You've done the hard work — seeds germinated, seedlings growing strong under your lights. Now comes one of the most critical (and most skipped) stages: getting your plants ready for the real world. Hardening off and potting up are two steps that make the difference between plants that thrive outdoors and plants that sulk, stall, or die. Here's exactly how to do both.
What Is Hardening Off?
Hardening off is the process of gradually acclimatising indoor-grown seedlings to outdoor conditions — sun, wind, temperature fluctuations, and rain. Plants raised indoors under grow lights live in a controlled, sheltered environment. Put them straight outside and the shock can cause wilting, sunscorch, or even death.
The process typically takes 7–14 days and costs nothing except a bit of attention. Skip it and you risk losing weeks of growth in a single afternoon.
Step-by-Step Hardening Off Schedule
Follow this 10-day plan for most seedlings:
Days 1–2: Place plants outside in a sheltered, shaded spot for 1–2 hours. Bring them back in before temperatures drop. No direct sun yet.
Days 3–4: Increase outdoor time to 3–4 hours. Introduce gentle morning sun (before 10am) for the last hour.
Days 5–6: 5–6 hours outside, including some direct sun. Watch for wilting — if leaves droop, bring them in and shade them the next day.
Days 7–8: Leave plants outside for most of the day. They can now handle several hours of direct sun. Bring in overnight if temperatures drop below 10°C.
Days 9–10: Plants can stay outside all day. Leave overnight only if frost is not forecast. After this, they're ready for permanent outdoor placement or final potting.
Key tip: Wind is as stressful as sun for young plants. Start in a sheltered spot and gradually expose them to breezier conditions.
Common Hardening Off Mistakes
- Going too fast — two days outside doesn't count as hardening off. Stick to the schedule.
- Forgetting to water — outdoor conditions dry compost out much faster than indoors. Check daily.
- Leaving plants out overnight too soon — even “hardy” plants can be set back by a cold night early in the process.
- Full midday sun on day one — this is the fastest way to sunscorch tender leaves. Start with morning light.
- Ignoring the wind — a breezy day can dehydrate seedlings faster than direct sun.
When to Pot Up Into Bigger Containers
Potting up — moving a plant into a larger container — should happen when your plant has outgrown its current pot, not on a fixed schedule. Look for these signs:
- Roots emerging from drainage holes — the plant has filled its current container
- Rapid drying out — if you're watering daily and the compost still dries out quickly, the roots have taken over
- Slowed growth despite regular feeding — a root-bound plant can't take up nutrients efficiently
- Visible root circling — when you ease the plant out, roots are tightly wound around the outside of the rootball
As a general rule, move up one pot size at a time — don't jump from a 1L to a 10L pot. Too much unused growing medium around the roots can hold excess moisture and cause problems.
How to Pot Up — Step by Step
1. Choose the Right Pot Size
Go one size up from the current container. For most seedlings, this means moving from a propagation cell or 9cm pot to an 11–13cm pot, then up to a 2–3L container as the plant establishes.
2. Prepare Your Growing Medium
Use a quality potting mix suited to your crop. Mix in 10–20% perlite to improve drainage and aeration — this is especially important if you're growing in a heavier soil-based compost. Moisten the mix before use so it's damp but not waterlogged.
3. Water Before You Repot
Water the plant an hour before potting up. A hydrated rootball holds together better and causes less root disturbance during the move.
4. Remove the Plant Carefully
Squeeze flexible pots gently to loosen the rootball, or run a knife around the inside edge of rigid pots. Tip the plant out and support the rootball — don't pull by the stem.
5. Place and Backfill
Add a layer of fresh growing medium to the base of the new pot, position the plant, and backfill around the sides. Firm gently — no need to compact heavily. Leave 2–3cm of space at the top for watering.
6. Water In and Feed
Water thoroughly after potting up to settle the medium around the roots. This is a good time to introduce a root stimulant or mycorrhizal inoculant to help the plant establish quickly in its new container. Hold off on heavy nutrient feeding for a few days to let the roots settle.
Recommended Products
- Propagation trays and cells — for raising seedlings before their first pot-up
- Perlite — mix into any growing medium to improve drainage and aeration
- Quality potting compost — a reliable base for potting up at every stage
- Pots and containers — a range of sizes so you can step up gradually
- Root stimulants / mycorrhizal inoculants — support root establishment after every repot
- Seedling and grow nutrients — gentle formulas suited to young plants transitioning outdoors
Browse our full range to find everything you need from propagation through to final container.
FAQ
Do all plants need hardening off?
Any plant raised indoors or in a greenhouse needs hardening off before going outside permanently. This includes tomatoes, peppers, courgettes, herbs, and most flowers started from seed indoors.
Can I harden off in a cold frame or greenhouse?
Yes — a cold frame is ideal. Open the lid progressively over the hardening off period to gradually expose plants to outdoor air. An unheated greenhouse works similarly.
What size pot should I use for potting up?
Go one size up each time. Jumping too many sizes at once leaves too much unused growing medium around the roots, which can stay wet and cause root problems.
How often should I feed after potting up?
Wait 3–5 days after potting up before resuming regular feeding. Fresh growing medium contains some nutrients, and the roots need time to settle before heavy feeding begins.
Can I harden off and pot up at the same time?
It's better to do one at a time. Pot up first, let the plant settle for a few days, then begin hardening off — or complete hardening off before the final pot-up into an outdoor container.
Final Thoughts
Hardening off and potting up are the unglamorous middle stages of growing — but get them right and your plants will reward you with stronger growth, better yields, and far fewer problems. Take your time, watch your plants, and don't rush the process.
Need help choosing the right pots or growing media for your setup? Browse our range or get in touch — we're here to help.