There are a variety of reasons to pinch back plants, and sometimes timing is very important
Most gardeners learn early that pinching back our plants encourages full, compact growth. But there is so much more to know! The reasons, results and timing for pinching back can be different for annuals, perennials, herbs, vegetables, summer blooming, fall blooming, etc.
It helps to understand exactly what pinching is doing to the plant. When we pinch out the growth tips on our plants it removes the production of auxin. Auxin is the growth hormone that encourages upward growth. Once removed the stem instead will produce more lateral branches. But pinching prompts a number of changes in plant growth.
So first we need to be aware of all the reasons for pinching so we can apply the knowledge for our preferred results.
Why pinch back our plants?
To shape a plant
To control size and fullness
To increase production of herbs
To control bloom size and quantity
How to pinch back
Pinching back is similar to pruning and deadheading, but is typically more precise to obtain specific objectives. For tender plants it is very easy to pinch off growth tips by simply pinching the stem between your fingertip and thumbnail. But certainly you can also use a garden scissor or shears if you have a lot to do or with sturdier stems.
The top couple of inches of stem is normally removed when pinching back, but you can remove up to half the length of a stem. Locate a spot on the stem where leaves join the stem. There will often be a pair of tiny buds forming just above the leaf stems. Pinch off the stem just above that spot being careful not to damage the new buds. This is where the plant will work to produce two new lateral stems. Pinching close to the leaf nodes will also help keep the plant looking neat.
Results of pinching back
Different plants will react differently to pinching back. It is important to understand the differences and best time to pinch certain plants to obtain your desired results. Most plants will grow compact and sturdy from being pinched back.
New side shoots will also produce additional blooms but those blooms may be smaller and later. If done properly you can extend the normal bloom time. By pinching back only half of the growth tips the untouched stems will produce blooms as usual. The pinched back stems will produce two blooms per stem a few weeks later. This extends the normal bloom period substantially. Conversely, if you prefer a few huge blooms, especially with mums, pinch of all side shoots very early in the season to encourage one large bloom per stem.
Some perennials should NOT be pinched as it may cause the plant to produce no blooms at all. Single blooms produced on a flower stalk rising from the plant base do not respond at all to pinching and will fail to produce. Plants with a single stalk that produce a spray of blooms also do not respond to pinching. Do not pinch back these perennials:
Red Hot Poker
Late blooming perennials should be pinched early in the season up until about the fourth of July. Pinch only the top tips to keep them tall.
Perennials that respond well to pinching back and can be pinched in May and June
Helianthus perennial sunflower
Joe Pye Weed
Annuals that are multi stemmed respond well to pinching. Pinching back annuals can be done immediately after planting and again when fast growth begins after establishing their roots. Pinching off the blooms of newly planted annuals will cause the plant to focus on root development. Pinching again when strong growth begins will create a compact well branched plant with twice as many blooms. Continuing to pinch off spent blooms is referred to as deadheading and in will cause the plant to repeatedly produce new blooms.
Annuals that are especially responsive to early pinching and produce blooms generously: Some are excellent cutting flowers so filling your vases is encouraged:
Many herbs should be pinched at the first sign of bloom buds to force continued herb production. Once a bloom is allowed to form the plant will focus on developing the bloom and seeds for reproduction. The herb foliage typically becomes bitter or strong flavored once a bloom develops. Some herbs respond to pinching by increasing the herb (foliage) production.
Herbs to pinch frequently throughout the season to increase production:
Herbs to pinch frequently in spring to control size, but later reduce the woody stems by only one third or less:
Herbs to hard pinch, removing half the stem length:
As you approach the end of growing season you may want to let everything flower and go to seed for the birds or for self seeding.