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

Here’s everything you need to know about how and when to start seeds indoors like a boss. These tips, tricks, and specific dates for Zone 5 gardening, plus how to calculate the dates in different growing seasons might make all the difference in your seed starting endeavors this year!

If you want to get a head start on your garden, consider learning how to start seeds indoors so you can transplant more mature plants to your garden. Sowing seeds like tomatoes and peppers directly in the garden is impossible in my zone 5 garden if I want to harvest any tomatoes and peppers before frost. 

A very important garden question, though, is when to start seeds indoors. 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 plant your vegetable 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. Knowing when to start seeds depends on:

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

And 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.

When to Start Seeds Indoors for Spring Planting

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. 

Frost sensitive seedlings started indoors can be planted outside after a process called hardening, when the danger of frost has passed. That date will depend on where you live – check which zone you’re in with Farmer’s Almanac. 

I am in Zone 5b which means 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.

To make the best decision about when to plant outdoors, start watching the extended forecast a couple weeks before your last frost date and look very carefully at overnight lows. I’ve been watching the weather for years and we almost always have a frost in mid-May, so 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. 

How to Start Seeds Indoors

The first step to starting seeds indoors is to decide on your target ready date by knowing your last frost date. Got it? Good!

Next, you need to find the right place to keep your pots and seedlings. Here are a few things to consider.

Where to start your seedlings indoors

• Do you have a space where plants grow without being disturbed?
• You’ll either need a sunny window or an electric light source. I always use a light source. I’ve found window light is never enough.
• You’ll need a table or shelf to house your containers. 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!

Seed starting materials you’ll need:

• Containers: 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. Whichever you choose, make sure they have drainage holes in the bottom. Read this for lots of free containers you can use for starting seedlings.
• Seed starting mix
• Seeds
• Water
• Labels or markers to keep track of what you’ve planted. Here are 20 cute and easy diy garden markers you can make easily at home!

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, you can 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

When to Plant Tomato Seeds Indoors for Spring Planting

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, so choose a very sunny spot that allows at least 8 hours of direct light. You can use a grow light, too.

When to Plant Herb Seeds Indoors for Spring Planting

Herbs, like basil, rosemary, cilantro, thyme, and oregano grow quickly and can be planted a little later than tomatoes and peppers. 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 too early they could die from the cold or just get leggy as they stretch for light. If you use grow lights and heat mats you can safely start some herbs earlier.

When to Plant Greens Indoors for Spring Planting

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

If you want to start from from seedlings then, aim to have them ready to plant three to four weeks before your last frost.

If the soil is warm enough, you can plant the seeds directly into raised beds instead of starting seedlings indoors. They may even germinate faster. So you might opt to directly sow cool weather leafy greens directly into your garden.

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, around the beginning of April.

When to Plant Peas Indoors for Spring Planting

Peas are tricky because they tend to rot in cold, wet soil before they even sprout.

They need to be planted six to eight weeks before your average last frost date and 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 for Spring Planting

You can plant onion and garlic 12-16 weeks before your last frost date. You can also plant 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. You can use pots or buckets and supplement with a plant light if you need to.

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

When to Plant Cool Season Veggies Indoors for Spring Planting

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 sewn directly into the garden up to two weeks before the last frost. I have never had good luck directly showing cool 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.

When to Plant Other Vegetables Indoors for Spring Planting

Here are some basic guidelines for some 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. I generally direct sow cucumber seeds instead of starting them. But if you want to start them for a zone 5 garden, late April is a good call.

• Plant zucchini seeds indoors 4-6 weeks before your average last frost date. I generally direct sow zucchini seeds instead of starting them. But if you want to start them for a zone 5 garden, late April is a good call.

• 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. But if you want to start them for a zone 5 garden, late April is a good call.

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 and transplanting.

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

Marigolds are the exception though. It’s best to give you marigolds a little extra time to germinate as they tend to be a bit slower. You might want to start marigold seeds indoors seven to eight 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, but 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 average 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 can 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!

More Gardening Posts You May Like:

How to Start Seeds Indoors to Save Sanity, Time, & Money

Get a Jump Start on Gardening with Winter Sowing

Learn How to Store Seeds + 15 Creative Seed Storage Ideas

How to Harden Off Your Seedlings

Printable Garden Planting Guide Zone 5