When to Start Seeds Indoors, Zone 5, for Spring Planting

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Were you shocked last growing season at the cost of seedlings in your local garden centers? If you’re wondering how and when to start seeds indoors, I’ve got some tips and tricks for you.

Having specific dates for Zone 5 gardening and knowing how to calculate the dates in different growing seasons will make all the difference in your garden this year. Starting seeds indoors is not too complicated.

when to start seedlings

Benefits of Starting Seeds Indoors

Starting seeds in early spring gives you a head start on your garden. Once you know how to start seeds indoors, you can transplant more mature plants to your garden. This gives you a more bountiful harvest earlier.

Depending on your growing zone, direct planting some vegetables like tomatoes and peppers is impossible. Extend your growing season by staring seeds indoors.  

Planting your own seeds also saves a ton of money compared to buying seedlings at a greenhouse or other market. Most seed packets contain more seeds than you’ll need and are $3-4. A seedling can be anywhere from $5-10 for one plant!

fresh seedlings sprouting in indoor seed container in windowsill

When to Start Seeds Indoors for Spring Planting in Zone 5

Zone 5, like all the growing zones, has a specific seed starting window that is important to understand for best garden success.

It’s generally a good idea to sow seeds in your garden as early as possible to get the biggest harvest throughout the season. But knowing when to actually start seeds indoors is a tricky question with a lot of variables such as:

  • Your garden zone (and first frost date and last frost date)
  • The type of seeds (cool season crops, warm weather vegetables, herbs, flowers)
  • When you want seedlings ready to plant outside (early, mid, late, fall garden?)

It’s oh-so tempting to get your seeds growing as soon as you can so your plants are lush and big, but is that really a good idea? Here’s a look at best practice for when to start seeds indoors.

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Calculate Your Frost Date

To find the best time to start seeds, the process is simple. You want to give the seeds enough time to grow so they’ll be mature enough when it’s time to plant them outside. 

Plant frost sensitive seedlings outside after a process called hardening, and when the danger of frost has passed. That date depends on where you live. Check which zone you’re in with Farmer’s Almanac

Once you’ve calculated your last frost date, count backwards 6-8 weeks. That date is when you want to start your seeds indoors.

I am in Zone 5b. My average last frost date is between April 29 and May 15. Ideally, I’d like my seedlings ready to go by end of April / early May. But having plants ready at the end of April sometimes fools me into planting too early too.

lettuce seedlings pots and garden shovel

When to Plant Seedlings Outdoors

Start watching the extended forecast a couple weeks before your last frost date. Pay attention to the overnight lows. I’ve been watching the weather for years, and we almost always have a frost in mid-May. Don’t be fooled into planting outdoors early to get a jumpstart on the growing season only to realize there’s a chance for frost. 

seed germinating kit in windowsill next to houseplant

How to Start Seeds Indoors

Once you have all your seeds and date calculations, it’s time to start planting them indoors. For the best results, make sure you have a good light source and a place for all those seedlings.

Best Location for Indoor Seedlings

Choose a quiet space where your seeds and baby plants will not be disturbed. Keep them off in a corner or against a wall or in a window sill where they will be out of the way. Avoid placing them in high traffic areas.

Choose a place in your house with lots of natural light. A south facing window is ideal for maximum sun exposure. If your house doesn’t get enough natural light, use an electric light source. I’ve always had to use electric lights.

To minimize the space needed for your plants, use a table or shelf with lots of vertical space. I’ve used tables, homemade seed shelves, and a store-bought metal shelf on wheels too. You can be as expensive or as frugal as you want!

metal seed starting shelf (1)

What Materials do I Need to Start Seeds?

To get your seeds off to a great start you’ll need containers, seed starting mix, seeds, labels, and water.


You can use most anything for starting seeds. Paper cups and egg cartons work well, but so do the plastic containers from the grocery store or store-bought seedling greenhouses. Peat pots are also a good choice.

I prefer a little bit larger container so they don’t dry out too quickly and there’s plenty of room for root growth.

The most important thing is the containers have drainage holes in the bottom. There are plenty of options for free containers you can use to start seedlings.

Seed Starting Mix

Seed starting mix is different than regular soil. It is lighter, finer, and has better aeration as a growing medium. Regular potting soil is formulated to support established plants. Use the right mix for starting seeds for the best results.


I prefer to save my own seeds from the previous year’s garden. However, when you’re starting out, try a variety of your favorite flowers and vegetables for indoor starts.

Choose good quality seed packages for your vegetable garden. I love this pack of Cucumber seeds with four kinds in it.


I absolutely recommend using labels or markers to keep track of what you’ve planted. (Ask me how I learned this lesson! :)) Here are 20 cute and easy diy garden markers you can make easily at home!

vegetable labels made from upcycled metal utensils to start seeds indoors

Start Zone 5 Indoor Seeds With Care

When you have everything you need, you can get started:

  • Fill your containers with soil and moisten thoroughly before filling them the rest of the way to leave a 1-inch space at the top. This is important because it prevents the seeds from drying out and allows room for root growth.
  • Once you’ve filled your containers with soil, plant your seeds. It’s recommended to plant them about an inch apart. Using a chopstick, make a small hole in the soil to the recommended depth found on the seed packet, place the seed, and cover loosely with soil.
  • Mark the planted containers with whatever information you need, then label it and put it somewhere that gets at least 6-7 hours of bright light each day. 
  • Sometimes placing your seed containers on heat mats will help the seeds to germinate. Just pay attention to soil temperatures and don’t allow the soil to dry out.
  • Now all that’s left is to keep the soil moist and watch until the seedlings emerge

What are the Best Seeds to Start Indoors?

Here in zone 5 I always start planting vegetable seeds indoors for a jump start on a short growing season. For example, tomatoes and peppers generally have a longer time until harvest. I must start those inside if I want fresh summer salads.

Start with the plants that have the longest time until harvest or are more difficult to germinate and might need a little “indoor babying”. I’ve listed the times for when to plant tomatoes and my other favorite garden vegetables below. This is for zone 5, so adjust the timing for your specific zone and weather patterns.

woman with gardening gloves preparing tomato seedlings indoors

When to Start Tomato Seeds Indoors Zone 5

Tomatoes should be planted at least six to eight weeks before the last frost date. Earlier is not necessarily better as it can lead to weak, spindly plants that don’t produce well.

In my zone 5 garden, targeting an April 29 ready date, I should start tomato seeds indoors any time from March 4 – 18.

Tomatoes require full sun. Choose a very sunny spot that allows at least 8 hours of direct light. You can use a grow light, too.

I really think tomatoes are one of the best seeds to start indoors for zone 5. I’ve never had them grow successfully when directly planted because the growing season here is not long enough.

When to Plant Herb Seeds Indoors for Spring Planting

Herbs like basil, rosemary, cilantro, thyme, and oregano grow quickly. Plant them a little later than tomatoes and pepper seeds. Six weeks before the last frost date is a great time to start your herbs indoors.

In my zone 5 garden, targeting an April 29 ready date, I should start herb seeds indoors around March 18.

If you plant them outside too early they could die from the cold or get leggy and go to seed too soon.

When to Start Leafy Greens Indoors

Cool weather leafy greens such as lettuce, swiss chard and kale tolerate a light frost. They are ready to plant in your garden as early as four weeks before your average last frost date if you have a cold frame or frost blankets.

Leafy greens actually prefer to be direct sown. If the soil is warm enough, plant them directly into beds 3-4 weeks before the last frost date.

If you want to start them indoors, aim to have the seedlings ready 3-4 weeks before last frost.

In my zone 5 garden, targeting an April 29 ready date, I should start cool weather leafy greens seeds indoors any time around March 11 so they’re ready to be planted outdoors, weather depending the beginning of April.

pea seedlings in tp rolls

When to Plant Peas Indoors for Spring Planting

Peas are tricky because they tend to rot in cold, wet soil before they even sprout. At the same time, they are one of the best vegetables for the early spring season.

They need to be planted six to eight weeks before your average last frost date. They also need a lot of light in order to stay warm and grow fast enough for spring planting.

When to Plant  Onion and Garlic Seeds Indoors

Plant onion and garlic 12-16 weeks before your last frost date. You can also direct sow shallots and garlic directly in the garden in the fall before your first frost.

Root vegetables need 12-16 hours of light each day and deeper containers than many other plants. Use pots or buckets and supplement with a plant light if you need to.

If you want to start onion or garlic seeds indoors, can plant them in January or February to give them a long enough growing season to mature in a Zone 5 garden.

When to Start Broccoli and Cauliflower Seeds Indoors

Cool-season crops are harvested later in the season and have a longer growing period. They are very hardy and tolerate cold better than many other plants.

Broccoli, cabbage, cauliflower and other brassicas can be started seven to nine weeks before your average last frost date or sown directly into the garden up to two weeks before the last frost. I have never had good luck directly sowing broccoli or cabbage so I always plant seedlings.

Brussels sprouts can be started three to four weeks before your average last frost date. Brassicas are tricky to start from seeds. Read my tips specific to starting brassica seeds here.

In my Zone 5 garden, I start broccoli and cabbages in early March so they’re ready to transplant before our last frost date.

How to Grow Cucumbers

When to Plant Other Vegetables Indoors for Spring Planting

Here are some basic guidelines for other vegetable seeds that haven’t been mentioned yet.

  • Pepper seeds can be planted 8-10 weeks before your average last frost date. I find peppers to be delicate plants that take a lot of babysitting. Start your pepper seeds early in a zone 5 garden, probably around mid February.
  • Plant eggplant seeds up to 10 weeks before your average last frost date. Start your eggplant seeds early in a zone 5 garden, probably around mid February.
  • Cucumber seeds can be planted indoors 3-6 weeks before your average last frost date. Be careful that your cucumbers don’t become root bound or they won’t transplant well.

  • Plant zucchini seeds indoors 4-6 weeks before your average last frost date.

Growing Tip: I usually direct sow both cucumbers and squash seeds instead of starting them indoors. For a zone 5 garden, start them indoors in late April for the best results.

  • Plant summer squash indoors 3-4 weeks before your average last frost date. This means start summer squash seeds around early – mid April. I generally direct sow summer squash seeds instead of starting them.
nasturtium seedlings

When to Plant Annual Flower Seeds Indoors for Spring Planting

Flower plants follow the same basic guidelines as vegetables when it comes to starting seeds indoors and transplanting.

Most flowers can be planted indoors six weeks before your average last frost date.

Marigolds are the exception. It’s best to give you marigolds a little extra time to germinate as they tend to be a bit slower. Start marigold seeds indoors 7-8 weeks before last frost.

Some flowers, like nasturtiums, cosmos, and zinnias, can even be planted a little later than that.

Many annuals can be planted directly into warm soil two weeks after the last frost, too.

Can You Plant Seeds Too Early Indoors?

Earlier is not necessarily better when it comes to starting your plants inside. Many gardeners make the mistake of planting earlier to have more mature plants and get a longer harvest period. Unfortunately, that’s not usually how it goes.

Too little light and cold soil will actually cause the plants to struggle far more than if you had waited until closer to your last frost date. They can become leggy and weak as they reach for sunlight.

Transplanting them in this state can often help them recover, but then they are using precious energy to get established instead of growing and producing. It’s definitely a delicate balance!

Starting seeds indoors is a great way to make the most of your growing season. Use these guidelines to get the timing right so you have plenty of healthy plants to start your garden off right this spring.

Now that you know everything you need to know about starting seeds indoors – make sure you let me know how it goes! Happy seed starting!

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About Michelle Marine

Michelle Marine is the author of How to Raise Chickens for Meat, a long-time green-living enthusiast, and rural Iowa mom of four. She empowers families to grow and eat seasonal, local foods; to reduce their ecological footprint; and to come together through impactful travel.

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