When Can I Start Seeds Indoors?

A hand planting cucumber seeds into black plastic pots filled with growing medium

text-box-multiplePart 6 in our Seed Starting Series link

All the info you need to start your seeds indoors!

Many of us gardeners start getting the urge to get something growing as the gardening season draws nearer. We start to wonder to ourselves ‘When should I start my seeds indoors?’…all the while knowing the answer is more than likely not yet. While it’s a common gardening question, there isn’t a one-size-fits-all answer for when you should start seeds indoors. There are several factors to consider when deciding the seed starting dates that will work best for you.

Frost dates and growing season length

Your local frost dates are key drivers for when to start frost-sensitive seeds indoors. Your last frost date in spring provides a rough indication of when it may be safe to start planting out frost-sensitive plants (which are commonly started indoors). Your first frost date in fall provides a rough indication of when your frost-sensitive plants may start dying back from the cold. Some frost-sensitive crops may need to be started early indoors to have enough time to mature and produce a harvest in your growing season (the time between the last frost date and first frost date).

In Planter, the Growing Calendar will show you the Start Inside window where you can start seeds indoors for various plants. The Start Inside window in Planter takes into account your local frost dates and common recommendations for how many weeks ahead that plant should be started indoors. It’s important to know that its not necessarily safe to transplant outdoors exactly on your last frost date- for more detail on this check out When Can I Plant Outdoors?

A screenshot of the Growing Calendar in Planter for some allium plants

The Start Inside window in Planter is a range and can be used as a guideline. You’ll still want to dial in your exact seed starting dates based on the factors we’re looking at in this article.

The seed varieties you choose

While there are good ‘rules of thumb’ we can follow for when to start specific plants indoors, it can also depend on the plant variety. Seed packets will usually give specific instructions for when that particular variety should be started indoors and this can sometimes vary for different varieties of the same plant. This guidance has to do with how long it takes the seeds to germinate, how fast the plant grows to a size where it is sturdy enough to be transplanted out, and how long it takes that variety to reach maturity and produce a harvest. As an example, some ‘superhot’ pepper varieties are slow to germinate and it’s often recommended to start them 2 weeks earlier than sweet peppers.

For cool-season crops it can be a bit tricky as the packet instructions for starting seeds indoors, if they’re available, will be based around when you plan to transplant out- not your last frost date. How early you can transplant out depends on the plant’s cold tolerance, the weather patterns, and whether you can protect it. If your seed packet doesn’t have adequate information on how early you can start seeds indoors (or if you’re using saved seeds), you’ll want to do some research and connect with local gardeners to see whether its advisable to start that type of seed indoors and if so how early.

Tomato seedlings in clear plastic containers beside a sunny window

Your indoor seed-starting set-up

A major consideration for how early to start seeds indoors comes down to how quickly the plant grows and how difficult it can become to keep it healthy indoors. Without planning for this it’s easy to run out of precious indoor growing space! Grow lights typically do not produce enough light to support a plant much beyond the seedling stage so they can become leggy and stunted (or they might just not fit under the lights!) Seedlings can quickly outgrow their trays and need to be potted up to larger pots to keep them from becoming root bound. If your seedlings become leggy or root bound, most if not all of the advantage from starting them indoors will be wasted. Starting seeds too early is a very common mistake as we’re always eager to get going- but it’s often unproductive and can even be detrimental to the plants. Plants grown from weak seedlings will not be as productive as those from healthy seedlings.

Tomato seedlings in a blue kiddie pool and pepper plants in a tray on a workbench
You’ll need both equipment and space to keep your seedlings healthy as they grow bigger!

Your goals and how much work you want to do

Your goals and the work you’re willing to put in should weigh into your decision on when to start seeds indoors. On one end of the spectrum, if you want a productive garden but don’t have a lot of time or space to commit to seed starting, buying seedlings is likely a better bet than doing any seed starting indoors. On the other end, if you’re an advanced-level home gardener or you’re planning on selling seedlings and produce, you may want to get your plants growing as early as possible and can invest the required time (and money) to make that happen. As your plants grow bigger you’ll need to commit more time moving trays, adjusting lighting, potting up, watering, monitoring, and managing any issues that might arise. Maintaining a fertilizing schedule also starts to become a bigger task as your seedlings grow and need more nutrients.

It also depends on how much effort you’re willing to put into protecting your crops once they’re outside. If you have a plastic hoophouse or greenhouse you can plan on transplanting outside much earlier (or may even be able to move some of your indoor seed starting operation outdoors). If you don’t have infrastructure but you can commit to taking the time to protect your plants in case of a late frost you might be able to plant them out a week or two earlier- and can plan to start them earlier indoors. Planning to plant early outdoors is a gamble so you’ll want to have a ‘plan B’ in case the weather doesn’t cooperate and you need to keep your plants inside another week or two.

Lettuce and herb seedlings in peat pots in an outdoor plastic shelter

Every year that you start seeds indoors, you’ll gain valuable experience on what works and what doesn’t. How fast did your seedlings grow? How difficult was it to keep everything healthy indoors? Was there anything you wished you had started earlier or later? Be sure to keep track of this information using notes and events in Planter so you can confidently answer the question ‘When is the best time for me to start my seeds indoors?’

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