11 September Garden Tasks

There’s just something about the end of summer that feels fresh and new, even though the growing season is drawing to a close. September is a great time to get outside and take care of some garden tasks that will cut down on work in winter and spring. 

September brings the beginning of fall to cool climates and even brings tolerable temperatures to warm climates. Here in Zone 9, I rarely work in my garden from June to August; it’s just too hot! But these -ember months feel like glorious relief.

If you’re feeling like going outside and getting your hands dirty now that the weather is beautiful again, you’re not alone. Many gardeners rejoice for the incoming cooler weather, and many plants do, too! Let’s discuss some tasks you can carry out in the garden in September to clean things up and set the stage for amazing autumn bloomers.   

Deadhead Summer Bloomers

Generally, you shouldn’t prune perennials in late summer and early fall. Pruning encourages a flush of growth that will be more susceptible to cold damage. For most summer blooming plants, all they need after blooming is a deadheading session. 

Deadheading is one of the most underrated gardening habits, in my opinion. It can be tedious, so I understand the lack of desire to carry out this task during the summer heat. 

A key to knowing which plants are important to deadhead at the end of summer is understanding their blooming habits. Some plants—shrubs, in particular—bloom on old growth. Old growth is that which begins to form immediately after it stops blooming. 

You should deadhead plants that bloom on old wood right after they finish blooming. For many plants, that time falls in or around September. Some hydrangeas, gardenias, many roses, and a handful of others fit in this category. 

While these are a must, you really can deadhead any summer bloomers as their flowering season ends. For those that bloom on new growth, you’ll want to do any major pruning in the spring. A light cleanup is good for everyone, though. 

Collect and Store Seeds

In most climates, this is the time when many annual flowers go to seed. Collecting and storing seeds is the perfect September task to help stretch your gardening budget next season. If you like growing sunflowers and have never harvested the seeds, you’re in for a real treat. 

You’ll be able to tell when your seeds are ripe when the pods begin to split. This looks different depending on the plant, as not all seed pods look the same. This is the time when it is easiest to separate the seeds from the pods. 

When it comes to storing seeds for the winter, it’s important to know which seeds need cold stratification. This means that they need a period of cold weather to initiate germination. Most flowering annuals don’t need this, but a few do. Nigella is one. Also, poppies, larkspur, bells of Ireland, and scabiosa, to name a few. 

Always allow your seeds to dry completely in a cool, dry, dark location before storing. Some seeds need moisture or warmth to germinate, and some require light. You don’t want your seeds to sprout in storage. Make sure they are dry before placing them in paper bags or envelopes for storage. 

Plant Trees and Shrubs

September and October are prime times for planting many trees and shrubs. By now, the ground is still warm, and it’s staying moist longer, but the harsh heat of summer has mellowed. Research shows that cooler weather makes planting a less stressful transition for most species. 

Aim to plant about six weeks ahead of your first freeze date. In cool climates, get this done as soon as possible. You want your trees and shrubs to establish roots before the ground freezes. In warmer climates, you’ve still got a little time, but you can start now if you please. 

The autumnal equinox falls on September 22nd, meaning that the days and nights are as close to equal as they will be for another six months. If you wait too much longer, the shortening days will begin to signal dormancy for many plants.

Cut or Pull Spent Annuals

This time of year, my annuals are looking pretty crunchy. After collecting seeds, it’s time to decide what to do with those beds that will need more attention in the spring. There is much debate surrounding this topic, so let’s explore it. 

If you prefer a neater appearance through the winter, you’ll probably want to clean up those dry, spent annuals right about now. If you don’t mind a bit of a wild look in the winter, there is an argument for leaving things alone, too. 

Leaving your spent annuals over the winter means a better chance of re-seeding. Leftover seeds also feed overwintering birds and other critters. It acts like natural mulch, and those dead plants will turn into compost for the garden. 

If you fall into the former group, you also have more than one option. If you simply want to clean up the surface, you can leave the roots in place. The roots will help to improve your soil and prevent erosion over the cool season. 

A pair of hand shears are your best friend for this task. Simply snip all of your plants at ground level and dispose of the dead foliage. You can also pull them out entirely, but it doesn’t benefit your soil in the same manner. 

Add Compost

September marks the beginning of the fall growing season in most climates. Between plantings is the perfect time to spread some of that magical compost you’ve been working on all year. 

Compost helps to rejuvenate your beds after the heavy feeders of summer have depleted the nutrients. Compost is a natural fertilizer, but it takes time to break down. It’s a great way to cut down on your fertilizer bill and chemical fertilizers in general. By spring, your compost will be feeding that soil nicely.

Compost also helps to loosen up and aerate the soil. If you have clay-heavy soil, it can compact over time, and mixing in some compost will do a world of good. In early fall, the soil will hold more moisture, making it easier to work with, which helps this task along. 

Mulch

After you incorporate that compost into your beds, cover it up with a nice, thick layer of mulch. Mulch does great things for the soil while it protects and insulates the ground. 

By tackling this garden task in September, your soil will hold warmth for longer, which is easier on the insects and microbes that contribute to a healthy environment. It also makes those cold winter nights a little less severe for some of your perennials. 

Avoid additional mulch in beds where you want to encourage self-sowing. The mulch could weigh things down and prevent sunlight from reaching seeds that need it to germinate. 

For plants that are only hardy as far north as your present zone, mulch is essential in those beds. For instance, I grow dahlias and ginger, which are both hardy to zone 8. I’m in zone 9a, so I make sure to mulch over those to keep them toasty. As a result, they come back stronger in the spring.

Mulch also breaks down over time. It helps to acidify the soil, based on the type you use (pine bark mulch is great for acid-loving plants). It adds organic material into the soil, which breaks down into valuable nutrients.

Prep Beds for Fall Bulb Planting

If you’re planning to plant your spring blooming bulbs this fall, it might not be quite time to put them in the ground. However, September is a great time to get outside and take care of the preparation tasks for those garden beds. 

Remove any weeds that have grown in these areas over the summer. Then loosen the soil about eight to twelve inches down. Bulbs like loose soil because it helps them to develop roots easily. 

Now is the perfect time to amend the soil with compost, worm castings, and other organic fertilizers. A sprinkling of bone meal is a great additive for the soil where you plan to put bulbs in the ground in the fall. 

Divide Perennials

You don’t have to divide all perennial plants, but it does benefit many of them to do so every two or three years. September is a great time to take care of this because the divisions will have ample time to settle back in before the cold comes through. 

Plants may start to look crowded or thin out in places over time. If you have clumping perennials that have begun to look like they need some help, dividing them can help. It also leaves you with more plants to spread around the garden, so it’s a double win!

Dividing your plants helps them in several ways. It reduces competition for nutrients, allowing the divisions to become more robust as individuals. It also stimulates new growth and promotes better blooming. 

Do this task on a cool, overcast day, and water your plants before you dig them up. This will reduce the amount of stress they have to recover from. Don’t divide while a plant is blooming! Wait until it can apply that energy to forming new roots. 

Begin to Transition Potted Plants Indoors

In areas with long, warm summers, some September garden tasks seem too early. But in cooler areas, these preliminary steps are essential. If you have plants in containers that won’t survive the winter in your climate, start planning to bring them indoors.

For many tropical plants, 40°F (4°C) is the minimum you want to expose them to. I mean the bare minimum. If your weather is anywhere close to this point in September, it’s time to transition things indoors. 

Before you bring anything inside, thoroughly inspect it for pests and diseases. I inadvertently brought spider mites into my house two years ago, and they seem like a pest that never quite goes away. Diseases can also negatively impact your houseplants.

Trim off any dead and damaged foliage, and clean them up by rinsing the plant with clean water. This is a great time to do some re-potting, as pests and diseases can lurk in potting soil.

Gradually acclimate plants to the indoors by moving them closer to the house. It’s a good idea to bring things indoors when the temperature outdoors is similar to that indoors to make adjustment easy. 

Harvest and Dry Herbs

During spring and summer, it’s easy to take fresh herbs for granted. Many herbs are cold tolerant, and some are even evergreen, but their growth slows for the winter. By September, many garden herbs should be robust and ready to harvest, making this task extra fragrant and enjoyable. 

Parsley, sage, and dill are some of the herbs that you can harvest in September. Basil, also, as it is not cold tolerant. If you don’t intend to bring it indoors, now is the time to harvest it. 

Dry your herbs in bundles held together with bands or strings. Hang them upside down in a cool, dry spot, out of direct sunlight. Hanging them upside down allows the oils to flow from the stems into the leaves, making them more flavorful. 

Allow your herbs to dry completely before storing them. If there is any residual moisture, you could end up with moldy oregano. Store fully dried herbs in airtight containers away from light and heat. Storing them in the freezer will make them last longer. 

Tidy Up Damaged Plants

Finally, take stock of everything in your garden, and do a little light cleanup of damaged plants. Remove any dead branches or foliage that have been affected by pests or diseases. 

Removing dead or damaged branches and foliage does more than just make things look nicer. It also tells the plant to redirect energy away from that part and invest that energy into new growth or root development. 

Don’t do any major pruning, as this encourages growth. Fall is not the ideal time for most plants to produce new growth. Newer growth is tender and doesn’t withstand cold weather as well as mature growth.