays under grow lights. — Chinese cabbage and bok choy seedlings benefit from an indoor start 4–6 weeks before transplanting outdoors.

Greens to Start Indoors

Most leafy greens can be direct-sown, but starting them indoors lets you transplant strong seedlings and get an earlier harvest.

Spinach

Spinach grows quickly and prefers cool conditions. Starting it indoors can give you a head start, especially in areas with unpredictable spring weather.

Swiss Chard

Swiss chard is hardy and adaptable. Starting it indoors produces sturdy transplants that establish quickly in the garden.

Collards

Collards are slow to bolt and tolerate cool temperatures well. An indoor start helps produce strong plants ready for early transplanting.

Review the Seed Packet Before You Start

Your seed packet is one of the best tools for successful seed starting. Before planting, take a moment to review the information on the packet, including:

Timing: When to sow seeds based on your last frost date.
Planting Depth: How deep to plant seeds for proper germination.
Germination Time: How long seeds typically take to sprout.
Spacing: Helpful when transplanting seedlings outdoors.

Always follow the seed packet instructions, especially if growing a specific variety.

Tips for Starting Seeds Indoors Successfully

Fast-growing seedlings started indoors need close attention to avoid stress.

Keep these tips in mind:

Provide bright, consistent light to prevent legginess.
Keep soil evenly moist, but not waterlogged.
Maintain good airflow around seedlings.
Harden off your seedlings before transplanting to the garden.
Transplant seedlings before they become crowded.

Helpful resources:

Can These Seeds Be Started Earlier or Later?

This timing window allows for some flexibility.

Most brassicas can be started slightly earlier if needed.
Starting these crops too early may result in oversized seedlings.
Starting a little late is often better than starting too early.

Aim for healthy, compact plants rather than large seedlings.

What to Start Next

As your last frost date approaches, warm-season crops and fast-growing plants are ready to be started both indoors and out.

Next in the indoor seed-starting schedule: Seeds to Start Indoors 2–4 Weeks Before Your Last Frost (Coming Soon).

This window includes fast-growing crops that benefit from a short indoor start before being planted outside.

And if your ready to sow seeds directly in the garden: Seeds to Direct Sow 4–6 Weeks Before Last Frost (Coming Soon).

Many cool-season crops can be planted directly in the garden at this stage as conditions continue to improve.

Starting seeds indoors during this window keeps your garden moving forward without overwhelming your growing space. With a little planning, these seedlings will be ready to thrive outdoors in just a few weeks.

Good planning is key to a successful vegetable garden

Whether you are new to growing your own food or have been growing a vegetable garden for years, you will benefit from some planning each year. You will find everything you need to organize and plan your vegetable garden in my PDF eBook, Grow a Good Life Guide to Planning Your Vegetable Garden.