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?

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 weeks earlier than sweet peppers.

For cool-season crops it can be a bit tricky as the packet instructions for starting seeds indoors will be sometimes be based around when you plan to transplant out- not strictly tied to your last frost date. How early you can transplant out cool-season crops depends on the plant’s tolerance for cold (especially while young), the weather patterns, and whether you can protect them if needed. 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.

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.

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.

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?’

What Is Hardening Off?

‘Hardening off’ seedlings is the process of gradually getting them acclimated to growing outdoors. Hardening off is especially important for seeds started indoors, as they have been growing in unnatural conditions their whole lives. Seedlings grown indoors take some time to build up their defenses to cope with more extreme sunlight, wind, temperatures, and drought outdoors. Hardening off is most crucial for seedlings raised indoors, but if you have enough time it can be helpful to harden off seedlings purchased from a garden center. Without hardening off, seedlings can suffer from sun scald, wilting, and wind damage.

Hardening off might seem like an extra step, but it provides some insurance that the seedlings you spent weeks nurturing won’t wither and die as soon as they get outdoors. Is hardening off necessary? No. Some plants might survive being transplanted directly into the garden. But skipping hardening off is a gamble, and plants that do survive the transition might never recover fully.

When and How to Harden Off Seedlings

To determine when to harden off seedlings, start by checking the ‘Transplant’ window in the Growing Calendar in Planter for each of the plants you plan on transplanting:

The transplant window in Planter is tailored to your growing location. Within the transplant window for each of your plants, think about when specifically you want to transplant. Consider recent and forecast weather conditions and when you have time in your schedule for planting. If you are succession planting, you may have multiple transplant dates to keep track of for each plant. Make sure your seedlings will be mature enough to be transplanted- they should have at least a couple sets of ‘true leaves’- but don’t leave it so late that they are outgrowing their containers.

With transplant dates in mind for each of your plants, aim to start the hardening off process about two weeks prior. To start the process, find a partially shady location in your garden, or start the process over a stretch of a few cloudy days. Be sure the selected location is somewhat sheltered from wind. Gather your seedlings together in trays or shallow containers - this will make carrying them outside and inside easier.

On the first day of hardening off, take your seedlings outside to the chosen location, leave them out for a few hours, then bring them back in. Each day, gradually increase the number of hours that the seedlings spend outside and the amount of sunlight they receive.

If the forecast calls for extreme weather (cold, heat, wind, storms), keep the seedlings inside. Continue the hardening off process once the risk of extreme weather has passed. Once the seedlings are spending most of the day outdoors you can start leaving them outside overnight, but watch the forecast to ensure it won’t get too cold for your plants. After a week or two of this process, your seedlings are ready for transplanting!

How to Transplant Seedlings

When transplanting seedlings the goal is to prevent ‘transplant shock’, which can stunt or kill them. To make the process as smooth as possible, plan your planting ahead of time using Planter, be sure your beds (or containers) are prepped, and have your tools handy.

Prior to transplanting, moisten the planting area and seedlings’ roots. Dig a hole deep enough so the plant stem will stay at the same depth as it was in its container (tomatoes, peppers, and eggplants can be planted deeper). Be careful not to damage the plant stem and roots in the process of taking the seedling out of its container and planting it. Do not break up or untangle the seedlings’ roots, unless they are especially root-bound (i.e., the roots are coiling around several times in the container). If you are using peat pots, rip the edges open when planting otherwise they can wick moisture away from the plant.

Place the plant in the hole and fill it with soil, pressing gently yet firmly until it is level with the surrounding soil. Water again gently so the plant settles in and has enough moisture.

Your plants will need some extra attention for the first few weeks after transplanting. Ensure they have consistent moisture and protect them from extreme weather conditions if needed. After this point your plants should be established and can be cared for as part of your regular garden maintenance.

It can be tempting to rush to get your seedlings planted outside, but taking the time to harden off and transplant your plants properly will set your garden up for a successful season!

Lighting and heat for seedlings

Seeds typically need warm, moist conditions for germination and most veggie plant seeds do not need any light for germination. Once the seeds germinate though, it’s a whole different ball game! Seedlings need ample light as soon as they germinate. If you’re using heat mats you can turn them off once the majority of your seeds on the mats have germinated. Refer to All About Grow Lights and Heat Mats for Starting Seeds for more information on how to use grow lights and heat mats.

How to water seedlings

How much to water your seedlings and when depends on many factors such as: how well your seed starting mix holds water, the type of containers you use, and how dry your house is. Check your seedlings often- multiple times a day- and as soon as the soil starts to appear a bit dry water them right away. While you never want the growing medium to go bone dry, you may not have to be as meticulous with misting and watering as when germinating seeds. Also, if you were using a humidity dome for germinating your seeds, be sure to remove it once they germinate. Many gardeners prefer to bottom water their seedlings, but no matter how you water them, be sure your containers have drainage and don’t let your seedlings sit in water. Over time you will develop a ‘feel’ for the right amount of water to give your seedlings.

How to fertilize seedlings

Seeds and newly emerged seedlings do not need any fertilizer- they have all the nutrients they need inside their ‘seed leaves’ (‘cotyledons’) or inside the seed. Because of this, seed starting mix often does not have any nutrients added (although some do). It’s usually recommended to start fertilizing seedlings once their first set of ‘true leaves’ emerge. When fertilizing young seedlings, use diluted quantities of fertilizer to avoid burning them. I like to use a liquid fish emulsion fertilizer diluted to a half or quarter of the usual strength. Fertilize about once a week as the seedlings grow (you can increase the frequency if the leaves start to turn yellow).

Thinning and potting up seedlings

Thinning seedlings is the practice of removing extra seedlings so you end up with only one seedling per growing container. Gardeners often sow more than one seed per container in case not all the seeds germinate However you’ll need to deal with any extra seedlings so they don’t crowd each other out. Identify the seedling that looks the most stocky and robust, and snip off any other seedlings in the container. Cutting is recommended instead of pulling so as not to damage the roots of the seedling you want to keep. If you don’t have the heart to kill the extra seedlings and you have space for the extras, you can very delicately use a chopstick to gently tease out the seedlings for potting up.

Potting up refers to moving seedlings into larger containers with more growing medium. Potting up gives seedlings room to continue growing so they don’t become stunted in a too-small container. To know when to pot up your seedlings, regularly check the roots. As soon as you see roots starting to poke out of the bottom of the container it’s time to pot up. Select a larger container, fill it partway with potting mix, then gently extract the seedling from its existing container (soil and all) and put it in the larger container. Fill in any gaps with more potting mix then water well.

Common seedling problems and remedies

Mold and damping off: Mold on seedlings is often just an eyesore, but it can be a sign of a deeper problem. ‘Damping off’ is a type of mold that can actually kill your seedlings. To prevent mold, make sure you are not overwatering your seedlings. Using a fan on a gentle setting to circulate air can also help keep mold at bay.

Fungus gnats: Fungus gnats are small, black flying insects. On mature plants they are a nuisance, but the larvae can inflict catastrophic damage on your seed starts. Sterilizing your seed starting mix with boiling water before sowing seeds will kill fungus gnat eggs. To treat fungus gnats in your seed starts, water them with a hydrogen peroxide solution of 4 parts water mixed with 1 part hydrogen peroxide. Multiple treatments may be needed to quell an infestation.

Leggy seedlings: Leggy seedlings are weak seedlings that are stretched too thin. To prevent leggy seedlings, make sure they are placed close enough to a light source to receive ample light, and use a fan on a gentle setting to help strengthen the stems.

Taking care of seedlings is not complicated, but it does require a bit of attentiveness. Keep a close eye on your seedlings to make sure they are not underwatered or overwatered, and that they have sufficient light and nutrients. Taking action as soon as you spot a problem can save your precious seedlings. You’ll be rewarded for your efforts when they turn into healthy plants you can transplant into your garden!

Why Start Seeds Indoors?

The main benefit of starting seeds indoors is extra time. Some plants take more time to reach maturity than you have in your growing season. These plants must be started indoors so they have enough time to produce a harvest before the frost comes in fall. For other plants, starting indoors is optional but it gives them a head start so you can harvest from them sooner! 

Buying seedlings from the garden center is an option if you don’t want to start seeds indoors, but the cost really adds up and you won’t find as many plant varieties to choose from.

Another benefit of starting seeds indoors is being able to control the environment and tend the plants more easily. Seeds that are direct sown outdoors are exposed to potential inclement weather and pests right away. It’s easier to micro-manage sensitive seedlings indoors versus having them exposed in the garden.

How to know which seeds to start indoors

Now that we know the reasons for starting seeds indoors, how do we know which seeds need to be started indoors?

• Plants where the number of ‘days to maturity’ are close to or greater than the total number of days in your growing season must be started indoors. 
• Plants where the days to maturity are less than the number of days in your growing season can be started indoors to get a head start on the season or to maximize the potential harvest, with a few exceptions (see below).
• There are some plants that don’t do well if started indoors and then transplanted. Common examples are root crops such as carrots, beets, and radishes (although some gardeners do start beets and radishes indoors). Corn, beans, peas and cucurbits (squash, cucumbers, melons, pumpkins) are also not usually started indoors- although it can be done!

The Growing Calendar in Planter helps you determine which plants should be started indoors and which should be sown outside.

Peppers should be started inside then transplanted (direct sowing outside is not recommended- no Sow Outside dates appear):

When to start seeds indoors

To find out when to start seeds indoors, first check the Growing Calendar in Planter. The Growing Calendar gives a good indication of when that plant should be started indoors, based on your local frost dates.

If the seed packet gives specific instructions on when to start seeds indoors you should follow them, as dates can vary for different varieties of the same plant. If the packet says “Start indoors 6-8 weeks before last frost”, count backwards 6-8 weeks from your Spring frost date in Planter to determine when to start those seeds indoors.

Starting seeds indoors too early can be almost as problematic as starting too late. Space and light start to become scarce as the seedlings get bigger, and they can be more prone to issues such as legginess and becoming root-bound. Resist the urge to plant earlier than recommended!

How to start seeds indoors

Starting seeds indoors involves a few basic steps:

• Pre-moisten your seed starting mix (don’t use regular garden soil!)
• Fill your containers or seed trays with the moistened mix and pat down gently.
• Place the seeds into the containers or cells. Gardeners often plant multiple seeds per cell as insurance in case one doesn’t germinate.
• Poke the seeds down into the soil to the depth recommended on the seed packet or in the How to Grow info in Planter (you might need to use a pencil or another tool for smaller seeds). Ensure they are covered over with soil.
• Label the containers so you know which seeds are where (this is very important!)
• Place the containers in a flat tray that can collect water.
• Place trays on heat mats and under grow lights if you’re using them (check out All About Grow Lights and Heat Mats for Starting Seeds for more info!)
• Keep the seed starting mix consistently moist but not soggy until you see germination (many gardeners prefer to bottom water seed starts).
• After the seeds germinate, turn off the heat mat if you were using one. If you have a fan, use it to gently circulate air around the seedlings for at least a few hours each day to help strengthen the seedlings and prevent disease.

As the seedlings grow you’ll need to continue watering and start fertilizing them. You might also need to ‘thin them out’ and ‘pot them up’. We’ll cover seedling care and troubleshooting in an upcoming post!

Having the ability to start seeds indoors will take your garden to the next level! It might seem intimidating, but over time you’ll hone in on a seed-starting approach that works for you. The time and investment is more than worth it to kick-start your gardening season!

Temperature requirements

Some veggies will have no problem germinating in a cold basement (where I start my seeds), while others need some warmth to sprout in a timely fashion.

Luckily, master gardeners have determined the optimal temperature for germinating most veggies. The following chart is pulled from this paper by the University of California:

Soil Temperature Conditions for Vegetable Seed Germination

You may have noticed the missing temperatures for celery. Celery requires diffused, non-direct light and a cooler night temperature than the day temperature for good germination. Its optimal conditions are 85°F during the day and 70°F at night.

Keeping the seed tray at the right temperature

Most heating mats don’t have an adjustable thermostat, so it’s hard to get the soil to the right temperature. And even if the heating mat works for one gardener, it might not work for you. This is because soil temperature can vary depending on a variety of factors:

• Wattage of the heating matt
• Amount of soil
• Amount of water in the soil
• Surrounding air temperature
• Air movement
• Heat from grow light(s)

That’s why we recommend a thermostat-controlled heating mat such as the Spider Farmer Heat Mat Kit. It comes with a temperature probe that you stick in the soil, so you know your seedlings are at the optimum temperature. No more guessing! However, if you already have a heat mat that is working for you and getting good germination rates, there’s no need to fix what isn’t broken.

One important thing to keep in mind: Not all seeds need a heat mat. Many cold-hardy crops such as lettuce, spinach, and peas will germinate just fine at room temperature. In fact, they might not germinate at all if the soil is too warm! Just take a look at the table above, the max germination temperature for spinach is just 75°F.

What next?

After your seeds germinate, unplug the heat mat. Besides wasting electricity, keeping the heat mat on can stress veggie roots and can cause some plants to bolt!

That’s about all there is to it!

Grow Light Types

There are two primary technologies for grow lights: LEDs and fluorescent lights. For a long time, fluorescent lights were the go-to bulb for gardeners, but LEDs have quickly become the standard.

Fluorescent grow lights come in a variety of shapes and sizes, including the traditional tube and the “compact” swirl style. They’re available in a wide range of color spectrums and intensities. They are also typically less expensive up-front than LEDs, but less energy-efficient.

LED grow lights, on the other hand, are more energy-efficient and come in even more shapes, sizes, intensities, and spectrums. Some even come in tubes like fluorescent lights.

There are a few other lighting technologies that are typically used in commercial applications, such as metal halide lights and sodium vapor lights, but we won’t be focusing on those.

Okay, time for some science!

Grow Light Color, AKA “Spectrum”

More important than the type of grow light is the color and intensity of the light that it produces. Plants use light in the photosynthetically active spectrum between 400 (violet) and 700 (red) nanometers. However, not all of that spectrum is used equally. Typically, plants use more blue-green light when they are seedlings and more red light later, when they begin to flower and produce fruit or seeds. Because of this, plants appear green/yellow because they reflect light in those wavelengths. Hence, many grow lights on the market try to mimic these wavelengths and are a mixture of blue and red, often appearing purple when combined.

For fluorescents and some LEDs, spectrum is often labeled as the color temperature or K (Kelvin). A higher color temperature, such as 6500K, is best for starting seeds. Many LED fixtures include a variety of colors, so instead of a Kelvin number you may see a spectrum graph, similar to the one above.

When deciding between the “purple” and “full spectrum” grow lights, one important factor to consider is where your seed-starting station is. If you’re starting your seeds in the basement or some other less-used space, go ahead and get the purple grow lights. On the contrary, if you’re starting them on the kitchen counter or some other common space, consider going for the full spectrum grow lights. The reason is, the purple grow lights aren’t very pleasant to be around for long periods of time. I start my seeds in the basement, so I don’t mind the purple hue and can deal with it in exchange for a bit more energy efficiency.

Grow Light Intensity

When buying grow lights, in general, more light intensity is better. Every light on the market has a watt measurement, which is the amount of energy that the unit uses.

Some grow lights will also include more helpful intensity units including:

• Lumens - When shopping for lighting fixtures, including grow lights, you may come across the term lumens. Lumens are a measure of the visible light emitted by the light source. In general, the brighter a light source appears, the higher its lumen output will be. While lumens can be a useful tool in determining the appropriate lighting for a room, they are not relevant to the needs of your plants. Lumens do not provide information about how well plants will respond to visible light.
• PPFD - Photosynthetic Photon Flux Density (PPFD) is a crucial measurement for your plants. Unlike lumens, which only measure the brightness of a light source, PPFD measures how much of the necessary light within the photosynthetic spectrum (400-700 nm) is reaching your plant.
• Foot-candles - A measure of how much light a square foot receives at a distance of 1 foot.

Grow Light Height

A good rule of thumb is to position your lights 6-24” away from your plants. Don’t just set it and forget it though! Pay attention to your seedlings, and if you notice them becoming “leggy”, they’re not receiving enough light, so you should lower your lights. On the contrary, if your lights are too close, plants can get damaged by the heat of the light. Try adjusting your lights to find the sweet spot, and periodically raising them as the plants grow.

Our Recommendations

There are so many grow light options out there, it can get overwhelming. So we’ve done the research and read the reviews for you. Here are our recommendations:

Barrina Grow Lights

Available in full spectrum, pink, or red/blue, these LED grow lights comes in both 2ft and 4ft lengths. Both lengths come in a 6 pack, perfect for gardeners who want to start a lot of seeds or are looking to start an indoor garden.

Barrina’s grow lights are highly rated, so if these options don’t work for you, check out their other options such as their 4 pack.

iGrowtek Seed Starter Kit

This kit contains everything you need to start seeds, minus the soil and the actual seeds. The light is full-spectrum.

iPower Stand and Grow Light

Perfect if you don’t need the seed starting tray but still want a stand.

Kullsinss LED Grow Light Strips

If you’re starting your seeds on the kitchen counter, this might be the grow light for you. You can stick these light strips to the underside of a cabinet or shelf, perfect for spaces where hanging a reflector wouldn’t work.

Adjustable Gooseneck Grow Lights

For the ultimate in adjustability, these grow lights are mounted on goosenecks. They are available in 4 different color options and are perfectly suited for lighting large houseplants as well as veggie seedlings!