What can still be planted in late autumn

Many plants can still go into the ground in late autumn, but the window narrows as the soil cools. The most reliable choices are spring-flowering bulbs such as tulips, daffodils, crocuses and alliums, garlic, dormant bare-root trees and shrubs, and hardy perennials that have time to root before winter. Timing depends less on the calendar than on how cold the soil is and how many weeks remain before the ground freezes in your area.

Root growth slows noticeably as soil temperatures fall toward 40 degrees Fahrenheit, and a common guideline is to plant trees and shrubs at least six weeks before the ground freezes. That head start lets roots anchor the plant before cold weather halts growth. A plant set in very late autumn may survive, but it has less time to build a root system and is more exposed to drying winter winds and repeated freeze-thaw cycles.

Spring bulbs that can still go in

Spring-flowering bulbs are a classic late autumn planting because they need a cold period to bloom well. About six weeks before the ground freezes is the ideal time to plant them, since the bulbs have time to root before the soil cools. They can still go in any time the soil can be dug, though, right up until it freezes. Bulbs planted late usually still flower, though the display may come later or be smaller. Set each bulb with its pointed end facing up, at a depth of about two to three times the height of the bulb. Bulbs set too shallow may flower poorly or be pushed up by frost movement in the soil.

Squirrels, voles and deer often dig up or eat certain bulbs, so planting in a spot they rarely visit, or covering the bed with wire mesh until the shoots appear, can reduce losses. Daffodils are generally less attractive to these animals than tulips, though no bulb is fully safe.

Garlic and hardy greens

Garlic is one of the most common crops planted in late autumn in regions with cold winters. Separate the cloves shortly before planting, keep the papery skins intact and set each clove about two inches deep with the pointed end up. In mild climates, hardy greens such as spinach and some lettuces can also be sown in late autumn for a winter or early spring harvest. Seeds germinate slowly in cold soil, though, so a row cover often helps them along.

Garlic needs well-drained ground more than rich soil, since waterlogged beds encourage rot in the cloves. Loose, well-drained soil or a raised bed suits it well.

Trees, shrubs and bare-root stock

Woody plants need enough root time before the soil freezes. Bare-root trees and shrubs, sold while dormant, are the classic late-autumn planting, but their roots are exposed and should go in promptly. Spread the roots into loose soil and set the root flare at or just above the soil surface. Container-grown shrubs can also go in while the soil is workable, but the potting-mix root ball dries out faster than the soil around it, so check it for moisture before planting and water it in thoroughly.

Before planting, dig the hole about two to three times as wide as the root ball and no deeper than the root ball itself, then loosen the sides so new roots can push outward. Avoid heavy amendments in the backfill, since roots that stay inside an enriched pocket can struggle to spread into the surrounding soil. Water thoroughly after planting, and keep watering whenever the soil is dry and not frozen, since plants that enter winter dry are more prone to damage.

Perennials and cool-season flowers

Hardy perennials can usually be planted or divided in autumn if several weeks of mild weather remain. Pansies and violas tolerate light frost and can brighten a bed or container into early winter, particularly in mild climates. Tender perennials, warm-season annuals and most vegetables that need heat should not be planted this late, because a sudden freeze can kill them before their roots form. If frost is forecast, a frost cloth over young plants offers short-term protection, but it cannot make up for missing root time.

Protecting new plantings through winter

After planting, water well and keep the root zone moist until the ground freezes. Hold off on heavy mulch until the soil has frozen several times. An early layer keeps the soil warm, which can keep plants growing when they should be settling into dormancy and can invite mice to nest in it. Once the ground has frozen, a layer of shredded leaves, straw or bark a few inches deep protects shallow roots and bulbs from repeated freeze-thaw cycles.

Winter dryness is a separate risk. A new planting can dry out during a mild, snowless spell even when the surface is frozen, so check the soil every few weeks and water on mild days when the ground is not frozen.

When to wait until spring

Some plantings are better delayed until spring. Plants that are not hardy in your area, tender bulbs and any plant that needs a long establishment period may do poorly if set out in cold soil. Most flower and vegetable seeds germinate poorly in cold ground, so sow them in spring unless the packet specifies cold sowing. Postpone planting when the soil is frozen, waterlogged or still covered with snow.

A quick check helps decide. Push a spade into the bed: if it slides in and lifts the soil in clean clumps, the ground is workable. If the blade will not enter, the soil is frozen. If the soil smears into a sticky, waterlogged mass instead, give it several dry days, because digging wet ground compacts it. Your local extension office can supply the average date of the first hard freeze and the date the ground usually freezes in your area, which shows how much planting time is left. Late autumn planting works best when the hardy material goes in first and the tender plants wait for warmer weather.