Growing Garlic, Onions & Alliums
When grown in loose, fertile soil rich in organic matter, alliums reward gardeners with large bulbs, excellent flavor, and long storage life. In many ways, alliums are indicators of how well the underground ecosystem is functioning.
What Are Alliums?
Garlic and onions are often casually grouped with root vegetables — but that is botanically incorrect. Garlic, onions, shallots, leeks, scallions, and chives all belong to the allium family: plants that grow as bulbs, which are modified stems surrounded by fleshy leaf bases. The underground bulb we harvest is not a root at all, but a tightly layered structure formed from swollen leaf bases attached to a small root plate.
This distinction matters for how we grow them. Root crops expand underground storage roots, while alliums build layered bulbs in response to seasonal signals — a combination of cold exposure, soil conditions, and day length. Because allium roots stay shallow, they cannot forage for nutrients deeper down and depend entirely on whatever is in the top layer of soil. Good soil structure and a healthy microbial environment are therefore essential.
When grown in loose, fertile soil rich in organic matter, alliums reward gardeners with large bulbs, excellent flavor, and long storage life. In many ways, alliums are indicators of how well the underground ecosystem is functioning.
What Garlic Needs to Grow Large Bulbs
Unlike true root crops — which often prefer lean, low-nitrogen soil — garlic is a heavy feeder that thrives in nutrient-rich conditions, but less nutrients than fruiting and leafy veggies above ground.
Garlic needs:
Fall planting (mid-October to early November) for the cold exposure that triggers proper bulb formation.
Fertile, well-drained soil — approximately 30% finished compost blended into 70% mineral topsoil (root veggie low nutrient soil), worked to a depth of 10–12 inches. Too much compost (above 40%) retains excess moisture and raises the risk of bulb rot. Deep Roots recently started selling this topsoil.
Soil organic matter content of approximately 5%.
Proper spacing of 6–7 inches between plants.
Don’t add nitrogen fertilizer (like the Deep Roots compost) at fall planting. Fall nitrogen pushes tender top growth that winter will kill.
Add a pinch of PhoSul in each planting hole at fall planting — a slow-release phosphorus and sulfur source that supports root and clove development without triggering leaf and shoot growth. Sulfur also contributes to garlic’s flavor and natural pest resistance.
In spring top-dress the bed with about a half inch of compost and worm castings when shoots reach 4–6 inches tall.
Soil pH between 6.0 and 7.0 is best
Garlic Planting & Care Calendar
Timing: Plant garlic about 3–4 weeks before the ground freezes, ideally after the first hard frost. In Zone 5, this typically means mid-October to early November.
Fall Planting: Mid-October to Early November. Garlic is one of the simplest and most rewarding crops to plant in fall. The goal is to give each clove time to root before the ground freezes, without encouraging much top growth before winter. Plant cloves pointed-end up, about 2 inches deep — a little deeper in sandy soil — and space them about 6 inches apart.
For colder climates like Zones 5b–6a, hardneck garlic is usually the best choice. It handles winter well and produces the bold flavor many gardeners are looking for.
Because Deep Roots soil already contains 30% nutrient- and microbe-rich thermophilic compost, including composted manure, blended with 70% sandy loam topsoil, there is no need to automatically add compost or worm castings into every planting hole. The cloves are already going into a fertile, biologically active soil with plenty of built-in organic matter.
PhoSul fertilizer: Add a small amount of PhoSul fertilizer – at most an 1/8 teaspoon in the clove hole. Phosphorus supports early root development, and sulfur is especially valuable for alliums, helping build the flavor and natural protective compounds that make garlic such a standout crop. PhoSul is gentle, so even slightly more won’t matter.
Straw mulch: After planting, cover the bed with 2 to 4 inches of straw mulch to help regulate soil temperature, reduce frost heaving, and suppress early weeds.
Spring Fertilizing: In spring, when garlic shoots reach about 4 to 6 inches tall, any feeding should be light and spread across the entire bed surface, not concentrated at the base of each plant. Garlic’s feeder roots reach outward, so a gentle bed-wide top-dress works better than feeding only at the stem.
Testing in Progress: Spring 2026 is our first season using the new soil for garlic. The Deep Roots team will be testing what amount of compost and worm castings that works best as a spring top-dress. We invite our gardeners to experiment with different small amounts as well and watch how their garlic responds. Our goal is to find the minimum effective amount for strong growth and healthy bulb development in a soil blend that is already rich and active.
With garlic, more nutrients is not always better. A strong start, a light touch, and healthy soil do most of the work.
June Scape Removal
Hardneck garlic sends up a curling flower stalk called a “scape” in early summer. Snap or cut it off as soon as it completes one full curl. Leaving the scape to flower diverts significant energy away from the bulb — removing it can increase bulb size by up to 30%. This is one of the most impactful and most commonly overlooked steps in garlic culture.
Scapes are a culinary treat: mild, sweet garlic flavor with a tender-crisp texture similar to asparagus. Use them sautéed, grilled, in stir-fries, or blended into pesto.
Spring and early summer watering
During spring and early summer, garlic benefits from consistent moisture while the bulbs are expanding. As harvest approaches, stop watering completely about two weeks before you plan to harvest. This allows the bulb’s outer wrapper layers to dry and paper over properly — essential for quality and long storage.
July Harvest
Harvest when one-third to one-half of the leaves have turned brown while the upper leaves are still green. Do not wait for full browning — by then the protective wrapper has often begun to deteriorate and cloves may be separating.
Loosen bulbs gently with a garden fork rather than pulling by the stem. Brush off loose soil but do not wash.
After Harvest Curing
Hang bulbs or lay them in a single layer in a shaded, well-ventilated spot for 3–4 weeks. Proper curing converts fresh-dug garlic into shelf-stable bulbs that store for months. Once fully cured, trim the roots and stems and store in a cool, dry location with good air circulation — never in sealed plastic.
Onions and Other Alliums
Garlic shares its family with onions, shallots, leeks, scallions, and chives. All share key characteristics: they grow from bulbs or bulb-like bases, have shallow root systems, and prefer loose, fertile soils rich in organic matter. Like garlic, they rely on temperature and day length to trigger bulb development. Well-prepared Deep Roots beds with balanced compost content serve all alliums well.
Companion Planting: Garlic & Spinach
Garlic and spinach make excellent companions in fall-planted beds. Garlic’s scent deters pests like aphids that bother leafy greens, providing natural protection for the spinach in spring.
How to do it:
Plant garlic cloves and spinach seeds in the same bed in mid-October to early November.
Spinach sown in fall will overwinter under mulch or germinate early in spring, maturing well before garlic needs the full bed.
Inter-plant spinach between rows of garlic. Spinach can be harvested by May or early June — long before the July garlic harvest.
Use hardneck garlic varieties, which are better suited to Zone 5 winters.