What to do in the garden in September

September is the start of autumn, and we can see the garden slowing down. Some herbaceous perennials and annuals have finished for the summer, others such as roses, sweet peas may keep on flowering. Exactly what is flowering and when depends on the sort of spring and summer we have enjoyed, but there is plenty of late colour in the garden.

Time to trim Lavender

By September Lavender has stopped flowering but the spent flowers remain on the plant. The advantage of pruning lavender in September is that it helps to keep it in a nice compact shape. If pruning is left until spring it runs the risk of cutting into the new growth . Lavender should be pruned lightly, trimmed really , into a tidy into compact shape for next year. It is best not to cut into the woody stems because they will not easily regenerate. Just a light prune.

Last chance to Prune Wisteria

Late August and early September is the last time to summer prune Wisteria. Click here for a video on how to prune wisteria.  This prune is to tie in any growth required for the framework and to prune out excessive growth, or long whippy shoots which are not needed or too long. The long shoots can be cut back to within half a dozen buds from the main framework. Pruning Wisteria twice a year encourages flowering, and some years it may even produce a second show of small flowers in August. 

Perennials :The question what to cut back and what to leave?

During September many perennials will look tired, with browning leaves. Later, as frosts strike, some of the herbaceous perennials will collapse into brown heaps, but some garden plants keep their shape.

The question is what to cut back and what to leave?

Some plants look good as they fade, such as sedum where the flower heads hold their own for weeks and look good with a dusting of frost. Others, such as Hosta  look a mess.
There is no hard and fast rule about what to do with herbaceous and perennial plants once summer is over. 

Some gardeners cut everything back, clearing out the borders, and others leave plants uncut to encourage wildlife and cultivate a less formal look. There is also a halfway house; cut back plants that look a mess, such as Hosta, Crocosmia, Delphinium leaving in place those which still add to the border, at least for a while, which also spreads the work in the garden over the winter months. As with many aspects of gardening, there is no right or wrong it's down to preference. I tend to cut quite a lot back, not least because I find all the summer weeds sheltering under the plants.  I try to get the borders weed-free over the autumn and winter and then mulch,  ready for next year; at least that's the plan. 

How to Overwinter and Protect tender plants

Overwintering plants on a trestle

By late September, the risk of frost is not far away and it is a good time to consider which of the tender plants are worth saving to overwinter in a frost free place. For example, Pelargoniums survive well in a conservatory or a sunny porch, or garden lean to,  and look
 lovely for months as they continue to flower.  Fuchsias, and Pelargoniums can be put under glass, others such as Petunias, Marguerite's, Diascias, Osteopermums are ready for taking cuttings-click here for more information.

If you are overwintering plants in a greenhouse, raise the plants off the ground because the base can be very cold, especially sitting on concrete slabs. Raising the plants off the ground is warmer, and it allows air to circulate as the damp, stagnant air aids disease, especially grey mould, which can so often be a problem when over wintering plants. If you can over winter the tender plants, it saves money for next year and you start the spring with a mature plant which should flower well. 

Time to think about Spring bulbs

Lovely pink Tulips

Autumn is the time for planting spring bulbs. It's best to plant daffodils during September whereas Tulips are best left until October/ November. The correct planting depth is very important to keep them flowering; too shallow and they will not flower. An easy rule of thumb is to plant the bulb 3xpreferably 4x times it's own depth. There are some great spring bulb combination. The planting depth is more important than when you plant. If you miss the window most bulbs (not snowdrops) can be planted as late as December or early January, especially late flowering varieties. 

For inspirations and images of spring bulbs, look at Pinterest Spring bulbs

It is not necessary when planting new spring bulbs in containers to use fresh compost. You can re use compost from any end of season containers being cleared out. A bulb is a complete power pack and has everything within the bulb to flower next year and needs no additional nutrients. Read more about using old compost and saving money.

Deadhead to prolong flowering

It is still worth dead heading perennials and annuals, especially the late flowering ones. Depending on the summer, even annuals such as sweet peas may still flower and, to keep them going, continue to dead head unless you want to collect the seed.

The late flowering perennials, such as Dahlias, will benefit greatly from continuous dead heading.

Seed collection

Saved flower  seeds

Late summer/early autumn is a good time to collect seed if you want to use it next year. Collect seed heads, carefully shaking or scraping out the small seeds and store in a packet in a dry place - a sealed tin is useful. Moisture is the very worst thing for seeds which need to be stored in a dry and cool, place, such as in the garage or in a fridge. I sometimes add grains of rice to absorb any lingering moisture, or those little packs of silica gel which come with some goods, (because they absorb water.)

A good starting place if you've not collected seeds before are Viola, Rocket, Nasturtiums, calendula, Cerinthe major, cornflower, and the bean family all of which are easy to collect and germinate. Given the cost of seeds, it is worth a try. 

Information on how to collect and save seed. 

Lawn Care

If your lawn has developed bare patches, this is a good time to seed. The soil is warm and we should have rain as we move into autumn,  which will help the seed to germinate and get going. If pigeons or birds are a problem in the garden, cover with sticks or a net to stop them eating the seeds.

September in the Veg plot

How to Store onions and garlic

The veg plot is full of crops ready for harvesting and September is time to pick, freeze and store. All the beans are ready and creating gluts, Sweet corn has ripened, onions and garlic are now fat bulbs. Ideas on what to do with a glut of Green Beans.

Onions will be ready either at the end of August or early September, depending on the weather. To harvest, bend over the top growth (if this hasn't already happened naturally) and harvest during a dry spell and ease the bulbs out of the earth. Traditionally, onions were dried by resting on soil during a sunny spell. With our variable weather  it can be difficult to find a dry spell to rest the onions outside.

An alternative, if the weather is poor, is to lay the onions out in the greenhouse - drop the foliage down between the slats and rest the onions on the slats which is an ideal way to dry them out, see the images below. Onions can also be dried in a shed placed in netting or even on newspaper in a conservatory. The onion bulbs must be completely dry before storing and the same applies to garlic.

Garlic is similar, wait till the top growth dries and turn brown usually in August/Sept. Store in a dry and light spot in the warmth rather than a cold area.

If you want to make onion or garlic strings when you harvest the bulb, keep as much of the top growth as possible to make into a plait. If there is not enough top growth, work in some raffia to help make a plait.

The images show garlic and onions drying in a greenhouse,  when summers are wet, this is the most practical way of drying onions and garlic ready for storage over winter. The right image onions strings storing overwinter in a conservatory. Onions and garlic easy crops to grow and for ideas for next year's crop advice on growing garlic and onions.

Harvest main crop Potatoes

Hessian sacks for storing Potatoes

Main crop potatoes can be harvested this month and need to be stored somewhere dry, and importantly, dark. The Hessian sacks sold by garden centres and on-line shops are ideal for storing potatoes.  Potatoes can go green very quickly when exposed to light, even if left in a green house for 24hours they will turn green. To prevent this, you need to exclude light. Potatoes also need to be dry, if they are wet when you dig them up, put them somewhere to dry off but quickly as soon as they are dry take out of the light and put in sacks/somewhere dry and dark. The Hessian sacks are good because they are a natural fibre, but you can use other mediums just to avoid plastic or polythene, which will tend to trap moisture.

This time of year it's hard to avoid a glut of something, ideas for what to do with a glut of beans. All the beans family,  French, broad, runners, and peas home freeze really well.   

Courgettes are more difficult as they don't really freeze very well and I make a mental note, each year, to grow fewer plants next year as there always seem to be too many. 

Tomato Care

ripening-toms (2)

Tomatoes continue to need a lot of attention and it is important to water either daily or every other day depending on conditions and feed regularly at least twice weekly. As the month moves on towards October and the plants are naturally slowing down, reduce the amount of watering and feed. When to do this will depend on where in the country the Tomatoes are being grown, the growing season is longer in the South of the country.  If the plants are still growing, they will continue to produce leaves and it is important to continue to thin these down to encourage the fruit to ripen. As the fruits ripen, the vines will need extra support and ties. Using soft raffia is good as it reduces damage to the tomato stems.

Sometimes it's difficult to get tomatoes to ripen late in the season Click here for a foolproof way to ripen tomatoes.

And sometimes we have a glut of tomatoes. Click here for recipe ideas for a glut of tomatoes.

For the best advice about growing tomatoes, including how to grow them from seed, which are the best types of tomatoes to grow, the right way to water tomatoes and much more, check out the Sunday Gardener's handy book " Success with Tomatoes" A concise, informative and practical guide containing all you need to know to ensure a tasty crop. 

Still time to sow Herbs and Salad


In many parts of the country, this is the last chance to sow rocket and salad and if the weather is cool, cover with a cloche to encourage germination and growth.

This is a good time to replenish dried herbs, which lose their pungency after a period of storage. Ideal herbs for drying are Oregano, Sage, Mint and also, Thyme and Rosemary, although as hardy perennials, we can pick them all year round.  

With the tender Herbs Basil, Coriander, dill & Mint, it is best to pot them up later in the month and bring in under glass ahead of any autumn chill.