For slow maturing plants and gardeners with short growing seasons, starting seeds indoors can be a crucial element to achieving success. To figure out the optimal time to start your seeds indoors, all you really need is a bit of backwards thinking.
Step 1: Getting Into Your Zone
Hardiness Zone maps are geographic divisions based on the average minimum temperatures of a particular region. For gardeners, knowing what zone you are in is key to knowing which types of plants will grow successfully in your area. Using hardiness zones to grow plants is not an absolute science, the maps are meant as a guide. Environmental stressors, cultivation techniques and artificial environments like buildings and elevated decks are all factors that play into whether a plant can be 'pushed' beyond its normal zone. To find your zone in North America, visit the National Arboretum's website at:
International gardeners can find maps of their zones here:
or here:
Step 2: Finding Your Last Frost Date
Now that you know your Hardiness Zone, you will need to find the average last expected frost date for your zone. If you live outside of North America, you can either compare your zone to the equivalent North American zone that matches your average lowest temperature, or check with your national climate data center.
Average Dates of Last Frost
(North American Zones)
|
| Zone 1: | June 1-June 30 |
| This zone remains vulnerable to frost 365 days per year. |
| Zone 2: | May 1-May 31 |
| Zone 3: | May 1-May 31 |
| Zone 4: | May 1- May 31 |
| Zone 5: | March 30-April 30 |
| Zone 6: | March 30-April 30 |
| Zone 7: | March 30-April 30 |
| Zone 8: | February 28-March 30 |
| Zone 9: | January 28-February 30 |
| Zone 10: | January 30 or before. |
| Zone 11: | Frost-free year round. |
As you can see, the above list illustrates a 4-week range of potential last frost dates for each zone. Many gardeners in lower zones like to use the earliest dates in the range. This is probably a good strategy if your growing season is on the shorter side. Higher zones may want to start seeds somewhere in the middle of the 4-week period. No matter what your zone, make sure you factor in an extra week for hardening seedlings off before moving them permanently outdoors.
To get the best picture of trends on your area, contact your county extension agency, or ask a local nursery. The best way to calculate the last average frost date for YOUR garden is to keep a journal and watch the trend over several seasons.
Step 3: Find the Number of Weeks Until Transplanting
The amount of time it takes from planting to maturity will vary greatly depending on the type of seed you are planting and what type of growing conditions you provide for it (soil, light, and water). New seed packages should tell you on the back the average number of weeks before the last frost they should be planted. Remember, the numbers given always assume that you will be providing optimal growing conditions. If your seeds didn't come information on how far in advance to sow them, use the chart at the end of this article as a guide:
Step Four: Think Backwards
Once you have your average last frost date and know how many weeks before it your seeds should be planted, simply find your last average frost date on calendar and count backwards the number of appropriate weeks and you'll have your date. Remember, when sowing seeds indoors, it's usually better to be too late, than too early. Start seeds too soon and you may end up with weak, spindly plants that can't hold up in the garden.
| Plant |
Number of Weeks Before Last Frost to Start Indoors |
| Artichoke | 10-12 |
| Ageratum | 8 |
| Alyssum | 8 |
| Aster | 6 |
| Balsam | 6 |
| Basil | 6-8 |
| Batchelor Button | 4-6 |
| Begonia | 12 or more |
| Broccoli | 8 |
| Browallia | 12 or more |
| Brussels Sprouts | 7-8 |
| Cabbage | 5-7 |
| Cauliflower | 8 |
| Catnip | 8-12 |
| Calendula | 6-8 |
| Cantaloupe | 3-4 |
| Celery | 7-12 |
| Celosia | 8 |
| Centurea | 8 |
| Chamomile | 8-12 |
| Chervil | 6-8 |
| Chives | 12-14 |
| Coleus | 8 |
| Collards | 5-7 |
| Columbine | 8 |
| Coriander | 6-8 |
| Cosmos | 4 |
| Cucumber | 4 |
| Dahlia | 8 |
| Daisy | 6-8 |
| Dianthus | 10 |
| Dill | 6-8 |
| Eggplant | 8 |
| Feverview | 8-12 |
| Fuchsia | 18-20 |
| Geranium | 12 or more |
| Impatiens | 10 |
| Kale | 4-6 |
| Lemon Balm | 6-8 |
| Larkspur | 12 or more |
| Leeks | 10-12 |
| Lettuce | 8 |
| Lobelia | 12-14 |
| Marigold | 6 |
| Mint | 12-14 |
| Muskmelon | 4 |
| Nasturtium | 4-6 |
| Nicotiana | 8 |
| Okra | 2-4 |
| Onion | 10-12 |
| Oregano | 12-14 |
| Pansy | 12 or more |
| Parsley | 12-14 |
| Peppers | 8 |
| Petunia | 10 |
| Phlox | 8 |
| Poppy | 12-14 |
| Portulaca | 10 |
| Pumpkin | 2-4 |
| Sage | 6-8 |
| Savory | 6-8 |
| Snapdragon | 10 |
| Spinach | 6-8 |
| Squash | 4 |
| Stock | 10 |
| Swiss Chard | 6-8 |
| Thyme | 8-12 |
| Tomato | 6 |
| Verbena | 10 |
| Vinca | 12 or more |
| Watermelon | 5-6 |
Tip: Don't discard your half-used seed packets from last year! Many of them will stay viable for several years if your keep them stored in a cool, dry place. Here is a brief list of the life expectancy of some common vegetable seeds.
| Plant |
Seed Life Exptectancy |
| Beans: | 3 years |
| Beets: | 4 y ears |
| Broccoli: | 3 years |
| Cabbage: | 4 years |
| Carrots: | 3 years |
| Cauliflower: | 4 years |
| Corn: | 2 years |
| Cucumber: | 5 years |
| Eggplant: | 4 years |
| Lettuce: | 4 years |
| Onions: | 1 year |
| Peas: | 3 years |
| Peppers: | 2 years |
| Radishes: | 5 years |
| Spinach: | 3 years |
| Squash: | 4 years |
About The Author: Ellen Brown is an environmental writer and photographer and the owner of Sustainable Media, an environmental media company that specializes in helping businesses and organizations promote eco-friendly products and services. Contact her on the web at http://www.sustainable-media.com
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