Starting Vegetable Seeds Indoors: I. Planning
Wesley Crump, Taun Beddes, Michael Caron, and Milena Oliveira
Introduction
For many Utah gardeners, starting vegetable seeds indoors offers both an exciting opportunity to experiment with new varieties and a practical necessity for ensuring the successful growth and production of certain crops in their unique environmental conditions. Vegetable gardening involves inherent risks, particularly for the plants, with weather being a significant and largely uncontrollable factor. Because of this, many gardeners are unsure about when to start certain crops and whether the crops should be started indoors or sown directly in the garden. This fact sheet, the first in a series of four, discusses planning considerations when starting seeds indoors. Topics include:
• Considering climatic information, including average first and last freeze dates, as well as the length of the growing season.
• Timing indoor planting so that transplants are ready at the appropriate time for planting outdoors.
• Deciding whether or not you should start seeds indoors. Many crops can be either started indoors or directly sown outdoors but some are not adapted to being grown as transplants, such as carrots and peas.
• Planning what to plant. This may not be important for casual gardeners who just want a few plants. However, planning and recordkeeping are necessary if you garden to preserve food or sell to the public.
Climate
When planning what vegetable seeds to purchase and grow in your garden, several climatic considerations are important, such as the average freeze dates (spring and fall) for your area, which dictate the average growing season length. When growing perennial vegetables such as asparagus, rhubarb, and artichokes or overwintering vegetables, another important climatic consideration is your area's hardiness zone. Average freeze dates and growing season length for many Utah locations are shown in Utah Weather Stations and Table 1 (available for download). Utah State University
(USU) uses weather stations across the state of Utah, with some having collected data on freeze dates for over 100 years (for more information, visit the Utah Climate Center).
USDA Hardiness Zones
The U.S. Department of Agriculture (USDA) has developed cold hardiness zones for the United States, varying from 1 (arctic) to 13 (tropical). These zones are based on a rolling average low temperature over the last several years. Hardiness zones help match plants with your region’s climate. For instance, a perennial vegetable that thrives in zones 4–9 can tolerate a broad spectrum of temperatures, whereas one adapted to zones 7–10 might not endure the colder winters of zone 5. The Wasatch Front is in Zones 6–7, with low temperatures averaging from -10 °F to 10 °F. Populated mountain valleys range from Zones 4–6, with average lows ranging from -30 °F through 0 °F Hardiness zones are more critical for perennials than annual vegetables because hardiness zones indicate the ability of the plant root system to survive and thrive through the winter and into the next growing season.
A Note on Averages
Each year has unique climatic characteristics that are out of your control as a gardener. The best you can do is look at historical weather patterns and base the current year on that information. When plotted together, years of data for a given measure (say, average freeze-free days) will often form a bell-shaped curve (Figure 1). The average number of freeze-free days lies in the middle of the bell curve. Because these are averages, in any given year, including this year, the number of freeze-free days could be greater or less (sometimes quite a bit) than the average number of freeze-free days reported. Additionally, because weather stations used to compute these statistics do not perfectly reflect your garden's climate (they may be in a neighboring city or area), consider keeping personal observations, as these will be the most relevant to your garden.
Figure 1. Bell Curve Showing the Average Freeze-Free Days From Weather Stations Throughout the State
Vegetable Climatic Considerations When Starting Seeds
When planning for your vegetable garden, there are several temperature-related considerations to determine what vegetables are more likely to survive and thrive in your garden. Some of these include the vegetable's cold tolerance, days to maturity, and whether you will direct sow your seeds or use transplants. Understanding these factors for the vegetables you want to plant will help you prepare to start your seeds. The planning process will clarify which seeds benefit from being started early indoors and which should not. Other general considerations, such as water and nutrient requirements for various crops, are not discussed here but can be found by searching for the individual crop on the USU Extension Yard and Garden website.
Cold Tolerance
In this fact sheet, vegetables are divided into four categories of cold tolerance:
1. Hardy plants can tolerate brief periods below 32 °F or extended periods at or near 32 °F.
2. Semi-hardy plants can tolerate some brief time at or near 32 °F
3. Tender plants may be significantly damaged or killed at or near 32 °F
4. Very tender plants may be stunted, damaged, or killed at temperatures between 50 °F–32 °F.
Many factors, including cultivar, growth stage, acclimation, and soil temperature influence a crop's cold tolerance (Figure 2). Table 2 (available for download) gives approximate cold tolerance groups for many vegetables commonly grown in Utah and information regarding sowing and transplanting.
Figure 2. Freeze Damage on a Protected Tomato (left) and Undamaged Lettuce (right) After a Night Briefly Below Freezing
Notes. Freezing temperatures can induce the formation of ice crystals within the cell walls of tomato tissues, leading to rupturing cell membranes. This rupture results in leaking cellular contents and a subsequent loss of cell integrity. As a consequence, affected tomatoes may exhibit discoloration, often manifesting as dark brown or black lesions on the fruit and stems due to tissue necrosis. In contrast, lettuce demonstrates a greater cellular tolerance to low temperatures. The resilience of lettuce is attributed to its ability to activate protective stress responses, such as accumulating compatible solutes, which play a critical role in stabilizing cellular structures and mitigating damage from freezing temperatures.
Days to Maturity
Days to maturity (DTM) is an approximate measure of time from outside planting, whether from seed or transplant, until harvest begins for a crop. DTM is measured using the most common method of growing a specific crop (direct sowing vs transplanting). The seed packet should list the most common method of growing, or you can refer to Table 2. DTM is often given on the seed packet or can be found online via various seed companies (note that the same cultivar may show slightly different DTM depending on the seed company). Be sure you are looking up the exact cultivar or variety (such as ‘Celebrity’ tomato versus ‘Brandywine’ tomato) you plan on planting. Ripening times vary significantly within certain crops, including tomatoes and sweet corn, by as much as 45+ days. DTM can be made even more ambiguous by variable harvesting stages, as with many greens (e.g., baby kale vs regular kale). Sometimes, seed packets list different DTM for different harvesting stages. Many other factors may affect ripening time, including irrigation patterns, air temperatures, soil temperatures, fertilization rates, and soil type.
Direct Sowing vs. Transplanting
For several reasons, it may be better to directly sow seeds or transplant. The biology of the crop may dictate this, as is the case with many root crops that do not do well if grown in trays and transplanted. More relevant in this discussion is the given crop's climatic requirements. For example, many of the crops designated as "hardy" or "semi-hardy" in this fact sheet will exhibit adverse reactions to temperatures above 75 °F to 85 °F (e.g., broccoli, cabbage, carrots, spinach, etc.). While these crops can and often are directly sown into the garden, in areas with a quick transition from spring to summer temperatures, starting some crops indoors and transplanting them outside may increase the window of appropriate weather for them to thrive. Conversely, transplanting tender plants outdoors after warm temperatures in a short growing season is often necessary to ensure a timely harvest (e.g., tomatoes, peppers, cucumbers, etc.). For some crops, sowing seed in mid to late summer will yield a fall harvest. Many of these crops are noted in the information from Table 2
Putting It All Together
Climate + Vegetable climatic considerations = Successful seed starting
Let's combine all this information! By better understanding your area's climate and the climatic requirements of the vegetables you wish to grow, you can create a plan for starting your seeds For those just growing a handful of plants, it may be simple enough to keep track of the considerations discussed here without writing them down. However, for larger gardens and more varieties, consider making a planning spreadsheet, as shown in Table 3 (available for download), to organize and plan your vegetable garden. This type of planning can also serve well for a flower garden.
For each crop (and cultivar/variety) you plan to grow, answer the following questions that correspond with columns on the planning spreadsheet (Table 3). Use the above figures and tables to aid in filling out the spreadsheet
1. What is the crop (or species) name?
2. What is the cultivar name?
3. Will you direct sow or transplant (start indoors)?
4. When is the average last freeze in your area? (Refer to Utah Weather Stations.)
5. How many weeks does it take to grow indoors before transplanting? For example, tomatoes usually take 6–8 weeks, while lettuce is shorter at 5–6 weeks. This information is often on seed packets. If not, check online seed companies or the USU Extension Yard and Garden website for general recommendations of specific crops.
6. When will you plant outside? Base this on the crop's cold tolerance group (refer to Table 2) and your average last spring freeze date (Step 4).
7. When should you start seeds indoors? Base this answer on the outside planting date (Step 6) and the weeks from sowing indoors until the transplant is ready (Step 5).
8. What is the average number of freeze-free days in your area? (Refer to Utah Weather Stations.)
9. What is the projected DTM for the crop? This information is usually printed on the seed packet. Consider using season-extension techniques if the DTM is longer than your average freeze-free days. See the USU Extension publication Extending the Gardening Season for information on season-extension techniques. If the average number of freeze-free days is significantly less than the expected DTM, consider growing a different cultivar or crop altogether.
10. Are there other essential descriptors, such as soil, watering, fertilization needs, you want to capture? If so, create additional columns for them. Table 3 includes examples such as seed vendor information, special instructions for growing that crop, and the number of plants you want to grow.
On the same or a separate spreadsheet, it is helpful for future years to track the progress of the crop to harvest and how you used the vegetables harvested. Suggestions include:
• Actual harvest dates over time
• Total yield of the crops grown.
• Diseases and pests that crops may have been susceptible to and how you managed them.
• Quantities of produce eaten, preserved, or sold as compared to what went unused
• Anecdotal observations of the quality traits of the produce (such as flavor, texture, color, shelf life, uniformity, shelf stability when picked fresh, how others like it, etc.).
Climate and Vegetable Seed Starting FAQs
Q. What's the difference between a frost and a freeze? Hard freeze vs light freeze?
A. A frost is when the dew point is below freezing (32 °F) and the air temperature drops below the dew point. Water then condenses from the air onto plants and other surfaces and quickly freezes, forming the visible, white, icy coating. A freeze is just when the temperature drops below freezing. For more information, see this publication from Michigan State University Extension
Meteorologists often define a "hard freeze" when the temperature drops below 28 °F and a "light freeze" when the temperature drops below 32 °F but stays above 28 °F.
Q. What about microclimates?
A. As mentioned, recommendations based on published temperatures will not perfectly suit your garden plot. Every area has subtle differences in landscape, elevation, and topography that contribute to its unique microclimate. Think of the climate differences between your home's north and south sides. The microclimates are different! It is essential to contextualize the information presented here in your microclimate(s).
Q. But my area is incredibly unique! How can I get help for my unique situation?
A. Unfortunately, the data collected and used for this publication cannot cover every area or situation. Collecting climate data for your location will be the most accurate. Consulting with your local Extension agent is a great way to start this process.
Q. What about season-extension techniques?
A. Any method or technique that manipulates your growing environment to "extend" the growing season can be considered a season-extension technique. These techniques range from inexpensive and straightforward, such as throwing a frost protection blanket over your plants if an expected freeze comes to expensive and complex, like a greenhouse. As mentioned, a season-extension technique will likely be necessary when any vegetable crop's expected DTM are significantly greater than the average number of freeze-free days in your area. See the USU Extension publication Extending the Garden Season for more information.
Q. Are "hardy" and "semi-hardy" crops the same as "cool-season" crops?
A. Yes and no. "Cool-season" crops generally refer to those crops that grow best in cooler temperatures. Cool-season crops generally prefer temperatures below about 80 °F. As used in this publication, a crop classified as "hardy" or "semi-hardy" refers to the crop's ability to deal with temperatures near freezing. Most cool-season crops are hardy or semi-hardy vegetables.
Q. I never follow published freeze dates for planting, and things usually work out fine. Is all this planning essential?
A. As mentioned, freeze dates and growing season length data are presented as averages, and each year will deviate from those averages. Similarly, there is variation in cold tolerance levels and DTM due to cultivar selection and other factors. Ideally, the information presented here will be used as a starting point. Knowing your garden plot and the cultivars you desire to grow will provide the most reliable information.
Photos
The authors contributed all photos.
References
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