The Ultimate Guide to Indoor Seed Starting

January 23, 2020

By Matt

While indoor seed starting may seem like something only expert gardeners are capable of, this task is one that can be learned by even the most novice horticulturist! Starting seeds indoors is a great way to ensure your plants will be successful upon transplant, as you will have ultimate control over the conditions your seedlings receive. 

You can start your seeds whenever it is most convenient for you, and you won’t be at the mercy of the local nursery as to when you choose to start your garden. Indoor seed starting also allows you to experiment with a greater variety of seeds, meaning you can try out new, hybrid, or heirloom species of plants that often aren’t available as seedlings when purchased locally. 

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Consider our ultimate guide to indoor seed starting, and you’ll get a jumpstart on the growing season when the weather warms this spring!

Advantages of Indoor Seed Starting 

The advantages of indoor seed starting cannot be overstated, and while there are many, these are some of the most prolific:

  • Better Spacing: When you direct sow your seeds into the ground, many of the seeds may not germinate. This can result in patchy spacing, making your time and space less efficient. Indoor seeding allows your plants to grow without competition for resources, and when you later transplant them into the garden, they will have stronger root systems and hardier stems and leaves.
  • Earlier Harvest: When you start your seeds indoors, ideally a month or two before you want to put them out in your garden, you will shorten the amount of time needed to help your plants reach full maturity. This early harvest is critical if you live in an area with a short growing season! 
  • Hardier Plants: Plants that have been started indoors will have more time to be hardened off. While we will talk about the hardening off process later, this is crucial for producing resilient plants. Plus, when you start your seeds indoors, you can cull the weaker seedlings from the group, which will help make them more likely to survive from challenges like weeds, pests, and poor weather.
  • Better Control: When you start seeds indoors, you will have more control over the temperature and moisture of your growing conditions. For finicky plants, this is a must-have factor in creating a successful garden. You also gain control over the fertilizers and conditions in which the plants are raised, meaning you won’t have to worry about any contamination.
  • Better Yields: Simply put, you will reap the benefits of everything we have already mentioned as being an advantage of indoor seed starting when harvest time rolls around. When you have plants that are resistant to changes in the weather, have been given adequate time to develop strong root systems, and are growing exactly where and how you want them to, the result is a better overall harvest. 
  • Cost Effectiveness: Starting seeds allows you to save a ton of money. Buying packets of seeds (which are usually less than a dollar or two, in most cases) is much cheaper than buying seedlings (which can cost as much as $12 for a six-pack). If you plan on growing a high volume, these costs can really add up. 
  • More Options: When you purchase seeds, you will likely be faced with dozens of options for different varieties of plants you can grow. You can grow heirloom or hybrid crops or experiment with multiple strands of the same vegetable. As a result, you will have a wider choice of textures, tastes, and options – a luxury that you simply don’t have access to when purchasing seedlings at the nursery, which typically has a much smaller inventory than the seed catalogs.

Challenges of Indoor Seed Starting 

While the advantages of indoor seed starting far outweigh any disadvantages, there are some challenges you need to keep in mind. 

  • Insects: Because plants provide a natural habitat to many organisms, you run the risk of bringing unwanted critters into your home. While this is less likely when starting seeds than if you were bringing in plants from other locations, remember that you will want to keep an eye out for flies, ants, and mold growths on your plants.
  • Lack of Variety: Not all plants can be grown indoors. While this is true in the outdoor setting as well, there are some plants that simply do not thrive when started from seed indoors. 
  • Difficulty in Providing Optimal Conditions: It can be difficult to provide the light, temperature, and moisture required for healthy plant growth. As a result, your plants could develop issues related to overwatering, underwatering, too much shade, or other inopportune conditions. However, starting seeds indoors is beneficial in this regard in that you can move the location of your containers if the first spot doesn’t work out.
  • Need for Vigilance: While your outdoor garden will probably be fine if you leave it for a week to go on vacation, if you leave your indoor plants without water for a week, they will likely die. They aren’t exposed to the outdoor climate, and while this is beneficial in many ways, it also means you need to take full responsibility for all of their needs.
  • Extra Mess: Watering indoor seedlings can be a major pain in that you need to do it often, and when you do, you will likely make a huge mess by scattering soil and water all over the floor. 

Best Types of Plants for Indoor Growing

Consider a planting guide like this when determining which plants should be started indoors –and when–based on your specific growing zone. Otherwise, consider these plants for your final list.

Broccoli

Broccoli is best started indoors so that it can be transplanted with enough time left to develop a good-sized crown before the weather warms up and the plant dies off. As a cold season crop, broccoli produces buds that will flower and make your plant taste bitter if the weather gets too hot. As the summer progresses, your plant will likely die off completely. While replanting broccoli will allow you to produce a second crop in the fall, it needs to be started early in the spring.  

Cauliflower

Start cauliflower no later than six weeks before the last expected frost. You can sow the seeds in individual pots, with four seeds per pot. 

Peas

Peas take about sixty days to produce pods, and they need to do this before it gets too hot. You can pre-sprout seeds indoors and then plant them outside as soon as the danger of frost has passed, which will allow them to thrive in the cool temperatures. 

Brussels Sprouts

In colder climates, Brussels sprouts can be started indoors for an early harvest (before the weather gets too hot). You can also direct sow seeds outside for a later harvest as well.

Cabbage

In general, cabbage seeds should be sown indoors about five to eight weeks before the last expected frost date. 

Eggplant

Eggplant is challenging to grow, but sowing your seeds indoors can help make it a little easier. The plants require ample sunshine and moist soil, conditions that are easier to provide when you are starting your seeds indoors. To encourage germination of your eggplant seeds, you will also need to soak the seeds in water for about 24 hours before you sow them. 

Peppers

Peppers can be started as many as eight weeks before the date of the last expected frost. They need a warm location (think 75 degrees or more) in order to grow successfully. 

Tomatoes

Tomatoes are one of the most popular plants to grow indoors, as they need plenty of heat to get going. Start them about eight weeks before the last expected date of frost and make sure you transplant them into new containers once they are about an inch tall.

Worst Plants for Indoor Growing

While most plants can be successfully started as seeds indoors, there are some that simply don’t hold up to the challenges presented by the transplanting process. The following plants should be started as seeds directly in your garden, and grown indoors only when you will be keeping them in containers permanently.

Beans

Beans do not transplant well and should be sown directly in the garden. When you plant beans in the garden, make sure you soak the seeds in warm water before planting. 

Corn

Corn is successfully started indoors by many gardeners, but most would advocate against it. Corn plants have more delicate root systems, and because you typically grow sweet corn in high volume (each plant will only yield a few ears), it might not be worth your time to experiment with indoor seeding. Instead, wait to direct sow. 

Carrots and other root vegetables

Under no circumstances should you try to start root vegetables indoors. Because these plants rely so heavily on their root systems, they will suffer and likely die if you transplant them outdoors. In addition to carrots, avoid beets, parsnips, potatoes, and radishes. 

Onions

Onions simply aren’t ideal crops for starting from seed, because they take too long to mature (up to five months, in many cases). You’re better off direct sowing them, but starting from sets instead of seed.

There are other plants that don’t necessarily fall into a “worst” or “best” category but keep in mind that you will need to transplant them with caution, as they have delicate root systems.

These include:

  • Celery
  • Cucumbers
  • Squashes
  • Pumpkins 
  • Peas 
  • Melons 
  • Chard 

If you’re looking for a plant that isn’t on either of our lists, you can follow this general rule of thumb. Typically, plants that are grown in rows are those that are best when direct-sown. This includes crops like carrots and corn, of course. You can sometimes start these plants indoors early if you have a very short growing season, but in our neck of the woods, waiting until the ground has warmed will provide plenty of time. 

On the other hand, plants that have a long time between seed and harvest are best started inside. Generally, think tomatoes, peppers, eggplant, and melons. Keep in mind that if you live in an area with a long growing season, you can start your seeds outdoors and don’t really need to worry about starting seeds indoors unless you experience some kind of a frost. 

How to Create Nutrient-Dense Soil for Indoor Growing

From their first day of growth, your seedlings will face challenges from the environment. Starting them off on the right foot by providing nutrient-dense soil can help them develop an immunity to these challenges and make them stronger as they grow. 

You can purchase a good seed-starting mix, one that will be formulated to discourage soilborne illnesses and to improve drainage and oxygen flow, from your local farm and garden store. A good organic seed starter, like this one manufactured by Espoma, will provide your plants with the structure and nutrients they need to get started.  

Alternatively, you can make your own. Simply mix together equal parts of degraded compost and peat moss to create a nutrient-dense brew. You can also use small amounts of vermicompost (worm compost) or worm tea, but this is often more efficient when applied to your seedlings after they have already germinated and started to grow. 

You might also consider adding vermiculite to your compost mix. Vermiculite is made out of naturally occurring minerals and helps your soil to better absorb and retain moisture. Many premade seed starting mixes already contain vermiculite or perlite, a similar additive.

Ideal Containers for Starting Seeds

If budget is a major concern when you’re starting your seeds, don’t panic just yet. There are several ways you can start seeds indoors, and homemade containers are easy to come by without requiring you to part with some serious coin. Consider these popular seed starting containers:

  • Toilet paper tubes: Cardboard tubes are found in most homes and can be used to make environmentally-friendly seed starting containers. These decompose quickly in the ground, and while they tend to rip easily when wet, they are a great alternative to store bought containers if you’re looking for a budget-friendly solution.
  • Homemade paper pots: You can fashion your own paper pots out of newspaper. This can be time-consuming if you want to make dozens of seed starting containers, but if you’re only making a few, newspaper is a great biodegradable option for starting your seeds. 
  • Egg cartons: Egg cartons are typically available in most households, making them easy to come by. These can limit you in terms of size, and you will have to work to maintain level moisture levels. They are also fragile when wet, meaning you won’t want to move them. 
  • Peat pots: Peat pots, like the ones made by Jolly Grow(SSP1), are a commercially available option that, while inexpensive, can add some expense to the cost of your planting. Since you can’t reuse these from year to year, as you can with other types of store bought pots, you may hesitate to purchase them. However, they are environmentally friendly and can be planted directly in the ground, making transplanting a little bit easier. If you use peat pots to start your seeds, remember that they tend to wick moisture from the soil, so you will need to water more frequently. 

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  • Plastic pots: Plastic seed-starting trays, like these 128-cell ones, (SSP2) are a more popular type of seed starting containers. These containers come pre-drilled to allow for water to drain. If you are using recycled cups or containers (or those not designed for plants), you should always poke holes in the bottom of the cups so you don’t have to worry about them becoming too waterlogged. While they have a higher upfront cost, you can reuse these between seasons as long as you handle them carefully. 
  • Soil blocks: Soil blocks are blocks made out of compressed potting soil, serving as both a container as well as a sowing tool. While soil blocks eliminate the need for a separate container, they can be a bit messy. However, they remove the likelihood of root compaction and are easy to use once you figure out all their intricacies. 
  • Mini greenhouse trays: You can purchase pre-made mini greenhouse kits that are comprised of black plastic trays with drainage grooves and a clear plastic dome. These units hold moisture nicely, but you have to make sure you remove the cover after the seeds sprout to prevent fungal growth. You can also DIY your own mini greenhouse trays by flipping plastic totes or food storage containers over your started seeds. 
  • Self-watering trays: These seed starting containers are ideal for the gardener with a less-than-green thumb. Particularly if you forget to water your plants, these are a good option. Self-watering trays consist of pots that rest on fiber pads. The pad wicks water from a reservoir, and while you must refill the reservoir from time to time, you don’t have to worry about your plants drying out, or about fungal growth, or overwatering. 
  • Other recycled materials: There are countless other options for DIY seed starting containers. Scour your house and rescue materials like milk cartons, plastic storage containers, cardboard boxes, yogurt cups, citrus peels, egg shells, and ice cube trays before they can hit the trash. While all of these offer their own disadvantages and benefits, they can offer a creative way to save a bit of money. 

No matter what kind of seed starting containers you use, make sure they are clean. Diseases and pests can be spread through used containers, and while these usually die off between growing seasons, this isn’t always the case. Ideally, start with fresh containers each season, but if you can’t, make sure you sterilize them, even if it’s just with a little bit of soap and water. 

In addition, if you are using a repurposed container or one that is not automatically porous, you will need to poke holes in the bottom for drainage. Swampy containers will lead to rotted roots and poor germination, something you definitely don’t want to deal with when starting seeds indoors. 

Location, Location, Location

Set-up is fairly straightforward when starting your seeds indoors, but location is so vital when it comes to producing healthy seedlings. 

For starters, you need to make sure your seeds have plenty of access to overhead light. While placing your seeds near a window is a start, window light is often weak and directional, particularly in the winter or early spring. Grow lights and overhead lights can help remedy some of these concerns.

Temperature is also key. Seeds germinate best at warmer temperatures. If you don’t have a room that is warm enough, consider planting your seeds and placing them in a location where there is natural bottom heat, like on top of the refrigerator. 

Room temperatures above 75 degrees for most of the day are best for encouraging germination, with a natural fluctuation between daytime and nighttime temperatures being ideal. After your seeds have germinated, they will need light for at least six hours a day – but twelve to fourteen is ideal. 

Keep in mind that you will also need to provide a steady flow of air to prevent your plants from becoming dried out. Outdoors, seedlings have access to a fresh breeze at all times. Inside, the air may become stagnant, increasing the likelihood of fungal issues and stunted growth. Avoid placing your plant near a drafty area or vent, where they will be subject to temperature fluctuations, and consider installing a fan near the seedlings so that they receive plenty of fresh air at all times. 

Supplemental Heat and Lighting

While grow lights aren’t necessary, many indoor seed starters use them as a way to improve germination and help their plants grow more fruitfully. Grow lights can be helpful if you are planting your seeds in late winter, when daylight hours are limited. Most plants need a minimum of six hours of direct sunlight, so if you are sowing earlier in the season, you may want to add a grow light to prevent your seedlings from under maturing. 

Grow lights can also help prevent your plants from becoming leggy. A leggy plant develops as it reaches toward the light, searching for extra sunlight to help fuel photosynthesis. If you have your plants even a small distance from the closest source of light, they can become bendy and deformed as a result. 

If you don’t use a grow light, make sure you rotate the containers every now and then to help them grow more uniformly. If you are using a grow light, raise it a few inches above the tallest seedling every few days to prevent your plants from becoming dried out.

Many growers use electric heat mats to help speed up germination. Because indoor soil temperatures, as well as temperatures in containers in general, tend to be a lot lower than outdoor soil temperatures (particularly once spring has arrived in full force), heat mats can help improve your success with indoor growing. These mats can reduce your germination time by several weeks in some cases.

For best results, you will want to use the heat mat only during the day and turn it off during the nighttime hours. This will simulate the natural environment in which the soil is cooler at night time. As soon as your seeds germinate, you should remove the heat mat and replace it with grow lights or some other supplementary source of heat (if necessary). 

Keep in mind that heat mats can be tough to get used to, and you don’t want to DIY your own heating mats with home heating pads (like the one you put on a sore knee or back). Regular heating pads reach higher temperatures than the ones you would use for seeds, running the risk of overcooking your plants or starting a fire. 

When to Start Your Seeds

Before you begin planting your garden, you will need to know the frost dates in your region. This will allow you to create a timeline for planting, transplanting, and finally, harvesting. Note the first and last frost dates in your area, as a frost has the potential to damage your newly established plants. 

Frost dates sound complicated and scientific, but really, a frost date is simply the day on which there is a fifty-percent chance of being frost-free. Ideally, you want to make sure the likelihood of a frost is less than fifty percent. For ultimate safety, consider adding two weeks on top of this date to make sure your plantings are protected.

There are several sources you can turn to in figuring out the most accurate frost dates. While your local gardening center or cooperative center will surely have this information, you can also determine your frost date by checking out a website like the Old Farmer’s Almanac or the National Gardening Association. These websites allow you to plug in your zip code to determine an approximate frost date for your area. 

You can also refer to your plant hardiness zone to help you figure out when to sow seeds. The Plant Hardiness Zone Map includes an accurate geographical map of the country, with each region broken down into zones based on climate and topography. Zones are commonly referred to when you are trying to figure out what plants will survive in your region. 

Paying attention to zones helps you realize that you can’t plant a cactus outdoors in Vermont, for example. While that example may be extreme, some less common plants may be more ambiguous in their growing requirements, which is why looking at a zone map can be helpful in planning out your garden. 

Zone maps, particularly those that are updated to reflect microclimates and other unique geographical features, are extremely helpful in figuring out first and last frost dates. These include multiple zones, many of which are broken down into “a” and “b” (northern and southern) subsets for added precision. Finally, the National Oceanic and Atmospheric Administration’s website also includes a list of freeze and frost data. These charts are broken down by probability as well as different temperature thresholds, making it easier to determine the best time to start your seeds. 

Purchasing Seeds


First, you need to consider where you will purchase your seeds. Request seed catalogs from as many different companies as possible, and comparison shop. This will allow you to find the best prices on seeds, as well as to determine which varieties will be suited to your growing zone and particular preferences. 

Once you have the list of options in front of you, begin to make a list of what you’d like to grow. Ideally, do this with a calendar handy. This will allow you to figure out a potential timeline for starting your seeds indoors. Generally, you will want to imagine your garden as one-quarter of the size that it actually is to help you conserve space. By mentally visualizing the amount of available space as being much smaller than it is, you will in turn purchase and cultivate fewer seeds, preventing overplanting later on which causes congestion in the garden.

When you’re purchasing seeds, prepare for losses and order a few extra packets of the seeds you select. Some seeds simply won’t germinate, while others may not produce a strong crop. Having some extra seeds on hand will give you some added insurance if this is the case. 

You might also consider working with your friends and neighbors to share seeds, tips, and catalogs. Many companies offer discounts when purchasing in bulk, so it might be worth your time and money to work with other indoor gardeners to save a bit of cash. You can also share seeds if you have leftovers from the prior season, making buying extra seeds a more worthwhile investment.


Planting Your Indoor Seeds

Most vegetables should be sown indoors around six weeks before the last expected date of frost, although this will vary depending on where you live. While it may be tempting to get a jump start by starting your seeds earlier than necessary, this can be detrimental as it will cause your plants to become rootbound and they will decline before being transplanted outside. 

When you’re ready to plant, begin by filling clean containers with potting mix. Try to use potting mix designed specifically for seedlings, as this will increase your germination success. Mix in a bit of peat, vermiculite, and perlite if your mix doesn’t already contain them to help retain water and allow for good oxygen flow. 

Then, insert your seeds as indicated by the seed packet. In some cases, you may need to soak, chill, or scratch seeds before planting. In general, all you need to do is gently press each seed into the potting mixture, placing two to three seeds per container. To help push the seeds into the soil without compacting it, you can use a small pointer, like the end of a pencil eraser. Make sure each seed is covered with a thin layer of soil. 

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Cover each container with a plastic lid or a thin layer of plastic wrap to prevent them from drying out. This will produce a pseudo-greenhouse effect. You can poke a few holes in the plastic for ventilation, just make sure you still take the plastic off once your seedlings have emerged to prevent fungal growth. 

Water your seeds carefully, using a light mist. This can take a bit of time, but it’s well worth it, as a pitcher can cause the water to compress the soil and scatter your seeds before they even get a chance to develop. Instead, use a spray bottle or even a turkey baster to dispense the water evenly and thoroughly. 

After you have planted, make sure you label each container carefully. This will eliminate a great deal of confusion later on, when you’re unsure of whether it’s a tomato plant or a pepper plant you’re looking at. You can use commercial plant labels or even make your own out of repurposed corks, Popsicle sticks, or cardboard. 

Cultivating Your Indoor Plants 

Water every few days while your seedlings are becoming established. There is no hard-and-fast rule on how often you should water your plants, as this will be determined by a range of factors such as the lighting, temperature, and humidity of the room they are in. However, you should make sure that the soil always stays somewhat moist–not waterlogged, and not dried out. 

Once your seedlings have emerged, you can remove any plastic that is covering them and prepare them for repotting. They will likely outgrow their first home within a few weeks of emerging, so you’ll need to put them in larger pots before transplanting. After transplanting them, keep them out of direct sunlight for a few days so they can adjust to their new homes. 

After your seedlings have grown to about an inch in height, consider adding an organic fertilizer to further spur their growth. A little goes a long way–you only need a pinch applied around the base of the plant. Worm tea (or compost tea, alternatively) also works well to help provide your plants with a dose of nutrients.

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Keep in mind that indoor seedlings face challenges when it comes to the lack of beneficial microbes in the soil, so you may need to provide a bit more fertilizer to encourage healthy growth. 

This is where compost, or other organically-based fertilizers, are so effective. While commercial fertilizers typically only include equal parts of nitrogen, potassium, and phosphorous, organic compost contains those major nutrients as well as several critical micronutrients and helpful microorganisms. These microorganisms will ensure that your soil stays healthy and active. 

Transplanting Your Seedlings

While you may be itching with excitement to get your seedlings transplanted outside as soon as the warm weather first appears, you shouldn’t rush to do so, even if the danger of frost has passed. This is one of the biggest gardening mistakes you can make. If you put your plants immediately in the ground without providing them with time to transition to the outdoor setting, you risk shocking them and killing them off altogether.

During the last week of indoor growth, withhold fertilizer and add water less frequently. About a week before you plan to transplant, put the seedlings outdoors in light shade for a few hours each day. Start off small, perhaps beginning with only an hour or two outside, before increasing their exposure to a full day’s worth of time outdoors. You can gradually increase the intensity of their exposure to conditions like the sunlight and wind, too. 

Throughout this transitional period, you will need to bring your plants indoors at night and continue to water them as normal. Dry air results in rapid water loss, meaning you may need to water even more frequently.

When you’re ready to transplant, try to do so on a calm, overcast day. If this isn’t possible, transplant in the early morning. This will help reduce your plants’ exposure to the harsh sun rays, which can still shock them even if they were hardened off correctly.

To get started, set your transplants into loose, aerated soil. Insert your seedlings, planting according to the specifications for each individual vegetable. Soak the soil around your seedlings right after you insert and pack them in, and then spread some mulch to help reduce moisture loss. 


Ready to get started? Even if spring is still a few months away, you can start today by ordering your seeds and starting them indoors. Happy planting!

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