Your August and September Sowing Calendar: What to Plant and When

The difference between a well-timed garden and a late one is usually two or three weeks. Seeds sown at the right moment establish before the weather turns; seeds sown even a little too late spend their energy surviving rather than growing.

Here is a practical sowing calendar for the next six weeks — what to start, when to start it, and what to expect.

Now Through August 31

The soil is still warm. This warmth is an asset for certain seeds and a liability for others. Use it deliberately.

Start Indoors (for October outdoor planting)

• Viola / Pansy — sow now indoors; they need 10–14 days to germinate and 4–6 weeks to develop into transplant-ready seedlings. The timing puts them in the ground right as autumn temperatures arrive. Expect flowers from October through the first hard frost
• Thyme — slower to germinate (14–21 days) but worth starting now. Seedlings established in October will overwinter and come back as robust perennials in spring

Direct Sow Outdoors (if you have an outdoor space)

• Catnip — direct sow into a warm garden bed or large outdoor container now; it will establish before cold and return next year as a full perennial

Continue Warm-Season Growing (for September harvest)

• Coleus — if you want a fresh indoor planting with autumn color, sow seeds indoors now. The plants will develop their characteristic foliage within 4–6 weeks and will look their best in the cooler, lower-light conditions of late autumn
• Basil — if you have seeds and a warm windowsill, a last indoor sowing now gives you 6–8 weeks of harvest before the cold affects flavor

September 1 — September 14

The transition window. Outdoor daytime temperatures are beginning to moderate; nights are noticeably cooler. This is prime time for cool-season establishment.

• Viola / Pansy — if you missed the August window, early September starts are still viable for November flowering; expect 2–3 fewer weeks of bloom before frost
• Thyme — still time to start from seed indoors for an October transplant; alternatively, take a cutting from an existing plant and propagate in a small pot
• Catnip — last window for direct outdoor sowing. After mid-September, the plants will not have enough growing time to establish before the first frost

September 15 — September 30

By mid-September, the sowing window for most annuals has closed. The focus shifts to what is already in the ground.

• Harvest and dry herbs — thyme, catnip, and any herbs approaching the end of their outdoor season should be harvested now. Cut them back by one-third, dry in small bundles, and store for winter use
• Pot up outdoor plants for indoor overwintering — catnip and thyme that have been growing outdoors can be brought inside in smaller pots before the first frost. They will continue growing slowly through winter and return to full vigor in spring
• Start planning for spring — what worked this season? What did not? Note what you want to grow differently next year while the details are fresh

The One Thing to Do This Week

If you have not already: sow one pot of viola seeds indoors today. Put them somewhere warm (65–70°F), cover lightly, and check daily. In two weeks you will have seedlings; in six weeks you will have plants ready for their autumn home — a window box, a balcony planter, or a single pot on a front step.

Viola is the most reliable cool-season payoff in container gardening. The flowers last until frost, they come in colors that look extraordinary in autumn light, and the whole thing starts with a seed and a pot of soil this week.

The calendar has an end, but the planning for next season begins now.